Chapter 7-Aerial Structures/Bridges

Table of Contents

7.1 INTRODUCTION 7-1

7.2 DESIGN CODES 7-l 7.3 VEHICLE FORCES 7-2

7.4 TRACK CONFIGURATION 7-2 7.5 RAIL/STRUCTURE INTERACTION 7-4 7.5.1 General 7-4 7.5.2 Bearing Arrangement at the Piers 7-6 7.5.3 Rail/Structure Interaction Analysis 7-6 7.54 Rail Break/Rail Gap Occurrences 7-7 7.5.5 Terminating CWR on Aerial Structures 7-10 7.5.6 Types of Deck Construction 7-l 1 7.5.6.1 Ballast Deck Construction 7-12 7.5.6.2 Direct Fixation Deck Construction 7-12

7.6 DIRECT FIXATION FASTENERS 7-13

7.7 TYPES OF SUPERSTRUCTURE 7-14

7.8 REFERENCES 7-17

List of Figures

Figure 7.21 Vehicle Bending Moments on Simple Spans 7-1 Figure 7.5.1 Radial Rail/Structure Interaction Forces 7-s Figure 7.5.2 Bearing Configurations for Elevated Structure Girders 7-6 Figure 7.53 Rail Break Gap Size predicted by Finite Computer Model 7-9 Figure 7.54 Tie Bar on Aerial Crossover 7-l I Figure 7.7.1 Typical Section of Elevated Structure Studied 7-15 Figure 7.7.2 Range of Deck Costs as a Function of Span Length and Beam Spacing of Structure 7-15 Figure 7.7.3 Range of Supporting Bent Costs as a Function of Span Length of Structure 7-15 Figure 7.7.4 Range of Foundation Costs for Different Soil Conditions as a Function of Span Length of Structure 7-15 Figure 7.7.5 Range of Total Costs of Elevated Structural System as a Function of Span Length for Different Soil Conditions 7-16 Figure 7.7.6 Average Ratio of Cost of Each Structural Subsystem to Total Cost of Structure-Founded in Good Soils 7-16

7-i Light Rail Track Design Handbook

Figure 7.7.7 Average Ratio of Cost of Each Structural Subsystem to Total Cost of Structure-Founded in Poor Soils 7-16 Figure 7.7.8 Average Ratio of Cost of Supporting Structure and Foundation to Cost of Deck Structure for Different Soil Conditions 7-17

List of Tables

Table 7.1 Effects of Unbroken Rail and Column Longitudinal Stiffness on Loads Transferred to the Substructure 7-8 Table 7.2 Comparison of Rail Break Gap Size by Different Formulas 7-10

7-ii CHAPTER 7-AERIAL STRUCTURES/BRIDGES

7.1 INTRODUCTION structures. In addition to local design codes, designers must choose between the Standard Railway aerial structures started as ballasted Specifications for Highway Bridges, published track structures that had little structural by the American Association of State Highway interaction between the rails and the structure. and Transportation Officials (AASHTO) and Urban railways and long span lift bridges have the Manual for Railway Engineering issued by been constructed with open deck designs. the American Railway Engineering and These lighter structures used jointed rail to Maintenance of Way Association (AREMA). limit the interaction between the rail and the Unfortunately, neither the AASHTO nor structure. CWR direct fixation track on a AREMA code accurately defines the concrete deck is typical of modern light rail requirements of an aerial structure to resist aerial structures. These structures can have light rail transit loads, although the AASHTO significant interaction between the rail, which code is probably more applicable. does not move, and the structure, which must expand and contract with changes in Most light rail loads are greater than the HS20 temperature. This chapter discusses the truck load used by AASHTO, but they are resolution of rail/structure interaction issues much less than the Cooper E80 railroad and presents the items to be considered loading cited in the AREMA code. Figure during the design of aerial structures. 7.2.1 plots bending moment versus span length for the Cooper E80 train load, the HS20 The design of aerial structures for light rail truck load, and the LRV load from the Dallas transit systems involves choosing a design and St. Louis transit systems. As shown in code, determining light rail vehicle (LRV) the figure, for a 305meter (lOO-foot) span, forces, confirming track configuration the LRV produces a bending moment requirements, and applying rail/structure approximately 50 percent higher than that interaction forces. This interaction is affected produced by the HS20 truck load, but less by such factors as the bearing arrangement at than 20 percent of the bending moment the substructure units, trackwork terminating caused by the Cooper E80 train load. on the aerial structure, type of deck construction, and type of rail fasteners.

The structural engineer must coordinate with the trackwork engineer to fully understand the s- issues that affect the design of an aerial z. trrm ! I ..=i;SZO HIGHWAY-\ structure. The details of the trackwork design _--’ --LIGHT RAIL MHICX - significantly affect the magnitude of the forces that must be resisted by the aerial structure. Figure 7.2.1 Vehicle Bending Moments on Simple Spans [‘I 7.2 DESIGN CODES The AREMA code, although applicable to At present there is no nationally accepted railroad structures, is too restrictive for light design code that has been developed rail transit structures due to the great specifically for light rail transit aerial difference in loadings. Wheel spacings for

7-1 Light Rail Track Design Handbook

AREMA loading do not correspond to those heavier vehicles in the design criteria for found on LRVs, and the AREMA impact aerial structures These alternative criterion is not consistent with the suspension maintenance/construction vehicles include a and drive systems used on LRVs. The crane car, maintenance car, work train with service conditions, frequencies, and types of locomotive, and even highway vehicles loading applicable to freight railroad bridges (during construction). On the other hand, are not consistent with those items on some transit properties establish the LRV as dedicated light rail transit systems. [‘,*I the basis of design for the aerial structures.

Alternately, a strong similarity exists between In addition to the LRV and alternative vehicle light rail transit design requirements and the live loads applied to the aerial structure, the AASHTO code. For light rail transit aerial following vehicle forces are considered: structures, the ratio of live load to dead load 0 Vertical impact more closely approximates that of highway l Transverse horizontal impact loadings than freight railroad loadings. In 0 Centrifugal force

addition, since the magnitude of the transit live l Rolling force (vertical force applied at load can be more accurately predicted, the each rail, one up and one down)

conservatism inherent in the AREMA code is l Longitudinal force from braking and not required in light rail transit structures. tractive effort

l Derailment force It is interesting to note that the older transit systems (Chicago, Philadelphia, New York) Combinations of vehicle forces, in conjunction often refer to the AREMA code for design of with dead loads, wind loads, and seismic their bridges, but the newer systems (Atlanta, loads, are developed to generate the load Washington, Baltimore) base their designs on cases that govern the design of an aerial AASHTO specifications. This is partly due to structure. an increased understanding of an aerial structure’s behavior and the designers confidence in the ability to more accurately 7.4 TRACK CONFIGURATION predict the transit loads. Both heavy rail and The majority of the early transit systems used light rail transit systems can use AASHTO as trackwork comprised of jointed rail supported a guide since their axle loads and car weights on elevated, simple-span guideway structures. are similar. Alternatives have been developed for light rail Although there is no current bridge design transit trackwork. Rather than the classical code that is completely applicable to light rail jointed rail with bolted connections every 12 transit bridges, the use of the AASHTO code meters (39 feet), the trackwork is normally will result in a conservative design that is not constructed with continuous welded rail. With overly restrictive or uneconomical. (‘,2,31 either rail configuration, the rails can be fastened directly to the aerial structure’s deck or installed on ties and ballast. 7.3 VEHICLE FORCES The bolted connections used with jointed rail The vehicle forces applied to an aerial allow sufficient longitudinal expansion and structure are often set by the transit agency’s contraction to reduce the accumulation of design criteria for site-specific circumstances. thermal stresses along the rails. But bolted Many transit properties include alternative joints have the following disadvantages:[41

7-2 Aerial Structures/Bridges

l Generate noise and vibration and contracts, while the CWR remains in l Are troublesome to maintain a fixed position l Contribute to derailments l Providing a connection between the CWR l Cause rail fatigue in the proximity of the and aerial structure (direct fixation rail joints fasteners) that is resilient enough to l Cause wear of the rolling stock permit the structure to expand and l Reduce ride quality contract without overstressing the l Increase the dynamic impact forces fasteners applied to the aerial structure An important element in the design of Over the past 20 years, CWR has been the trackwork using CWR is the consideration of most common track configuration for light rail rail breaks. Rail breaks often occur at transit systems. This is mainly due to its structural expansion joints in the aerial ability to overcome many of the disadvantages structure and must be accommodated without of jointed rail. Specifically, CWR? 61 catastrophic effects such as derailment of the l Minimizes noise and vibration vehicle. Depending on the length of the aerial l Reduces track maintenance structure, the CWR has to be sufficiently l Improves track safety restrained on the aerial structure to limit the l Eliminates the joints that cause rail fatigue length of the gap if the rail does break. l Limits wear of the rolling stock l Provides a smooth, quiet ride CWR is a standard now employed in the l Limits the dynamic impact forces applied transit industry. Therefore, transit system to the aerial structure designers must understand how it interacts with aerial structures as the temperature The use of CWR, combined with direct fixation changes in order to provide a safe track and of the rails to the supporting structure, is an structure. improvement in the support and geometric stability of the trackwork. As a result, rider Expansion (sliding) rail joints are used in comfort and safety is enhanced and track certain circumstances to reduce the maintenance requirements are decreased. interactive forces between the CWR and the structure. These include locations where The use of CWR requires designers of special trackwork is installed on the aerial trackwork and aerial structures to consider structure, where signal track circuits need to items that are neglected with the use of be accommodated, and where the aerial jointed rail, such as? **‘I structure includes very long spans. Providing sufficient rail restraint to prevent horizontal or vertical buckling of the rails Rails can be attached to the structure in a Providing anchorage of the CWR to variety of ways. The most common prevent excessive rail gaps from forming if mechanism is the use of direct fixation the rail breaks at low temperature fasteners with spring clips. Rigid rail clips have also been used in the vicinity of Determining the effect a rail break could substructure units (piers and abutments) with have on an aerial structure fixed bearings, as well as adjacent to special Calculating the thermal forces applied to trackwork. Also, zero longitudinal restraint the aerial structure, the rail, and the fasteners have been installed to minimize the fasteners as the aerial structure expands

7-3 Light Rail Track Design Handbook interaction forces between CWR and an installation temperature cause tensile forces existing aerial structure. that increase the probability of a rail break (pull-apart). A rail break creates unbalanced forces and moments in the aerial structure 7.5 RAIL/STRUCTURE INTERACTION and results in a gap in the rail that could cause a derailment. Rail breaks are discussed in further detail in Section 7.5.4. 7.51 General Based on these thermal effects, there are With widespread use of CWR, the designer of three problems to address in the design of an aerial structure must be aware of trackwork aerial structures with CWR: design and installation procedures, as well as 1. Controlling the stresses in the rail vehicle performance and ride comfort issues. attributed to the differential longitudinal Trackwork design and installation procedures motions between the rail and the are especially critical in establishing the superstructure because of temperature magnitude of the interaction forces between changes or other causes the rail and aerial structure. 2. Controlling the rail break gap size and As the temperature changes, the resulting loads into the superstructure superstructure (deck and girders) expands or 3. Transferring of superstructure loads and contracts. The rails are basically stationary moments into the substructure because of their continuity throughout the length of the bridge and because they are A structural system is formed when CWR anchored off the bridge. The movement of the track is installed on an aerial structure. The superstructure as the temperature changes major components of this system include:‘61 imposes deformation on the fastening system Long, elastic CWR, whose ends are that attaches the rails to the bridge deck. anchored in ballasted track beyond the abutments This thermal action exerts additional interactive axial forces and deformations on Elastic rail fasteners that attach the rails the rails and superstructure. Reaction loads directly to the superstructure are applied to the substructure (piers and The elastic superstructure abutments) through the fixed bearings and by shear or friction through the expansion Elastic bearings connecting the girders to bearings. The aerial structure must also resist the substructure lateral components of the longitudinal loads The elastic substructure anchored to rigid on curved track. When the cumulative foundations resistance of the fastening devices (rail clips) along a length of superstructure is overcome, There are a number of principal design factors the superstructure slides relative to the rail. that affect the magnitude of the interaction movement and forces between the rails and Since CWR is not able to expand or contract, the structure, including:nO~ “I temperature increases above the rail l The composition of the girder material installation temperature cause compressive (steel or concrete) will affect the forces that could buckle the rail. Rail expansion/contraction response to fasteners prevent buckling of the rail. temperature changes Temperature decreases below the rail

7-4 Aerial Structures/Bridges

The girder length and type (simple span where: F, = thermal rail force or continuous) will affect the magnitude of A, = cross sectional area of the rail the structure’s thermal movement that the E, = modulus of elasticity of steel rail fasteners must accommodate a = coefficient of thermal expansion Ti = final rail temperature The girders support pattern of fixed and T, = effective construction expansion bearings from adjacent spans temperature of the rail on the piers (refer to Section 7.5.2) The magnitude of the temperature change On horizontal curves, the axial forces in the rail and superstructure result in radial forces. The rail fastener layout and longitudinal These radial forces are transferred to the restraint characteristics; there are at least substructure by the bearings. The magnitude four concepts of fastener and restraint of the radial force is a function of rail 1. Frictional restraint developed in temperature, rail size, curve radius, and mechanical fasteners longitudinal fastener restraint. Refer to Figure 2. Elastic restraint developed in elastic 7.5.1 as well as other pertinent publications fasteners for the equation to calculate the radial 3. Elastic restraint developed in elastic rail/structure interaction force. fasteners with controlled rail slip

4. Elastic and slip fasteners installed in RPSIN FORCEPER FWT accordance with the expected relative r movements between girder and rail; install sufficient elastic fasteners near the fixed bearing to control rail creep;

install slip fasteners over the balance RAOIALFORCE PER FOOT PERRAJL = E aAT ‘k : KF ‘CCNT R 4 of the girder length to provide full MEINN + INTERACTlCN lateral restraint and minimal CWWT CCMPONENT WERE: E = YMJULUSff ELASTICITYOF RNL STEEL longitudinal restraint a = CWFICJWT OF THERUN EXPANSCtNOF RAi!. STEEL AT = OIANCXR RfiL TENPERAT’JREFROU STRESS-FREE TEYPERATURE Depending on the method used to attach the + = AREAff RAlL SECTION R = RADIUSff HCRIZONTALC’JRX rails to the structure, the structural engineer KF = FASiENERsup VALUEMWDEO BY FASTENERSPAONG must design the structure for longitudinal I~,= LENGWIOF WW3UP.E 8ETWEN EXPANSCNJOINTS restraint loads induced by the fasteners, Figure 7.5.1 Radial Rail/Structure Inter- horizontal forces due to a rail break, and radial action Forces WI forces caused by thermal changes in rails on curved alignments. Today’s designer can use Various solutions have been implemented in computer models to simulate the entire an attempt to minimize the interaction forces structure/trackwork system to account for caused by placing CWR on aerial structures, variations in the stiffness of the substructure including the use of and the dissipation of rail/structure interaction l Ballasted track instead of direct fixation forces due to the substructure’s deflection track (refer to Section 7.56)

(see Section 7.53). l Zero longitudinal restraint fasteners (refer to Section 7.6) The thermal force in the rail is calculated by the following equation: r4,‘, ‘I l High-restraint fasteners near the structure’s point of fixity and low-restraint Fr =ArEra(Ti-To) (Ew 1) fasteners on the remainder of the

7-5 Light Rail Track Design Handbook

structure (Note: it has been reported that this solution results in problems with rail creep and excessive rail gaps at breaks in the rail) (DNFIGURATION A l A series of rail expansion joints and low- restraint fasteners to allow the rail to move independent of the structure; requires highly restrained zones to transfer traction and braking forces to the

structure. CONFIGURATION 6

/-RAIL (‘X7) ,-QRMR ,-FASTENERS 7.5.2 Bearing Arrangement at the Piers

The magnitude of rail/structure interaction forces transferred to the substructure depends 0 EXPANSION BEARING heavily on the bearing arrangement used. As A FIXED BEARING ~FIGURATION C shown in Figure 7.5.2, there are three commonly used bearing arrangements. Figure 7.5.2 Bearing Configurations for Configuration A is a symmetrical bearing Elevated Structure Girders WI arrangement, with fixed bearings (or expansion bearings) from adjacent spans at engineer must still design the bearings and the same pier. Configurations A and B are their anchor bolts to resist these forces. commonly used on modern transit systems that utilize CWR. Configuration C is a non- 7.5.3 Rail/Structure interaction Analysis symmetrical bearing arrangement typically used on railroad and highway bridges. Opinions differ throughout the transit design “community” regarding the level of complexity As a guideline for light rail transit systems with required to design aerial structures subjected CWR, the symmetrical bearing arrangement is to thermal interaction forces from CWR. The the most desirable. In this arrangement, the thermal interactive forces induced into the rail interaction of the rails and supporting structure tend to cancel out each other. This is true as involves the control of rail creep, broken rail long as the adjacent spans are of similar gaps, stresses induced in the CWR, axial stresses induced in the guideway structure, length and geometry. On the contrary, if an expansion bearing at the end of one span is and longitudinal and transverse forces coupled with a fixed bearing at the end of the developed in the supporting substructure.[*l adjacent span on the shared pier Some suggest that hand calculations are (Configuration C), then the thermal interactive adequate and provide a good understanding forces would have a cumulative effect. of the important considerations of rail/structure interaction. Today’s structural engineer has Although the interactive forces at symmetrical bearing arrangements tend to cancel out the advantage of being able to use computer software to more “exactly” analyze this before loading the piers, the structural complex interaction.

7-6 Aerial Structures/Bridges

Others have found that simpler analysis flexural stresses in the rail and the tensile methods are unreliable in predicting stresses stress already in the rail is likely to be at its and structural behavior critical to significant maximum value at this location.” ‘. I21 W/R-related design elements.[51 These design elements include: A broken rail on a light rail transit bridge is an The control of stresses in rails attributed important consideration because of the to thermally induced differential potential to transfer a large force to the bridge movements between the rail and or for a derailment because of the formation of supporting superstructure a rail gap. As a result, aerial structure designers must consider the rail break The control of the rail break gap size and condition. Limits on the size of the rail gap the resulting loads transferred into have to be established, usually based on the structures during low-temperature rail pull- light rail vehicle’s wheel diameter. It is apart failures commonly assumed that only one rail of a The transfer of thermally induced loads single- or double-track alignment will break at from the superstructure, through the any one time. bearings, into the substructure When the rail breaks, the pads of the The choice of the method used to analyze fasteners situated between the break and the rail/structure interaction forces is clearly at the thermal neutral point are realigned in the discretion of the experienced structural opposite direction. Then, the rail slips through engineer. Depending on the length of the the fasteners whose pads have deformed aerial structure and other considerations, beyond their elastic limit, engaging enough simple formulas may be used to determine the fasteners to resist the remaining thermal structural requirements. Alternately, force. Once the required number of fasteners complexities such as curved alignments, is engaged to balance the thermal force in the varying span lengths, and the type of rail, the rail ceases to move. structural elements may require that a rigorous three-dimensional structural analysis The unbalanced force from the broken rail is be performed. At times, the transit agency’s resisted by the other unbroken rail(s) and the design criteria will include the required aerial structure. The portion of the rail break analysis methodology. force that is resisted by the unbroken rail(s) versus the aerial structure is significantly affected by the substructure’s longitudinal 7.5.4 Rail Break/Rail Gap Occurrences stiffness (the force required to induce a unit deformation in a component), the bearing A rail break occurs when a thermally induced configuration, and the rail fasteners restraint tensile force, resulting from a significant characteristics.[51 decrease in temperature, exceeds the ultimate tensile strength of the rail. The rail break is Refer to Table 7.1 for a comparison of the rail likely to occur at or near an expansion joint in gap size for different column stiffnesses and the superstructure or at a bad weld, a rail flaw, levels of fastener restraint. Note that or other weak spot in the rail. progressively lower loads are transferred to the columns as column stiffness decreases. The structure’s expansion joint is a likely As a result, higher loads are transferred to the general area where a rail break can occur unbroken rails. This increases the thermally because the girder’s end rotations increase

7-7 Light Rail Track Design Handbook induced stress in this rail and raises the where: possibility of a second rail break. With higher G = rail gap, cm (in.) restraint fasteners, more load is transferred to X Cl = P,& the maximum longitudinal the unbroken rail and less to the column than deflection of the non-slip fastener with medium-restraint fasteners. X c2 = aATL,, the nominal rail contraction

X c3 = W, + Vfns) WNC, the Researchers found that the superstructure’s reduction in rail contraction bearing arrangement, as discussed in Section caused by fastener constraint 7.52, has little effect on rail gap size. But a = coefficient of expansion, 1.17x1 Oe5 decreasing the fastener’s longitudinal stiffness cm/cm/% (6.5 x IO” in.lin./“F) for or slip force limit, or both, will result in an steel increased rail gap size. AT = temperature change, “C (“F) L = length of span (fixed to expansion The redistribution of the rail break force to the point), cm (in.) substructure causes a longitudinal deflection P fS = minimum longitudinal restraint in the substructure. The resulting force in controlled slip fastener substructure deflection, with the thermal slip of kg (lb.1 the broken rail, combine to create the total P fns = minimum longitudinal restraint gap in the broken rail. force in non-stip fastener, kg (lb.)

& = fastener longitudinal stiffness Several methods can be used to calculate the kg/cm (lb./in.) potential rail gap size. Following are the n ns = number of non-slip fasteners in equations discussed herein?] span = number of controlled-slip Rail gap size is generally estimated by the n, fasteners in span following equation: A, = cross-sectional area of rail (72.58 G=~(XCI+XC~-XC~) (Eqn. 2) cm2 [I 1.25 in.2] for 115 RE rail) 6 = rail modulus of elasticity, 2.1 X lo6 kg/cm2 (30 X lo6 Ib./in.2)

TABLE 7.1 EFFECTS OF UNBROKEN RAIL AND COLUMN LONGITUDINAL STIFFNESS ON LOADS TRANSFERRED TO THE SUBSTRUCTURE [S]

* Assuming a symmetrical girder bearing configuration of E-F/F-E/E-F and a 600 F temperature drop.

7-8 Aerial Structures/Bridges

A simplified form of Equation 2 has been used I I I I I I Y to estimate rail gap size, based on a length, L, 5 I I I I I I I I1I II on either side of the break over which full rail :ASE 6 (btt.dAT~)/I , , , , , anchorage is provided, so that: I ! ! ! !I! ! ! ! ! ! ! 1

G = (wIT)~ AEJR, (Ew. 3) where R, is the longitudinal restraint per centimeter of rail in kilograms per centimeter I i i i i i iii i :F (pounds per inch).

CL Equation 2 provides a reasonable estimate of :2 rail gap size for medium- and high-restraint a cc fasteners, but significantly underestimates the rail gap size for low-restraint fasteners. Low- 1 restraint fasteners generally do not adequately I i i i control the size of the rail gap. Equation 3 provides relatively accurate estimates in many I I I I I I I I I I I 0 cases, except where high-restraint fasteners 0 20 40 60 80 100 120 140 160 are used. Improved accuracy can be obtained TEMPERATUREDROP, DEGREE F with Equation 2 if the term &, is modified to Figure 7.5.3 Rail Break Gap Size Predicted .use the estimated total number of fasteners by Finite Computer Model fg over which the locked-in load is distributed. Therefore: Table 7.2 summarizes estimated rail gap size G = WL + &, - W 0% 4) using different equations and software. where: Once the rail gap size has been estimated, &2 = 0.5 aAT nxLs the variables affecting the magnitude of the n, = PJPfmax = PfmcJwPTKf gap (such as rail fastener spacing and PT = aAT AE,, the thermal load, kg (lb.) stiffness) should be adjusted to limit the size P fmax = hsPfns + n,h)4n,, + n,>, the average fastener restraint limit of the gap. This will minimize the chance of a light rail vehicle derailment caused by a rail kg (lb-1 Y = AE&, the rail spring, kg/cm gap. The size of the rail gap is usually limited (lb./in.) based on the diameter of the vehicle’s wheel. Typically accepted rail gaps are in the range Kf = fastener longitudinal stiffness kg/cm (Ib.in.) of 50 millimeters (2 inches) for a 400- millimeter (16-inch) diameter wheel. [41 Equations 2 and 3 estimate rail gap size assuming linear load distributions. Typically, It is interesting to note that efforts to control finite-element computer models show the rail gap size offer opposing solutions. For fastener load distributions to be nonlinear. safety reasons, the length of the rail gap Refer to Figure 7.5.3 for the rail gap sizes should be minimized to reduce the possibility predicted using a finite-element model. of a derailment. In addition, the forces and

7-9 Light Rail Track Design Handbook

TABLE 7.2 COMPARISON OF RAIL BREAK GAP SIZE BY DIFFERENT FORMULASm

Note: AT, = Temperature change in the girder; the girder bearing configuration = E-F/F-E/E-F; the length of the span = 80 ft.; the length of the fastener = 30 in.; and the temperature change in the rail = 60” F (temperature drop).

a Using average of R, = n,h + nnsPhsY(ns + nns) where n, = the number of slip fasteners, and nns = the number of non-slip fasteners. bTBTRACK and TRKTHRM are programs developed to calculate rail-break gap size.

moments transferred to the structure due to a specialwork locations due to discontinuities in rail break should be minimized to achieve an the rail. Standard turnout units, by design, economical structure. To resolve safety transfer high forces through the units on an issues, fasteners with relatively high aerial structure which causes misalignment longitudinal restraint should be used. To and wear.h2] address the structural issues, fasteners with a relatively low longitudinal restraint should be To accommodate the large forces occurring at used. The trackwork and structural engineers locations of specialwork, rail anchors or rail must coordinate the opposing design expansion joints could be used. Rail anchors requirements to balance the needs for each create a zero force condition through the transit system. specialwork, but pass the rail termination force to the structure. The massiveness of the resulting substructure, however, may be 7.55 Terminating CWR on Aerial aesthetically and economically undesirable. Structures The use of sliding rail expansion joints must consider the following: As much as possible, CWR should not be l The construction length of the sliding rail terminated on an aerial structure due to the joints large termination force transferred to the structure. Problems arise when specialwork l The length of structure required to must be located on an aerial structure due to accommodate the specialwork and sliding the length of the structure, the needs of the rail joint

transit operations, or other occurrences. l The design, location, and installation Unbalanced thermal forces exist in details of the rail anchors

7-l 0 Aerial Structures/Bridges

Some transit systems have used a tie bar girder. An equal and opposite thermal force is device to accommodate specialwork on their developed in the tie bar and transferred to the aerial structures. See Figure 7.5.4 for a AX0 girder through a welded connection. picture of a tie bar installation at an aerial Therefore, the net longitudinal thermal force is structure crossover. directed through the tie bar instead of the piers or the specialwork, where the trackwork could be damaged.

Designers should avoid specialwork on aerial structures. When this cannot be avoided, there are ways to accommodate the specialwork without causing it to malfunction.

7.56 Types of Deck Construction

Traditionally, three distinctly different types of deck construction have been used in rail transit construction. The earliest elevated transit track featured open deck construction, where timber crossties were attached directly to the steel superstructure. This type of construction was used to eliminate the cost and dead load of the ballast, as well as the deck structure required to support/contain the Figure 7.54 Tie Bar on Aerial Crossover Iq ballast. Ballast deck construction was then used to address the public’s complaints about With a tie bar system, the CWR is interrupted the noise and vibration generated by the at the crossover and the rail ends are transit vehicles as they traveled along the attached as rigidly as possible to special open deck structures, among other issues. “AXO” girders adjacent to the outer ends of Over the last 30 years, a mixture of ballast the specialwork. The AX0 girders are similar deck and direct fixation deck construction has to standard girders except for the addition of been used. The direct fixation deck was an embedded steel plate to which the tie bar developed to resolve the shortcomings of the is attached by welding. The tie bar, a ballast deck. structural steel member with a cross section equal to two rails, is located on the centerline The decision concerning which type of deck of each track and is welded to the embedded construction to use with CVVR has profound plates on the centerline of the two AX0 construction cost implications. Based on the girders. The tie bar rests on Teflon bearing difference in cost of aerial structures with and pads placed directly on the concrete deck for without CWR and the resultant thermal effects the length of the crossover. considered in the structural design, the most conservative design using CWR could When the temperature changes, the thermal increase structure costs by 23 percent.i51 But force built up at the end of the CWR is there are many variables to consider when transferred to an AX0 girder through a group of rail fasteners equally spaced along the

7-11 Light Rail Track Design Handbook choosing the type of deck to use on any many transit properties. Developed in the particular transit structure. 1960s for new light rail transit projects, the rails are attached directly to the concrete deck by elastic fasteners. The advantages of this 7.5.6.1 Ballast Deck Construction type of construction include?*, “I Ballast deck construction is still considered a l Elastic fasteners absorb noise and valuable choice by some transit agencies. It vibration and provide vertical flexibility is usually used on moderate length bridges, l Improves aesthetics by using shallower, generally 91 meters (300 feet) or less. less massive structures Advantages of the ballast deck incIude:[2~4.‘01 l Generates a relatively low dead load l Provides an intermediate cushion l Rail fasteners provide electrical isolation between the rails and the structure to and a means to efficiently adjust the line enhance ride quality and grade of the track l Limits the thermal forces associated with l Requires less maintenance and is easier rail/structure interaction to maintain l Uses typical rail track fasteners l Retains track geometry much longer than l Reduces noise and vibration ballasted track l Permits standard track maintenance to l Provides relatively good ride quality adjust alignment and profile l Offers relatively good live load distribution l Provides good live load distribution l Offers good track support The use of direct fixation track construction has been credited with saving millions of Disadvantages of the ballast deck include: dollars on a transit project by eliminating the l The cost of deck waterproofing and the need for crossties and ballast.r’41 MTA New ballast layer York City Transit discusses the difficulty in identifying any specific increased cost for the l The heavy deck load rail/structure interaction associated with the l The greater depth of deck required thermal effects.r’] The construction cost l The cost of maintenance of the ballast impacts are unclear since thermal forces are layer, including cleaning and tamping combined with live loads, dead loads, and (although not light rail, some Japanese other loads in various combinations according railways require maintenance and to the design codes and criteria. tamping operations on their ballast deck structures two to three times a year. In Disadvantages of direct fixation deck include: addition, their overall maintenance costs l Rail/structure interaction must address for ballast deck structures is thermal forces approximately five times greater than for l High initial cost direct-fixation structures 1131) 0 Tight construction control required

l Specialized rail fasteners required l The development of rail breaks with horizontal, vertical, and angular Although direct fixation deck is presently the displacements most common construction method on light rail transit structures, it is clear that the decision to use ballast deck or direct fixation 7.5.6.2 Direct Fixation Deck Construction deck construction on a transit property’s aerial Direct fixation deck construction has now structures is based on technical requirements, become the accepted standard practice for

7-12 Aerial Structures/Bridges

aesthetics, construction cost, maintenance l Vertical fastener stiffness: cost, and individual preference. 13,300 to 26,600 N/mm (75,000 to 150,000 lb./in.)

7.6 DIRECT FIXATION FASTENERS l Lateral fastener stiffness: 3,900 to 11,400 N/mm Since the majority of transit properties now (22,000 to 64,000 lb./in.) use CWR with direct fixation deck l Longitudinal fastener stiffness construction, the aerial structure designer 600 to 3,200 N/mm should understand the types of rail fasteners (3,400 to 18,000 lb./in.) presently available. Rail fasteners secure the CWR to the deck of the aerial structure; the l Longitudinal restraint bottom portion of the fastener is bolted to the 9,000 to 15,750 N deck and the top portion is bolted or clipped to (2,000 to 3,500 lb.) the bottom flange of the rail. Direct fixation fasteners are commonly spaced Low-restraint, moderate-restraint, and high- at 762 millimeters (30 inches) on center. This restraint fastener clips are available. In spacing is determined by analysis of rail addition, some transit properties have utilized bending stresses, interaction forces of the rait zero longitudinal restraint (ZLR) fasteners in and rail fasteners, and the rail gap size at a certain circumstances. Although ZLR rail break location. Trackwork and structural fasteners allow the superstructure to move engineers need to carefully coordinate longitudinally without generating thermal fastener spacing on sharply skewed bridges to interaction forces, the rail gap size at a rail ensure that the fasteners are adequately break has to be carefully considered when it is supported on each side of the joints in the used. deck.

With a conventional direct fixation fastener, Most light rail transit systems use a concrete the elastomer provides isolation of the high pad, or plinth, to support the direct fixation wheel/rail impact forces from the deck; fasteners and attach them to the electrical isolation; vertical elasticity to superstructure. Intermittent gaps are provided dampen noise and vibration; longitudinal along the length of the plinths to elasticity to accommodate rail/structure accommodate deck drainage and to provide interaction movements; and distribution of the openings for electrical (systems) conduits wheel loads longitudinally along the rail. The placed on the deck. fastener also provides full restraint in the lateral direction, maintains the desired rail Reinforcing steel dowels project from the tolerances, and prevents rail buckling under bridge deck, anchoring the second-pour concrete plinths to the deck. high temperature. The level of longitudinal Alternately, restraint chosen for the fastener is a threaded female inserts are embedded in the compromise between the restraint required to concrete deck and threaded reinforcing steel limit the rail gap size and the desire to is installed prior to pouring the plinths. In minimize rail/structure interaction forces.r6~*l addition, the deck slab is usually recessed for the second-pour plinths, forming a shear key The following are typical ranges of direct to help resist the lateral loads from the rail and fixation fastener properties: vehicles. The installation of direct fixation trackwork requires tight tolerances for the

7-13 Light Rail Track Design Handbook

support structure The second-pour concrete l Site working conditions, including plinths are carefully constructed to meet the weather, local ordinances, and working alignment and profile requirements of the restrictions CWR and fasteners. l Aesthetics 0 Owners preference 7.7 TYPES OF SUPERSTRUCTURE l Urban constraints

During the early stages of design, the l Durability designer must determine the type of l Construction schedule superstructure to be used for a specific aerial transit structure. Whether the superstructure For comparison of the many variables is comprised of steel or concrete girders, as involved in evaluating a type of well as the configuration of the girders, must superstructure, a structural system was be evaluated with respect to the project and selected that includes a cast-in-place site constraints. reinforced concrete slab supported by standard precast, prestressed concrete Commonly considered superstructure types girders, whose substructure included concrete include: pier columns and a concrete footing (see 0 Cast-in-place concrete Figure 7.7.1). The goal is to select a span 0 Precast concrete length that minimizes the sum of the 0 Segmental precast concrete construction costs for the deck, girders, and l Steel girders with cast-in-place or precast substructure. The cost optimization effort can deck slab be based on a typical span or an entire transit l Steel box section with either cast-in-place line.r161 or precast deck The relative costs of different structural The following factors are used to components considered for each span are comparatively study superstructure types:r’r 4, 151 shown as plots of cost versus span length in Figures 7.7.2 through 7.7.8. Although this . Effectiveness of structural function (span comparison was performed in 1976, the lengths, vertical clearances, span-to- following conclusions still apply to present depth ratio, etc.) aerial structure design efforts:

. Constructibility issues, such as erection l Economically attractive span lengths vary and construction convenience, including from 9 meters (30 feet) to 21.4 meters (70 transportation of the structural elements to feet). the site l The effect of beam spacing increases with . Production schedule constraints span length; within other design . Capital cost constraints, the largest beam spacing possible should be used. . Maintenance cost . Availability of materials and finished product . Availability of construction expertise

7-14 Aerial Structures/Bridges

80 - I $/FT. = 328 f/M

0 I 40 60 80 100 120

SPAN. FEET (10 FT = 305 M) Figure 7.7.3 Range of Supporting Bent Costs as a Function of Span Length of l-l v II (1FT. =0.305 “I I Structure r16l 10 Figure 7.7.1 Typical Section of Elevated 200 1 f/FT. = 328 S/M Structure Studied rw 1 s/F?= 1075 $/HZ

500 1 f/FT. = 3.28 t/M = 10.75 $/M2 -20 1I f/FT2 5 g 400

8 s z 300

2 ii = 200 (1 .1 SPACING 0 z 40 60 80 loo 120 -5 * 100 SPAN. FEET (10 FT. = 305 M) Figure 7.7.4 Range of Foundation Costs

40 60 80 100 120 for Different Soil Conditions as a Function SPAN, FEET (10 FT. = 3.05 M) of Span Length of Structuren61 Figure 7.7.2 Range of Deck Costs as a Function of Span Length and Beam soil conditions exist and spread footer Spacing of Structure rw foundations are more economical.

l The minimum cost span derived from l In poor soil conditions, foundation costs minimizing the total construction cost is increase sharply with increasing span generally different than that obtained by length, up to a point where deep minimizing the cost of one component foundations should be considered instead (deck, girders, substructure, or of spread footings; therefore, shorter spans foundations). are recommended when unfavorable

7-15 Light Rail Track Design Handbook

600 . EARING CAFAUTY W FCWDATION SW 5 KSF (2.5 KG / CN2) -25 BEARINGCAPAClM OF 500 FOUNDATIONSW 2 KSF (1 KG / OA2) 1 -20 -20 400 R BEAM NG -15 -15 300

BEARINGCAPACITY OF FOUNDATIONON PkES 25 130 g FOUNDAWN SOIL: 10 KSF (5 KG / CU*) 1 cnn -

HIGHER BEAM SPACIN

u 40 60 80 100 120 --- 1 40 60 80 100 120

SPAN. FEET Figure 7.7.5 Range of Total Costs of Elevated Structural System as a Function of $oan Length for Different Soil Conditions f761

I- BEARING CAPACITY OF 'OIJNDATION ON PILES FOUNDATION SOIL = 10 KSF (5 KG/CM’) f DECK

FOOTING + PILES

BENT - PIER

BENT - PIER

40 60 80 loo 120

SPAN, FEET (10 FT = 3.05 M) SPAN, FEET (10 FT. = 3.05 M) Figure 7.7.6 Average Ratio of Cost of Each Figure 7.7.7 Average Ratio of Cost of Each Structural Subsystem to Total Cost of Structural Subsystem to Total Cost of Structur+Founded in Good Soils [W Structure-Founded in Poor Soils WV

7-16 Aerial Structures/Bridges

7.8 REFERENCES

PI Harrington, G., Dunn, P.C , Investigation of Design Standards for Urban Rail Transit Elevated Structures, UMTA, June, 1981. PI Niemietz, R.D., Neimeyer, A.W., Light Rail Transit Bridge Design Issues, Transportation Research Board, Light Rail Transit: Planning, Design, and Operating Experience, Transportation Research Record No. 1361, 1992.

131 Nowak, AS., Grouni, H.N., I “Development of Design Criteria for 80 100 120 Transit Guideways”, AC/ Journal, SPAN, FEET (10 FT. = 3.05 M) September-October, 1983. Figure 7.7.8 Average Ratio of Cost of Supporting Structure and Foundation to I41 AC1 Committee 358, Analysis and Cost of Deck Structure for Different Soil Design of Reinforced Concrete Conditions f’s1 Guideway Structures, ACI 358.1 R-86. El Ahlbeck, D.R., Kish, A., Sluz, A., An It is important to note that in planning for aerial Assessment of Design Criteria for structures, any economical span range can be Continuous- Welded Rail on Elevated considered in the design. The final span Transit Structures, Transportation length selection should be weighted by other Research Board, Rail Track and considerations such as aesthetics and Structures, Transportation Research community factors. Record No. 1071, 1986.

Many times in an urban setting, the span I31 Clemons, R.E., Continuous Welded Rail lengths are specified that provide the required on BART Aerial Structures, horizontal and vertical clearances to existing Transportation Research Board, Rail facilities along the light rail system’s Track and Structures, Transportation alignment. The location of existing railroad Research Record No. 1071, 1986. tracks, roadways, highway bridges, [71 Grouni, H.N., Sadler, C., Thermal waterways, and major utilities can restrict interaction of Continuously Welded Rail substructure locations, thereby limiting the and Elevated Transit Guideways, choices for span lengths. Ontario Ministry of Transportation and Communications. As part of a preliminary design effort for an aerial structure, a study should be performed PI Guarre, J.S., Gathard, D R., to determine the most desirable structure implications of Continuously Welded configuration based on economic, social, Rail on Aerial Structure Design and environmental, and technical needs. Construction, June, 1985.

7-l 7 Light Rail Track Design Handbook

PI New York City Transit Authority, v91 Beaver, J.F., Southern Railway Metropolitan Transportation Authority, System’s Use of Sliding Joints, AREA Continuous Welded Rail on Elevated Bulletin 584, February, 1964 Structures, August, 1991. PO1 Billing, J.R., Grouni, H.N., Design of 1101 Clemons, R.E., Continuous Welded Rail Elevated Guideway Structures for Light on Aerial Structure: Examples of Rail Transit, Transportation Research Transit Practice, APTA, January, 1985. Record, Journal 627, 1977. 1111 Fine, D.F., Design and Construction of WI Casey, J., “Green Light”, Civil Aerial Structures of the Washington Engineering, May, 1996. Metropolitan Area Rapid Transit PI Deenik, J.F., Eisses, J.A., Fastening System, Concrete International, July, Rails to Concrete Deck, The Railway 1980. Gazette, March 18, 1966. Lee, R. J., Designing Precast Aerial t121 t231 Dorton, R.A., Grouni, H.N., Review of Structures to Meet Track and Vehicle Guideway Design Criteria in Existing Geometry Needs, 1994 Rail Transit Transit System Codes, ACI Jounral, Conference. April 1978. Eisenmann, J., Leykauf, G., Mattner, L., iI31 [241 Fox, G.F., Design of Steel Bridges for “Recent Developments in German Rapid Transit Systems, Canadian Railway Track Design,” Proceedings of Structural Engineering Conference, the Institution of Civil Engineers, 1982. Transport. Vol. 105, No. 2 (May, 1994) 1251 International Civil Engineering Meyers, B.L., Tso, S.H., “Bay Area [I41 Consultants, Inc., Task Report on a Rapid Transit: Concrete in the 196Os”, Study to Determine the Dynamic Rail Concrete International, February, 1993. Rupture Gaps Resulting from a [151 Desai, D.B., Sharma, M., Chang, B., Temperature Drop for BART Extension Design of Aerial Structure for the Program, July 26, 1991. Baltimore Metro, APTA Rapid Transit 1261 Jackson, B., “Ballastless Track, A Rapid Conference, June 1986. Transit Wave of the Future?“, Railway [If31 Naaman, A.E., Silver, M.L., “Minimum Track and Structures, April, 1984. Cost Design of Elevated Transit v71 Kaess, G., Schultheiss, H., “Germany’s Structures”, Journal of Construction New High-Speed Railways, DB Division, March, 1976. Chooses Tried and Tested Track iI71 Fassmann, S., Merali, A.S., Light Rail Design”, International Railway Journal, Transit Direct Fixation Track September, 1985. Rehabilitation: The Calgary Experience, PI Magee, G.M., Welded Rail on Bridges, Transportation Research Board, Light Railway Track and Structures, Rail Transit: Planning, Design, and November, 1965. Operating Experience, Transportation Research Record No., 1361, 1992. WI Mansfield, D.J., “Segmental Aerial Structures for Atlanta’s Rail Transit AREA Manual for Railway Engineering, 1181 System”, Transportation Research Section 8.3, “Anchorage of Decks and Board, Rail Track and Structures, Rails on Steel Bridges,” 1995

7-18 Aerial Structures/Bridges

Transportation Research Record No. 1361 PBQD, Thermal Study of Bridge- 1071, 1986. Continuous Rail Interaction, Metro Pasadena Project, Los Angeles River [301 “Philadelphia’s El Gets Major Facelift”, Bridge, August, 1994. Mass Transit, May/June, 1995. I371 Swindlehurst, J., “Frankford Elevated 1311 McLachlan, L.J., “University Boosts Reconstruction Project,” lntemational Light Rail Traffic”, Developing Metros, Bridge Conference, June, 1984. 1994. 1381 Thorpe, R.D., San Diego LRT System: ~321 Middleton, W.D., “Engineering the Ten Years of Design Lessons, Renaissance of Transit in Southern Transportation Research Board, Light California”, Railway Track and Rail Transit, Planning, Design, and Structures, March, 1993. Operating Experience, Transportation c331 Middleton, W.D., “DART: Innovative Research Record No. 1361, 1992. Engineering, innovative Construction”, Varga, O.H., The Thermal Elongation of Railway Track and Structures, Rails on Elastic Mountings, AREA December, 1994. Bulletin 626, February, 1970. Patel, N.P., Brach, J R., “Atlanta Transit WI Yu, s., “Closing the Gaps in Track Structures”, Concfe te lntema tional, Design,” Railway Gazette International, February, 1993. January, 1981. PBQD, Rail/Structure interaction t351 Zellner, W., Saul, R., “Long Span Analysis - Retrofit of Direct Fixation Bridges of the New Railroad Lines in Fasteners with Spring C/ips, WMATA - Germany, Bridges: interaction Between Rhode Island Avenue, February, 1995. Construction Technology and Design.”

7-l 9 Chapter 8-Corrosion Control Table of Contents

8.1 GENERAL 8-l

8.2 TRANSIT STRAY CURRENT 8-2 8.2.1 Stray Current Circuitry 8-2 8.2.2 Stray Current Effects 8-2 8.2.3 Design Protection Components 8-3 8.2.3.1 Traction Power 8-4 8.2.3.2 Track and Structure Bonding 8-4 8.2.3.3 Drain Cables 8-4 8.2.3.4 Trackwork 8-5

8.3 TRACKWORK DESIGN 8-5 8.3.1 Rail Continuity 8-6 8.3.2 Crossties 8-6 8.3.2.1 Concrete Crossties 8-6 8.3.2.2 Timber Crossties 8-6 8.3.3 Ballast 8-7 8.3.4 Embedded Track 8-7 8.35 Cross Bonds 8-7 8.3.6 Direct Fixation Track 8-8 8.3.7 Impedance Bonds 8-8 8.3.8 Rigid Bumping Post 8-8 8.3.9 Stray Current Tests and Procedures 8-9

8.4 SUMMARY 8-9

8.5 REFERENCES 88

8-i CHAPTER 8-CORROSION CONTROL

8.1 GENERAL The problem with stray currents evolves from the fact that whenever electric current leaves Electrified rail transit systems, both light and a metallic conductor (i.e., a water pipe) and heavy rail, typically utilize the track system as returns to the soil (perhaps because it is the negative side of an electrical circuit in the attracted to a nearby gas line), it causes system’s traction power network. In light rail corrosion on the surface of the conductor it is transit systems, the positive side, which leaving. This is the same phenomenon that carries DC electrical current from the occurs when a metallic object is electroplated, substation to the transit vehicle, is typically an such as when construction materials are zinc overhead contact wire system or catenary. plated. In the case of stray currents, the Because perfect electrical insulators do not typical current path can involve several exist, electrical currents will leak out of this different conductors as the electricity wends circuit and escape into the soil to find the path its way back to the substation; therefore of least resistance back to the substation. corrosion can occur at multiple locations. This The amount of such stray currents will be can create conditions that range from leaking inversely proportional to the efficiency of the water lines to gas line explosions. The rail electrical insulation provided and directly itself will also corrode wherever the current related to the conductivity of the soil and any jumps from it to reach the first alternative alternative current paths back to the conductor. Structures along the transit line, substation such as pipes, cables, reinforcing particularly steel bridges and embedded steel, etc. reinforcing steel, are also at risk. Hence, multiple parties have an interest in controlling Typically, unless a fault has occurred in an or eliminating the leakage of stray currents insulator, stray currents from the positive side and minimizing the damage they inflict. of the light rail transit traction power circuit are minuscule. Stray currents from the track, on Stray currents are common on a light rail the other hand, are common and can get quite transit system because its track structures are large due in no small part to the proximity of typically close to the ground. Grade the track to the ground. Once in the soil, stray crossings, embedded track, and fouled or currents will follow any available conductor to muddy ballast are common locations for get back to the traction power substation. propagation of stray currents. Because of the These paths can include the soil itself, buried maze of underground utility lines typically utility pipelines and cables, or other metallic found in urban and suburban areas where structures, such as bridges, along the way. If light rail transit systems are built, abundant an alternative path offers less electrical alternative electrical paths exist. Predicting resistance than another route, then the better the likely path of potential stray currents and conductor will carry proportionally more of the defining methods to protect against them can stray current. In extreme examples, be extremely complex. Because of this particularly when the electrical continuity of complexity, it is essential that the advice of a the track structure is poor, more electricity will certified corrosion control specialist with stray return as stray current than through the current experience be sought from the running rails. Some older elevated systems beginning of design. were actually designed for this occurrence.

8-1 Corrosion Con fro/

In his book Corrosion Engineering, Mars G. and perform regular monitoring and Fontana states: maintenance afterwards. ‘. . . The term stray current refers l Provide auxiliary conductors to improve to extraneous direct currents in the ampacity of the rail return system. the earth. If a metallic object is This can be accomplished by connecting p/aced in a strong current field, a all rails together or by adding cable potential difference develops conductors. across it and accelerated corrosion occurs at points where Existing pipes and cables in the vicinity of the current leaves the object and tracks must be investigated and protective enters the soil. Stray current action taken as necessary to protect them problems were quite common in from stray current corrosion. previous years due to current leakage from trolley tracks. Whether the light rail operator or the local Pipelines and tanks under tracks utility takes responsibility, it is imperative that were rapidly corroded. However, strategic action is required to mitigate the since this type of transportation is effects of stray current corrosion in the design now obsolete, stray currents from phase and during construction. This will avoid this source are no longer a corrosion from becoming a costly and problem.” [‘I dangerous maintenance issue later.

This text requires updating since the “trolley tracks” have evolved into light rail transit 8.2 TRANSIT STRAY CURRENT (LRT) lines and the stray currents from LRT will re-introduce potential corrosion problems. 8.2.1 Stray Current Circuitry Some of the principal measures that can be taken to minimize traction current leakage Traction power is normally supplied to light rail include: vehicles (LRV) by a positive overhead If jointed track is used, install electrical catenary system. The direct current is picked bonding across the joints. One of the up by a vehicle pantograph to power the many advantages of continuous welded motor and then returns to the substation via rail (CWR) is that it offers a superior the running rails, which become the negative traction power return. part of the circuit. Unfortunately, a portion of the current strays from the running rails and insulate rails from their fastenings and flows onto parallel metallic structures such as encase rails in embedded track in an reinforcing steel, utility pipes and cables, and insulating material. The steel other structures such as pilings, ground grids, reinforcement in the underlying concrete and foundation reinforcing bars. slab can be continuously welded to act as a stray current collector. 8.2.2 Stray Current Effects In ballasted track areas the ballast should be clean, well-drained and not in contact Corrosion of metallic structures is an with the rail. electrochemical process that usually involves Conduct corrosion surveys on susceptible small amounts of direct electrical current (dc). metal structures before service begins It is an “electro” process because of the flow

8-2 Light Rail Track Design Handbook of electrical current. It is a “chemical” process is, some portion of the traction power current because of the chemical reaction that occurs will always seek an alternative path back to on the surface and corrodes the metal. One the substation. ampere of direct current flowing for 1 year will corrode 20 pounds of iron, 46 pounds of Utility companies fought this problem, both in copper, or 74 pounds of lead. Natural the courts and in the field. Once the legal galvanic corrosion involves milliamperes of issues were resolved, the most effective current so many buried structures can last means of minimizing stray current damage several years before structural failure. was to make the buried utility network as electrically continuous as possible. Copper Unlike the very small currents associated with bonds were placed around joints in buried galvanic corrosion, stray current corrosion pipes and crossing utility lines were from a transit system can involve several electrically bonded to each other. Finally, the hundred amperes. The same physical laws entire utility network was directly connected to apply for corrosion of the metal, electron flow, the negative bus of the traction power chemical reactions, etc., but metal loss is substation by “drain cables” so that any stray much faster because of the larger amounts of currents could return without causing current involved. For example, with 200 significant corrosion along the way. All big amperes of current discharging from an city utility companies participated in a underground steel structure, 2 tons of metal “corrosion control committee” with the trolley will be corroded in 1 year (20 pounds per company to ensure that all new facilities were ampere per year x 200 amperes = 4,000 properly integrated into the system, thereby pounds of steel corroded). preserving the delicate balance of the network. (Since in many cities, a single Thus, stray current from a light rail system will holding company might own most of the utility corrode transit rails, rebar, and steel structural companies and the trolley company as well, members and all adjacent underground such committees were not necessarily metallic structures unless protective measures combative congregations.) Such methods are provided. were generally effective; however a side effect of the improved underground electrical continuity was that the utility grid typically 82.3 Design Protection Components became better bonded than the track structure. As such, a significant portion of the The phenomenon of stray currents from traction power current would perversely elect electrified street railways was first observed to stray from the rails and use the buried when trolley systems were constructed in the utilities to get back to the substation. 1880s. The importance of maintaining good electrical continuity of the rails was quickly When trolley systems were abandoned in recognized and many trolley systems welded most cities, the corrosion committees were rail joints 60 years before the process was disbanded and the utility companies became widely accepted on “steam” railroads. Where less zealous about bonding their networks. In rails could not be welded, they were many cases, the introduction of non-metallic electrically bonded to each other with copper piping created significant electrical cables. These measures reduce stray discontinuities in utility systems. Such gaps currents, but cannot eliminate them. No were of no consequence in a city without a matter how good a conductor the track system local originator of significant stray currents

8-3 Corrosion Control

and associated corrosion protection improvement in stray current control. measures. With no trolley network in the Nevertheless, stray currents are still possible neighborhood, corrosion potential could in an ungrounded system as the electricity can typically be neutralized using sacrificial leave and return to the track structure from the anodes. However, if a light rail system is ground. It is entirely possible for current to introduced into such a city, the sacrificial leak out of the track, travel along alternative anodes are insufficient. The result can be paths in the ground, and then return to the corrosion problems not unlike those that track at another location. Since the track itself occurred a hundred years ago with stray must eventually be directly connected to the currents leaping off metal pipes when they negative substation bus, stray currents can reach an electrical dead-end at a non-metallic circumvent substation isolation systems. conduit.

Reverting to the continuous utility bonding and 8.2.3.2 Track and Structure Bonding drain cable methods of the past is typically not Achieving electrical continuity of the track a completely effective methodology of structure is of paramount importance in achieving stray current control. Because of keeping negative return current in the rails. the widespread use of non-metallic buried The use of continuously welded rail, together pipe, and the subsequent high expense of re- with the installation of bonding cables around creating an electrically continuous path unavoidable bolted joints, provides most rail through the utilities, it is typically much transit systems with an excellent current path cheaper-and arguably easier-to attempt to through the rails. Stray current corrosion of effectively insulate the track structure from the transit structures can typically be controlled ground so that stray currents are minimized through electrical bonding. Since the 1960s it from the beginning. Such insulation, coupled has been common practice to also bond with other protective measures, including reinforcing steel in concrete structures so as selective bonding of utilities and drain cabling, to provide a continuous electrical path. The is the foundation of stray current corrosion bonding is typically concentrated in reinforcing control measures of modern light rail transit bars in the lowest portions of the structure and systems. This controlled approach also those surfaces in contact with rail such as protects rails and other transit structures that retaining walls. would be subjected to these stray currents. Many light rail systems have been built with heavily reinforced slabs beneath the track to 8.2.3.1 Traction Power provide both structural support and a barrier Since the 1960s increased efforts to reduce against migration of stray currents into the stray currents have been made through ground. Bonded reinforcing steel networks modifications to traction power substations. can provide a shielding effect for outside utility Typical modern substations are either structures. ungrounded or “floating” above ground potential, or are grounded through diodes that prevent stray currents from passing from the 8.2.3.3 Drain Cables negative bus to the ground. This frequently Drain cables are sometimes provided for reduces stray currents from hundreds of future use on modern light rail systems, but amperes to near zero. Completely are not necessarily connected to the utility ungrounded systems exhibit the greatest system. Utility companies monitor their

8-4 Light Rail Track Design Handbook pipelines for any stray currents and, if each require individual attention. Electrical problems are detected, they have the option isolation of the rail using insulation is of connecting to the drain cable as a last necessary for utility pipelines and steel resort. Coupled with other protective systems, structures.[21 In addition, if the track is shared such cabling provides a secondary approach by railroad freight traffic during non-revenue to corrosion protection in the event that the hours, insulated rail joints are required at all primary measures are ineffective at locations rail sidings and connections to adjacent rail where excessive leakage from the rails facilities. occurs. The essence of state-of-the-art technology in the design of modern transit systems is the 8.2.3.4 Trackwork concept of controlling stray current at the rails. Ultimately, electrical insulation of the track Operation of the traction power system with structure offers the first line of defense against the substation negatives isolated from ground stray currents. Keeping the rails clean and (floating) will result in a higher overall system- dry is important, as is good insulators between to-earth resistance. The goal is to maximize the rail and the ties. Good drainage is also the conductivity of the rail return system and critical. Rail laid in streets may also have the electrical isolation between the rails and insulating coatings to maintain electrical their support systems. isolation. Since track design is the focus of this handbook, track insulation will be The following are generally accepted design discussed in detail in Section 8.3. It must be measures for the various track types to create emphasized, however, that track insulation is an electrically isolated rail system that not a panacea, particularly if the track controls stray currents at the source: insulation systems are not regularly Continuous welded rail maintained and cleaned. If track insulation Rail bond jumpers at mechanical rail systems are compromised, such as by fouled connections (especially special trackwork) ballast or dirty insulators, stray current leakage is inevitable. Thus, the required level Insulating pads and clips on concrete of maintenance should be considered during crossties design. Insulated rail fastening system for timber crossties and switch timber

8.3 TRACKWORK DESIGN Maintaining a minimum separation of 25 millimeters (1 inch) between the bottom of LRT systems utilize dc electrical power that is the rail and the ballast on ballasted track normally returned from the LRV to the Insulated direct fixation fasteners on substations through the rails. Stray current concrete structures control is a necessary element in the design of the track system. Modern designs for dc Coating the rail with coal tar epoxy or transit systems include the concept of “source other insulating material at all roadway control” at the base of the rail or rail surface to and pedestrian crossings minimize the generation of stray currents. Coating embedded rails with an The route of an LRT system is not generally insulating material and encasing the track right-of-way; within a totally dedicated slab with an insulating membrane therefore, the various types of rail construction

8-S Corrosion Control

Providing an insulated rubber boot around special trackwork components. The use of the rail in embedded sections jumpers must be carefully coordinated with the design of the signal system _ Cross-bonding cables installed between the rails to maintain equal potentials of all rails and reduce resistance back to the 8.3.2 Crossties substation Insulation of the impedance bond tap 8.3.2.1 Concrete Crossties connections from the housing case Concrete crossties with an insulating base consisting of a rail pad and clip insulators Insulation of switch machines at the provide good rail insulation. The rail seat pad switch rods is generally constructed of thermo-plastic Installation of rail insulated joints to isolate rubber, ethylvinyl acetate, or natural rubber. It rail-mounted bumping posts is approximately 6 to 16 millimeters (0.25 to 0.62 inches) thick and is formed to fit around Installation of insulated rail joints to isolate the iron shoulder embedded in the concrete the main line from the yard and the yard crosstie. The clip insulator may be a glass from the usually grounded maintenance reinforced nylon material formed to sit on the shop area rail and under the steel anchoring clip. This Separate traction power substations to affords electrical insulation between the rail supply operating currents for the main and the concrete tie anchoring clip. Insulating line, yard and shop the rail base is important because concrete crossties, with their reinforcing steel, are not Rail insulated joints to isolate the main good insulators. line rails from freight sidings or connections to other rail systems 8.3.2.2 Timber Crossties 8.3.1 Rail Continuity While wood is generally a good insulating material, timber crossties are only marginal Continuous welded rail is the generally insulators when they are treated with accepted standard for main line light rail preservative chemicals or as they age and construction. CWR creates an electrically absorb moisture. While they provide sufficient continuous negative return path to the insulation against low-voltage, low-amperage substation, in addition to other well-known signal system currents, they also offer a benefits. leakage path for high-voltage, high-amperage traction power current. Timber crossties with The rail configuration at special trackwork, insulating components at the fastening plate, turnouts, sharp curves, or crossovers may as shown in Chapter 5 (Figure 5.4.2), can be require jointed rails. Jumper cables, used on main line track and at special exothermically welded to the rail on either side trackwork turnouts and crossovers to reduce of the bolted rail joint connections, ensure a leakage. continuous electrical path across the mechanical connections. Jumper cables may Electrical insulation can be achieved by be used to bypass complex special trackwork inserting a polyethylene pad between the to provide continuity. Jumpers can also metal rail plate and the timber tie, installing an protect track maintenance workers from insulating collar thimble to electrically isolate electrical shock when they are replacing the steel plate from the anchoring lag screw,

8-6 Light Rail Track Design Handbook and applying coal tar epoxy to the hole for the requirement pertains to rail on both concrete lag screw. The insulating pad and collar and timber crossties for both main line and thimble afford insulation directly between the yard trackage. This is essential to increase two materials. Coal tar epoxy applied to the the rail-to-earth resistance and assist in drilled tie hole fills any void between the end minimizing the stray current leakage to earth. of the collar thimble and timber tie and affords Ballast should be clean and well-drained. The some insulation between the lag screw and use of metallic slags as ballast is not wood tie. The insulated tie plate pad should recommended. Rail grinding should be done extend a minimum of 12 millimeters (0.5 with vacuum systems to minimize inches) beyond the tie plate edges to afford a contaminating ballast with metallic grindings. higher resistance path for surface tracking of stray currents. Chemical compatibility between the pad and epoxy material must be 8.3.4 Embedded Track verified during design. Embedded track is generally located in the central business district (CBD) street-running Maintenance shop tracks are grounded to section of a light rail system. Electrical protect workers. Maintenance yard tracks are isolation of the rails can be provided by generally floating or non-grounded, and insulating the rail face and rail base, insulating insulation is rarely included between the rails the trough that the rail sits in, or a combination and the timber crossties. This design decision of both. Track may also be isolated by is based on economic considerations, as well insulating the perimeter of the entire concrete as the fact that the rails are only used base slab, using the “bathtub” stray current sporadically and a separate traction power isolation concept. The materials used to substation is used to supply operating current provide this insulation generally consist of for train movement in the yard. The only time polyethylene sheeting, epoxy coal tar coating, the yard rails become electrically connected to polyurethane grout (Icosit), or natural rubber the main line or shop rails is when a train sheeting, such as pads or rail boots. All these enters or leaves the yard or shop. This is a materials have been used successfully. The short period and does not result in any specific design for stray current control is harmful sustained current leaking into the selected by the track designer with earth. Note that transit system structures recommendations from the corrosion control within a yard complex may have to be specialists. protected against locally originated stray currents between yard trackage and the yard 8.3.5 Cross Bonds substation. Consequently, underground Periodic cross bonding of the rails and parallel utilities in yards are constructed with non- tracks provides equivalent rail-to-earth metallic materials such as PVC, FRE, and potentials for all rails along the system. Using polyethylene. all parallel rails to return current provides a lower negative return resistance to the 8.3.3 Ballast substation, since the return circuit consists of multiple paths rather than individual rails. To eliminate the path for stray current leakage from rail to ballast, the ballast section should Cross bonds are generally installed at be a minimum of 25 millimeters (1 inch) below impedance bond locations on rails to avoid the bottom of the rails. The clearance interference with rail signal circuitry. Cross

8-7 Corrosion Con fro/

bonding is accomplished by exothermically are subject to seepage. This coats the welding insulated cables to the rails. Both fastener with a wet conductive film, which can rails are connected in single-track locations, be mitigated by periodic cleaning. with all four rails cross bonded in double-track areas. 8.3.7 Impedance Bonds Cross bonding in embedded track sections requires an alternative design approach since Leakage of stray currents into the earth can the signaling system is not carried through the be a significant problem if the cables from the embedded track area. This is typically the rails are electrically connected to an case as most embedded track light rail impedance bond housing case that is in systems run on “line-of-sight” operating rules contact with the earth. This type of grounded coordinated with street traffic signal patterns. installation can result in a continuous maintenance problem if an effectively high To provide cross bonding of embedded tracks, rail-to-earth resistance is to be achieved. insulated conduits are generally installed Instead, the housing case should be mounted between track rail troughs prior to installation clear of any concrete slab conduits, of the concrete for the initial track slab. reinforcing bar and contact with the earth. Insulated cables are exothermicaiiy welded to each rail to obtain electrical continuity. Impedance bond housing cases for a light rail Smaller cables may be used to provide an transit line are generally located at-grade easier turning radius to the rails in the rail along the right-of-way. The cases are trough zone and facilitate exothermic welding mounted on timber tie supports in the of the cables to the rails in constrained ballasted area either between or directly spaces. It is common design practice to adjacent to the rails. In order to eliminate install the cables at 305-meter (l,OOO-foot) possible points of contact with the earth, the intervals throughout the CBD, with one center taps of the impedance bonds are location being directly adjacent to each insulated from the mounting case by installing substation. a clear adhesive silicone sealant between the center taps and the case.

8.3.6 Direct Fixation Track 8.3.8 Rigid Bumping Post Direct fixation (DF) track is generally located on aerial sections or in tunnels in light rail In order to reduce the frequency of transit systems. The direct fixation fasteners maintenance required and maintain a higher provide electrical insulation between the rails degree of rail-to-earth resistance, rail and the concrete structure. The elastomer insulating joints are installed in the rails to design consists of a component of natural isolate the bumping post. The insulating joints rubber bonded between the metal base plate eliminate the electrical connection between and the top surface metal plate. An elastomer the bumping posts and the running rails and of the proper resistivity provides excellent prevent leakage of stray currents into the insulation and deters current leakage. earth. Fastener inserts are often epoxy coated to further isolate the rails from the concrete slab. Most of the methods discussed above Leakage may occur in DF track in tunnels that (Sections 8.3.2 through 8.3.8) provide good initial values of rail-to-earth resistance. As

8-8 Light Rail Track Design Handbook these components deteriorate, they become system operators and builders to either avoid dirty and require maintenance to maintain or mitigate the effects of stray current their original resistivity. Periodic tests are corrosion. The designer must seek the advice also required to locate and remove direct of experts in this complex field, as well as shorts that occasionally occur as discussed in coordinate with the utility companies and the the following section. Stray currents can rise signal system designer. It is also important to to harmful levels if short circuits to ground are recognize that track component specifications not detected and removed. should include appropriate electrical resistance features to accomplish the corrosion protection plan. If such 8.3.9 Stray Current Tests and Procedures specifications are provided, the designer should not specify performance requirements Regularly scheduled tests are required to for earth-to-ground resistance of the entire maintain the integrity of stray current control track system. systems once they are in operation. The most common tests are rail-to-earth resistance tests, substation-to-earth voltage tests, and 8.5 REFERENCES structure-to-earth tests. Research shows a broad spectrum of approaches are used PI Fontana, Mars G. Corrosion ranging from infrequent use of consultants to Engineering, McGraw-Hill Book permanent in-house corrosion control Company, Third Edition, Fontana personnel. The greatest efforts seem to be Corrosion Center, Ohio State University put forth when stray current problems have 1988. already damaged piping, utility structures, PI Sidoriak, William & McCaffrey Kevin, trackwork components, or signal circuits. Source Control for Stray Current Such troubleshooting can be effective, but Mitigation, APTA Rapid Transit regularly scheduled, routine conducting Conference 1992, Los Angeles, monitoring for stray currents problems can California, June 1992. allow detection and correction before they manifest themselves in the form of 131 Moody, Kenneth J., A Cookbook for measurable corrosion or degraded signal Transit System Stray Current Control system performance. NACE Corrosion 93, paper No 14, New Orleans, Louisiana, February 1993. NACE International, “Stray Current 8.4 SUMMARY t41 Corrosion: The Fast, Present, and Corrosion from stray electrical currents is an Future of Rail Transit Systems,” NACE important issue that requires the attention of International Handbook, Houston, the design team. There are several effective Texas, 1994. methods that have been used by the light rail

8-9 Chapter g-Noise and Vibration Control

Table of Contents

9.1. INTRODUCTION 9-l 9.1 .I Acoustics 9-l 9.1.2 Scope 9-2

9.2. NOISE AND VIBRATION CONTROL DESIGN GUIDELINES 9-2 9.2.1 Groundborne Noise and Vibration Criteria 9-4 9.2.2 Wheel/Rail Rolling Noise 9-6 9.2.2.1 Normal Rolling Noise 9-7 9.2.2.1 .I Generating Mechanisms 9-7 9.2.2.1.2 Wheel Dynamics 9-9 9.2.2.1.3 Rail Dynamics 9-9 9.2.2.1.4 Resilient Direct Fixation Fasteners 9-10 9.2.2.1.5 Contact Stiffness 9-10 9.2.2.2 Impact Noise 9-l 1 9.2.2.3 Rail Corrugation Noise 9-11 9.2.2.4 Treatments for Rolling Noise Control 9-11 9.2.2.4.1 Continuous Welded Rail 9-12 9.2.2.4.2 Rail Grinding 9-12 9.2.2.4.3 Rail Support Spacing 9-13 9.2.2.4.4 Direct Fixation Fastener Design 9-l 3 9.2.2.4.5 Trackbed Acoustical Absorption 9-14 9.2.2.4.6 Rail Vibration Absorbers 9-14 9.2.2.4.7 Wear-Resistant Hardfacing 9-15 9.2.2.4.8 Low Height Sound Barriers 9-15 9.2.3 Special Trackwork Noise 9-15 9.2.3.1 Frogs 9-15 9.2.3.1.1 Solid Manganese Frog 9-15 9.2.3.1.2 Flange-Bearing Frog 9-l 5 9.2.3.1.3 Liftover Frog 9-l 6 9.2.3.1.4 Railbound Manganese Frogs 9-16 9.2.3.1.5 Movable Point Frogs 9-16 9.2.3.1.6 Spring Frogs 9-16 9.2.4 Wheel Squeal Noise 9-16 9.2.4.1 Causes of Wheel Squeal 9-17 9.2.4.2 Treatments 9-18 9.2.4.2.1 Dry-Stick Friction Modifiers 9-18 9.2.4.2.2 Lubrication 9-18 9.2.4.2.3 Water Sprays 9-l 9 9.2.4.2.4 Rail Head Inlays 9-19 9.2.4.2.5 Rail Head Damping Inlays 9-19 9.2.4.2.6 Track Gauge 9-l 9 9.2.4.2.7 Asymmetrical Rail Profile 9-20

9-i Light Rail Track Design Handbook

9.2.4.2.8 Rail Vibration Dampers 9-20 9.2.4.2.9 Rail Vibration Absorbers 9-20 9.2.4.2.10 Double Restrained Curves 9-20 9.2 5 Groundborne Noise and Vibration Mitigation 9-21 9.251 Vibration Generation 9-21 9.2.5.2 Groundborne Noise and Vibration Prediction 9-22 9.2.5.3 Vibration Control Provisions 9-22 9.2.5.3.1 Floating Slab Track 9-22 9.2.5.3.2 Resiliently Supported Two-Block Ties 9-23 9.2.5.3.3 Ballast Mats 9-24 9.2.5.3.4 Resilient Direct Fixation Fasteners 9-25 9.2.5.3.5 Rail Grinding 9-26 9.2.5.3.6 Rail Straightness 9-26 9.2.5.3.7 Vehicle Primary Suspension Design 9-27 9.2.5.3.8 Resilient Wheels and Rail Head Ball Radius 9-27 9.2.5.3.9 Subgrade Treatment 9-27 9.2.5.3.10 Special Trackwork 9-28 9.2.5.3.11 Distance 9-28 9.2.5.3.12 Trenching and Barriers 9-28 9.2.5.3.13 Pile-Supported Track 9-28 9.3 REFERENCES 9-28

List of Figures

Figure 9.1 Change in Elastic Modulus and Rail Head Curvature Required to Generate Wheel/Rail Excitation Equivalent to Roughness Excitation 98 Figure 9.2 Vertical Pinned-Pinned Resonance Frequency vs. Rail Support Separation for Various Rails 9-9 Figure 9.3 Geometry of Curve Negotiation and Lateral Slip 9-18 Figure 9.4 Track Crabbing Under Actual Conditions 9-18

List of Tables

Table 9.1 Criteria For Maximum Airborne Noise From Train Operations 9-3 Table 9.2 Guidelines For Noise From Transit System Ancillary Facilities 94 Table 9.3 Groundborne Vibration And Noise Impact Criteria 9-5 Table 9.4 Criteria For Maximum Groundborne Vibration From Train Operations By Land-Use Category 9-8 Table 9.5 Criteria For Maximum Groundborne Noise From Train Operations 9-7

g-ii CHAPTER 9: NOISE AND VIBRATION CONTROL

9.1. INTRODUCTION prepared in Transit Cooperative Research Program (TCRP) Report 23, which Noise and vibration can cause significant includes numerous references to adverse environmental impacts on wayside technical reports and other literature.[*] communities and, as a result, noise and l A review of groundborne noise and vibration impact mitigation must be vibration prediction and control was considered in track design. With appropriate performed in 1980, including preparation design and maintenance provisions, noise of an annotated bibliography 13] and vibration from light rail transit can usually l A handbook on all aspects of rail transit be held to acceptable levels at reasonable noise and vibration control has also been cost. Effective noise control must consider the prepared.‘” vehicle and track as a system rather than as separate, independent components. For example, expensive track vibration isolation 9.1 .I Acoustics systems might be avoided where vehicles with low primary suspension vertical stiffness are Sound in the form of noise is often included used, whereas vehicles with high primary among the most significant negative suspension stiffness might produce vibration environmental effects of new transit systems. that can only be controlled by a floating slab The impact of noise will increase with track-an expensive proposition. The track tightness of track curvature, operation in city and vehicle design teams must coordinate centers, speed, and other general track and their designs in the early stages of any operating conditions, unless noise and project. Mitigation could involve considerable vibration control provisions are implemented expense, weight, space, or special in the track and vehicle designs. procurements Late consideration of noise Wayside noise primarily originates at the and vibration isolation may preclude some treatments simply because insufficient time wheel/rail interface. During the passage of a train, the surface roughness of both the exists to obtain them or to implement design wheels and rails combined at the point of changes. rolling contact (the contact patch) generates Many studies of rail transportation noise and vibration in the rails, crossties, supporting vibration have been conducted, producing track structure, wheels, and other vehicle detailed technical reports containing components. These vibrating surfaces radiate comprehensive information concerning rail sound to a greater or lesser extent, depending transit noise and vibration prediction and on the magnitude of vibration and the control. Particularly useful sources of radiation or sending efficiency of the literature include: component. This is an area of active research in the European community, though primarily l The proceedings of the International Workshop on Railway and Tracked with respect to high-speed rail. The physics Transit System Noise (IWRN), which are of noise and vibration generation and usually published in the Journal of Sound transmission for transit systems is similar or and Vibration.t’f identical to that of high-speed rail. l A review of the state-of-the-art in wheel/rail noise control has been

9-l Light Rail Track Design Handbook

Noise or sound pressure is conventionally wheel/rail rolling noise, and wheel squeal. described with a logarithmic decibel scale Rolling noise and wheel squeal are (dB). An approximation of the response of the fundamentally different processes, hence their human ear can be imposed on this scale by separation The final section concerns applying the ‘A’ frequency weighting network, groundbome noise and vibration. which results in the A-weighted sound level (dBA). 9.2. NOISE AND VIBRATION CONTROL The wheels and rails radiate approximately DESIGN GUIDELINES equal amounts of sound energy to the wayside or surrounding areas. The nature of Guidelines have been developed by the sound is such that halving a sound energy Federal Transit Administration (FTA) and the emission produces only a 3-dB reduction in American Public Transit Association (APTA). noise level, a difference that may be barely These standards or guidelines can be used as perceptible if frequency characteristics remain criteria for both airborne and groundborne unchanged. This condition would be equal to noise in a transit corridor. The APTA no sound energy transmitted by the wheels guidelines recommend limits on maximum while leaving the rails untreated, or vice versa. passby noise levels (i.e. the maximum noise Therefore, noise control techniques have to levels that occur during an individual vehicle be applied to both components to achieve a or train passby), as well as limits on the noise satisfactory reduction in sound level. caused by ancillary facilities (i.e., fixed services associated with the transit system). The FTA guidance manual provides criteria 9.1.2 Scope for environmental impact analysis and mitigation in terms of the day-night level (L& The purpose of this chapter is to provide for both pre- and post-build conditions.t5’ The guidelines with respect to track design for FTA guidelines integrate the noise impact acceptable levels of noise and vibration. analysis for rail operations with that for other While many of the treatments considered here modes of transportation, such as highway or can be designed by the transit track engineer, aircraft. The FTA guidance manual should be the design of specific noise and vibration used to assess impacts for federally funded treatments, such as floating slabs, should be projects, and is recommended by the FTA for conducted by those who have considerable all rail transit projects. Refer to the FTA experience with designing and specifying guidance manual for detailed description of vibration isolation systems. The noise and the standards. For most practical situations, vibration designer should have an engineering the wayside noise levels resulting from or physics background and understand basic applying the FTA and APTA guidelines are concepts in noise and vibration control. very similar, though not identical.

The design of vehicle “on-board” and wayside The APTA guidelines are discussed below, treatments such as sound barriers are not because they may be used immediately by included here, as these are beyond the limits the track designer without detailed knowledge of track design. of existing ambient noise levels. The APTA guidelines pertain to standards that are The following sections include guidelines for typically adopted by transit agencies for the criteria on noise and vibration control at both design of new rail facilities to determine the the track wayside and vehicle interior,

9-2 Noise and Vibration Control location and extent of mitigation measures III High-density urban residential, average necessary to avoid noise impacts. The design semi-residential/commercial areas, parks, goals can be used directly without any museums, and non-commercial public assumptions regarding schedule, total number building areas. of trains or pre-existing ambient noise, as IV Commercial areas with office buildings, would be required when criteria are stated in retail stores, etc., with primarily daytime terms of a noise exposure level metric. This occupancy; central business districts. results in a consistent design with similar V Industrial areas or freeway/highway mitigation being installed in areas with similar corridors land uses or occupancies. The guidelines in Table 9.1 indicate maximum The various track structure types-ballast noise emissions from trains applicable to the track, direct fixation track and embedded land uses and types of buildings and track-must be considered in meeting the occupancies along the transit route. The criteria as each track type responds differently maximum passby noise level is the level in to wheel passage, and potential noise and decibels relative to 20 micro-Pascal of the vibration issues must be considered during average root-mean-square (RMS) A-weighted the initial planning stages. The services of a sound pressure amplitude occurring during a recognized noise, vibration and acoustical train passby, usually for a I- to 4-second expert in this field are recommended. average period.

The APTA guidelines as listed in Table 9.1 This is not to be confused with the single- apply to different types of communities along event noise exposure level (SENEL). Specific the transit alignment as follows: criteria are provided for various building types I Low-density urban residential, open in the APTA guidelines. space park, suburban residential, or quiet recreational areas with no nearby The guidelines in Table 9.2 indicate criteria highways or boulevards. for transit system ancillary facilities. Transient II Average urban residential, quiet noise criteria apply to short duration events apartments and hotels, open space, such as train passby noise transmitted suburban residential or occupied outdoor through tunnel vent shaft openings. areas near busy streets. Table 9.1 Criteria for Maximum Airborne Noise from Train Operations* Maximum Single Event Noise Levels (dBA) Single-Family Multi-Family Hotels and Category Community Area Dwellings Buildings Motels I Low-Density Residential 70 75 80 II Average Residential 75 75 80 Ill High-Density Residential 75 80 85 IV Commercial 80 80 85 V Industrial/Highway 80 85 85 * These criteria are generally applicable at the near side of the nearest dwelling or occupied building under consideration or 50 feet from the track centerline, whichever is furthest from the track center. Source. “Guidelines and Principles for Design of Rapid Transit Facilities; Noise and Vibration,” APTA 1979

9-3 Light Rail Track Design Handbook

Continuous noise design criteria apply to The environmental impact criteria noises such as fans, cooling towers, and other recommended by the FTA for ground vibration long duration or stationary noises, except are similar to American National Standards electrical transformers or substation facilities Institute (ANSI) standards for vibration in buildings.‘*’ The ANSI standard gives a For transformers and substation facilities (i.e., baseline criterion curve for l/3 octave band noise with tonal quality) the design criteria RMS vibration velocity of 100 micro- should be lowered by 5 dBA from the values meters/second (4,000 micro-inches/ second), in Table 9.2. corresponding to a vibration velocity level of 72 dBV re 1 micro-inch/second. Where l/3 octave analyses are not performed, the 9.2.1 Groundborne Noise and Vibration standard recommends a limit of 72 dB re 1 Criteria micro-inch/second for a frequency weighting that approximates the criterion curve for 113 Guidelines are presented below for octave levels. This latter limit is very similar to groundborne vibration impacts in buildings. the vibration impact criterion of 72 dBV The guidelines use the RMS vibration velocity recommended by the FTA for floor vibration level in dBV relative to 1 micro-inch/second as velocity levels in residences. For rail transit the principal descriptor of vibration impacts on ground vibration, there is no practical building occupants. The RMS vibration difference between the weighted vibration velocity metric is incorporated in various velocity described in ANSI standard S3.29 standards and specifications.t6.71 Vibration and the overall, or unweighted, vibration prediction procedures are described in the velocity level, because most of the vibration FTA guidance manual and other literature. energy occurs at frequencies above 8 Hz. The FTA guidance manual recommends The vibration should be measured as the criteria for wayside overall vibration velocity. RMS vibration velocity occurring during These criteria are presented in Table 9.3 vehicle consist passage. Thus, if a vehicle consist requires 4 seconds to pass, the RMS

Table 9.2 Guidelines for Noise from Transit System Ancillary Facilities*

Maximum Noise Level Design Criterion (dBA) Category Community Area Transient Noises Continuous Noises I Low-Density Residential 50 40 II Average Residential 55 45 III High-Density Residential 60 50 IV Commercial 65 55 V Industrial/Highway 70 65

* The design goal noise levels should be applied at 50 feet from the shaft outlet or other ancillary facility or should be applied at the setback line of the nearest line of the nearest buildings or occupied area, whichever is closer. For transfomlers or substation noise, reduce “Continuous Noises” by 5 dB.

9-4 Noise and Vibration Control

Table 9.3 Groundborne Vibration and Noise Impact Criteria

Groundborne Vibration Impact Levels (dBV re Groundborne Noise 1 micro-inch/second) Impact Levels (dBA re 20 micro-Pascal) Frequevt l;frequent Frequeyt InfrequeFt Land Use Category Events Events Events Events 5 3 4 4 - Category 1: Buildings where low 65 65 NA NA ambient vibration is essential for interior operations Category 2: Residences and buildings 72 80 35 43 where people normally sleep Category 3: Institutional land uses with 75 83 40 48 primarily daytime use

Notes ’ “Frequent Events” is defined as more than 70 vibration events per day. * “infrequent Events” is defined as fewer than 70 vibration events per day 3 This criterion limit is based on levels that are acceptable for most moderately sensitive equipment such as optical microscopes Vibration-sensitive manufacturing or research will require detailed evaluation to define the acceptable vibration levels. 4 Vibration-sensitive equipment is not sensitive to groundbome noise. Source: Transit Noise and Vibration Impact Assessment, Federal Transit Administration, USDOT, April 1995 vibration should be measured over a duration Typical design criteria for floor vibration are approximately equal to 4 seconds. (The listed in Table 9.4, for the land use categories actual duration will depend on the integration identified in the APTA Guidelines. These times available from the analyzer.) The result design criteria are not part of the APTA obtained for the maximum vibration using a Guidelines, but have been applied to several vibration meter with a slow response, transit systems, both heavy rail and light rail, equivalent to a l-second averaging time, in the United States. They are very similar to would be slightly higher than that obtained the FTA criteria described above. The over the train passby duration by a fraction of guidelines for maximum groundborne a decibel at distances greater than about 15 vibration are presented in terms of dBV meters (50 feet) from the track, and should be relative to 1 .O micro-inch/second. acceptable. A “fast” meter response, or integrating time shorter than 1 second, should Groundborne vibration that complies with not be used, because the vibration level may these design criteria would not be fluctuate considerably during vehicle passage, imperceptible in all cases. However, the level giving an unrepresentative reading. would be sufficiently low so that no significant Fluctuation of vibration amplitudes and levels intrusion or annoyance should occur. In most is a normal result of the random nature of low cases, there would be vibration from street frequency ground vibration. traffic, other occupants of a building, or other sources that would create vibration that is

9-5 Light Rail Track Design Handbook

Table 9.4 Criteria for Maximum Groundborne Vibration from Train Operations by Land-Use Category*

* Criteria apply to the vertical vibration of floor surfaces within buildings.

equivalent to or greater than vibration from l Normal roiling noise transit train passbys. l Impact noise due to loss of contact between the wheel and rail

The APTA guidelines recommend limits on l Rail corrugation noise groundborne noise transmitted into building l Grinding artifact noise structures (see Table 9.5). They have been employed as design criteria for many heavy Normal rolling noise is broadband noise and light rail transit systems in the United produced by reasonably smooth rail and States and are similar to the FTA criteria. wheel treads. Departures from this “normal” condition include impact noise, corrugation noise, and grinding artifact noise. Impact and 9.2.2 Wheel/Rail Rolling Noise corrugation noise are more raucous and are usually the cause of community concerns Rolling noise is associated with the action of about transit noise. While impact noise the wheel rolling over tangent or curved track, occurs at special trackwork, flat wheels, and is produced primarily by rail and wheel excessive rail roughness, undulation, surface roughness. Rolling noise is distinct corrugation, and rail joints also cause impact from wheel squeal, which may occur at noise. Rail corrugation involves periodic rail curves, both in nature and in generating roughness with wavelengths from 25 mechanism. Rolling noise may be radiated by millimeters to 150 millimeters (1 to 6 inches), the wheels and rails, and may also be may be low amplitude, as during its initial radiated by the structure supporting the track, stages, or may involve deep corrugation and such as elevated steel or concrete structures. contact separation. Grinding artifact noise is caused by a grinding pattern left in the rail by For the purpose of this chapter, wheel/rail rail grinding machines, and has been noise is categorized into confused with corrugation noise.

9-6 Noise and Vibration Control

Table 9.5 Criteria for Maximum Groundborne Noise from Train Operations*

*Source: Guidelines and Principles for Design of Rapid Transit Facilities, Noise and Vibration, APTA 1979

9.2.2.1 Normal Rolling Noise should not produce as much noise as longer wavelength components, unless the milling 9.2-2.7.7 Generating Mechanisms marks are non-uniform. The following generating mechanisms have been identified as sources of normal roiling Increasing the conformity of the wheel and rail noise: contact has been proposed as a noise l Wheel and rail roughness reduction technique that takes advantage of l Parameter variation of rail head geometry uncorrelated roughnesses between various or moduli parallel paths along the rail in the longitudinal l Dynamic creep direction. rgl Significant noise reductions on l Aerodynamic noise the order of 3 to 5 dB are predicted for frequencies on the order of 500 Hz. However, Wheel and Rail Roughness. Wheel and rail excessive wheel/rail conformity due to wear surface roughnesses are believed to be the has been identified as a cause of spin-creep most significant cause of wheel/rail noise. corrugation, leading to increased noise.“01 The greater the roughness amplitudes, the Therefore, care should be exercised before greater the wayside noise and vibration. increasing the conformity of wheel and rail Assuming that the contact stiffness is infinite, profiles. Excessive wheel/rail conformity from the rail and wheel would displace relative to wear (and false flanging) will result if wheel each other by an amplitude equal to their profile truing is not conducted frequently. combined roughness amplitudes. The ratio of Further, good low-noise performance has rail motion relative to wheel motion at a been obtained with 115RE rail with a 250- specific frequency will depend on the dynamic millimeter (IO-inch) head radius and characteristics of the rail and wheel. cylindrical wheels.

At short wavelengths relative to the contact Parameter Variation. Parameter variation patch dimension, the surface roughness is refers to the variation of rail and wheel steel attenuated by averaging the roughness moduli, rail support stiffness, and contact across the contact patch in a direction parallel stiffness due to variation in rail head ball with the rail, an effect known as contact patch radius. The influence of fractional changes in filtering. Thus, fine regular grinding or milling Young’s elastic modulus and of radius-of- marks less than 1 or 2 millimeters wide, curvature of the rail head as a function of

9-7 Light Rail Track Design Handbook wavelength necessary to generate wheel/rail simple grinding with a parallel axis grinder or noise equivalent to that generated by surface block grinder is preferred. roughness are illustrated in Figure 9.1. The wavelength of greatest interest is 25 to 50 Dynamic Creep. Dynamic creep may include millimeters (1 to 2 inches), corresponding to a both longitudinal and lateral dynamic creep, frequency of about 500 to 1,000 Hz for a roll-slip parallel with the rail, and spin-creep of vehicle speed of about 97 km/hr (60 mph). the wheel about a vertical axis normal to the Over this range, a variation in modulus of 3 to wheel/rail contact area. Longitudinal creep is 10% is required to produce the same noise as wheel creep in a direction parallel with the rail that produced by rail roughness. and is not considered significant by some researchers, who claim that rolling noise 10 levels do not increase significantly during --w-- MODULUS braking or acceleration on smooth ground rail. However, qualitative changes in wheel/rail noise on newly ground rail with an irregular transverse grinding pattern in the rail surface are audible as a train accelerates or decelerates, suggesting that longitudinal creep may play a role. Lateral creep is wheel slip across the rail running surface in a direction transverse to the rail during curve negotiation and is often accompanied by wheel squeal. Lateral creep may not be significant at tangent track, but may occur

0.001 during unloading cycles at high frequencies 1 10 100 1000 on abnormally rough or corrugated rail, and WAVELENGTH - mm may be responsible for short-pitch corrugation Figure 9.1 Change in Elastic Modulus and at tangent track. Spin-creep is caused by Rail Head Curvature Required to Generate wheel taper that produces a rolling radius Wheel/Rail Excitation Equivalent to differential between the field and gauge sides Roughness Excitation of the contact patch. Roll-slip refers to rolling contact with slip at the edges of the contact Rail head ball radius variation also induces a zone. Some slip, continuous or otherwise, is dynamic response in the wheel and rail. A required at the edges of the contact zone, as variation of rail head curvature of the order of with Heathcote slip of a bearing in its groove, IO to 50% produces noise levels similar to required by the conformal contact of curved those produced by rail height variation alone. contact surfaces. Rail head ball radius variation will normally accompany rail height variation. Maintaining Aerodynamic Noise. Aerodynamic noise a uniform rail head ball radius is necessary to due to high velocity air jets emanating from realize the advantages of grinding rail to grinding grooves in the rail has been claimed maintain uniform head height. Irregular to produce a high frequency whistling noise. definition of the contact wear strip is indicative No test data have been obtained to confirm of excessive ball radius variation. Thus, rail this claim. It is further claimed that fine rail profile grinding with a vertical axis grinder to grinding removes course grinding marks and produce a distinct head curvature rather than thus the noise. This is important if grinding is

9-8 Noise and Vibration Control specified during construction to eliminate mill respectively. It has not been determined if the scale from the rail to obtain better traction and pinned-pinned mode is directly responsible for electrical conductivity. The grinding must peaks in the wayside noise spectrum, but it is have the fine quality mentioned previously expected to have a bearing on wayside noise and must maintain the design head radius for and possibly rail corrugation. the rail.

Other sources of aerodynamic noise include air turbulence about the wheels and trucks, and traction motor blower noise. Neither of these is controllable by the track designer, but traction motor blower noise can, under certain circumstances, dominate the wayside noise spectrum if not properly treated. Aerodynamic noise due to air turbulence about the wheels and trucks at light rail transit speeds is not significant. cl -12 18 24 30 36 42

RAIL WF’ORT SEPARATION- INCHES 9.2.2.1.2 Wheel Dynamics The dynamic response of the wheel has a O 132 tB/uo Aloo LB/M substantial effect on rolling noise and l 115 LB/M) O 90 LB/Yu vibration. The response is affected by axle Figure 9.2 Vertical Pinned-Pinned bending modes beginning at about 80 or 90 Resonance Frequency vs. Rail Support Hz, tire resonances, spring-mass resonances Separation for Various Rails of resilient wheels, and so forth. Up to about 400 Hz, the wheel is considered a rigid mass. Bending waves will propagate in the rail up to At higher frequencies, these resonances a frequency corresponding to l/2 the pinned- cause a very complex response that is not pinned mode frequency in the case of rigid rail easily described here. supports. Between this frequency and the pinned-pinned mode frequency, vibration transmissions along the rail may be 9.2.2.q.3 Rail Dynamics attenuated, depending on the rail support The dynamic response of the rail also dynamic characteristics, producing what is influences the radiation of noise. Up to about termed a “stop band.” Between the pinned- 500 Hz, the rail behaves as a simple beam on pinned mode frequency and another cutoff an elastic foundation. At higher frequencies, frequency, bending waves may propagate standing waves may occur in the rail due to freely, resulting in a “pass band.” The resonance between the rail supports. The response of the rail and its ability to radiate first of these is the pinned-pinned mode of rail noise will be affected by the widths of the stop vibration. Estimates of the pinned-pinned and pass bands, A slight randomness in the mode resonance frequencies based on support separation may significantly alter the Timoshenko beam theory are presented in stop and pass band characteristics. The pinned-pinned mode Figure 9.2. Shortening the rail support pitch will increase resonance frequencies of a rail supported at the stop band frequency range, and thus 900- and 750-millimeter (36- and 30-inch) reduce noise. Thus, 600 millimeter (24 inch) spacing are about 500 Hz and 800 Hz,

9-9 Light Rail Track Design Handbook spacing is probably preferable to 750- or 900- of 650 Hz and higher. The fastener behaves millimeter (30-or 364nch) spacing. as a pure spring below the single-degree-of- freedom resonance frequency. At higher The main point here is that the response of frequencies, top plate bending amplifies the the wheel and rail above 500 Hz is very transmission of forces to the invert and complicated, and that the propensity for produces a high reaction to rail motion that adverse interaction between these elements, tends to “pin” the rail at this frequency, leading to tonal components of wayside noise possibly interacting with the “pinned-pinned” and possibly corrugation, is high. Track mode. At higher frequencies the transmitted design should, ideally, be directed toward force declines significantly. minimizing this possible interaction by ensuring that the pinned-pinned mode As with the pinned-pinned mode, the frequency is not coincident with an anti- significance of fastener top plate bending on resonance or resonance of the wheel. rail radiated wayside noise has not been Reducing rail support spacing and introducing determined. However, from the standpoint of damping into the track support system may be track design, introduction of damping into the useful for this purpose. system and exploiting the top plate resonance may be beneficial. This would be achieved by incorporating a neoprene elastomer with high 9.2.2.1.4 Resilient Direct Fixation loss factor and tuning the top plate resonance Fasteners to absorb vibration energy at the pinned- Resilient direct fixation fasteners are used for pinned mode frequency. Tuning the plate can rail support and provide modest vibration be accomplished by changing the thickness. isolation. The most common form of resilient More research and testing are required to direct fixation fastener consists of top and determine which approach is best. bottom steel plates bonded to an elastomer pad. Modern designs incorporate anchor bolts that engage the bottom plate, so that the 9.2.2.1.5 Contact Stiffness top plate is retained entirely by the elastomer Contact stiffness is the ratio of the contact vulcanized bond. The top plate contains vertical force to the relative vertical deflection recesses to retain the rail clips. of the wheel and rail running surface. If the contact stiffness is small relative to the A direct fixation fastener is a complex stiffness of the wheel or rail, wheel/rail forces mechanical element, even when considering will be controlled partially by the contact only vertical motion. There are two stiffness, in which case both the wheel and frequencies that affect performance. One is rail vibration will decrease in response to the top plate resonating on the elastomer pad roughness. The contact stiffness does not in rigid body motion and the other is the vary greatly over the range of rail head ball bending resonance of the top plate. The first radii. The ball contact stiffness varies about of these can be thought of as a single-degree- 16% for radii between 150 millimeters (6 of-freedom oscillator with mass equal to the inches) and 375 millimeters (15 inches). top plate mass and spring equal to the top Under the most optimistic scenario, this plate stiffness, and may occur at frequencies variation would increase contact forces, and as low as 250 Hz. The second is influenced thus noise, by at most 1.5 dB. However, by the vertical stiffness per unit area of the contact stresses may also increase as a result elastomer and the bending stiffness of the top of a smaller contact area, and rail head plate, and occurs at frequencies on the order

9-10 Noise and Vibration Control geometry should be designed to minimize associated impact noise. Impact noise due to stress and wear. Also, some investigators rough wheels and rails is probably the most have identified that high wheel/rail conformity significant and irritating noise on older transit with spin-slip corrugation and large ball radii systems where rail grinding and wheel truing may promote conformity. Corrugation are not practiced. notwithstanding, rail wear is not considered a serious problem at tangent track due to low transit wheel loads. Wheel tread concavity 9.2.2.3 Rail Corrugation Noise due to wear increases the lateral contact Rail corrugation is a series of longitudinal high patch dimension. Although the rail head and low points or a wave formed in the rail radius may be optimized for noise control, head surface. Rail corrugation causes wheel tread wear may frustrate maintaining a excessive rolling noise of a particularly harsh specific contact geometry unless a vigorous character and very high sound level. The wheel truing program is in place. terms “roaring rail,” “roar,” “wheel howl” or “wheel/rail howl” describe noise produced by corrugated rail. If rail corrugation exists, the 9.2.2.2 Impact Noise wayside noise level will be much higher than Impact noise is a special type of wheel/rail that of normal rolling noise, and the frequency noise occurring on tangent track with high spectrum will contain discrete frequency amplitude roughness, rail joints, rail defects, components and associated harmonics. or other discontinuities in the rail running surface and wheel flats. Impact noise is Rail corrugation is more difficult to control on probably the most apparent noise on older rail transit systems than railroads because of transit systems that do not practice regular rail the lower contact static loads and uniformity of grinding and wheel truing. transit vehicle types and speeds, which prevent randomization of wheel/rail force Remingtont”1 provides a summary of impact signatures. Thus, maintaining rail noise generation that involves non-linear smoothness is probably more important for wheel/rail interaction due to contact transit systems than heavy freight systems. separation, and is closely related to impact Rail corrugation is the principal cause of noise generation theory at special trackwork. excessive noise levels on many transit I. L. Verns categorizes impact noise by type systems, and controlling rail corrugation is key of rail irregularity, train direction, and speed. to minimizing rail transit system noise. At present, the most effective means of Modern transit systems employing continuous controlling rail corrugation is rail grinding. welded rail will likely not be concerned with Detailed discussions of rail corrugation noise impact noise generated by rail joints, though are included in TCRP Report 23.“’ impact noise will be generated by rough rail, wheel flats, turnout frogs, and crossover diamonds. Even with continuous welded rail, 9.2.2.4 Treatments for Rolling Noise rail welds and insulating joints must be Control carefully formed to reduce impact noise Continuous welded rail, rail grinding, fastener generation. Further, rail joint maintenance is support spacing, rail vibration absorbers and important on older systems employing jointed dampers, and rail head hardfacing are track- rail. All systems must be concerned with rail oriented treatments for controlling rolling grinding and wheel truing to eliminate noise Rail grinding is included because it pertains to track maintenance. Even though

9-l 1 Light RailTrack Design Handbook rail grinding is usually the task of the transit solutions to providing access to work system operator, the initial grind may be equipment. performed during track construction to remove mill scale from the rail for better traction and Some grinders may have difficulty negotiating electrical conductivity. The grinding must curves in tunnels or may be unable to grind have the fine quality mentioned previously, rail on very short radius curves. Adequate and must maintain the design or specified clearance must be included in track and head radius for the rail. system designs to accommodate rail grinding machines. Rail grinding can be performed only if there is adequate access to the track 9.2.2.4.1 Continuous Welded Rail during non-revenue hours. Grinding time can Rolling noise levels with properly ground be optimized by minimizing travel time to and continuous welded rail and trued wheels in from the grinder storage location and the good condition are the lowest that can be treatment section. Pocket tracks capable of achieved without resorting to extraordinary storing the grinder during revenue periods will noise control measures. There are no rail minimize travel time. joints to produce impact noise, which can be clearly audible with moderately maintained Vertical axis grinders with special provision track. Noise from jointed rail may be as much may be able to grind embedded girder rail. as 5 dB higher than from continuous welded Using standard T-rail sections provides the rail. Continuous welded rail requires less greatest flexibility with respect to grinding, maintenance than jointed rail, so that the especially on embedded curves. benefits of low noise are more easily obtained. The optimal grinding procedure includes grinding the rail to achieve a head radius profile with a 12- to 16-millimeter (l/2- to 5/8- 9.2.2.4.2 Rail Grinding inch) contact zone. This should be achieved Rail grinding combined with wheel truing is with grinding facets of about 2 millimeters the most effective method for controlling (l/l6 inch). Multiple head grinders reduce the wheel/rail noise and maintaining track in good grinding time necessary to produce the working condition. With ground rail and trued desired contour. Computer controlled grinders wheels, wheel/rail noise levels at tangent with various grinding profiles stored in ballasted track are comparable with the memory can simplify setup and further combined noise levels from traction motors, increase grinding time. The gauge corner can gears, and fans. be finished in a manner consistent with the wheel profile. Grinding car speeds should be As a track designer, it is important to plan for as slow as possible to reduce the wavelength maintenance activities that must be performed of grinding patterns to a minimum. However, to keep the system working well. Rail grinding the speed should not be so slow as to to control noise is one of these activities. produce excessive heating of the rail. Consideration should be given to where grinding equipment (as well as other track Rail grinding should be performed at intervals maintenance equipment) can be staged to short enough to avoid the development of rail access the system. Short track shutdowns for corrugation. Periodic track inspections for maintenance are the norm in the industry. corrugation growth and noise increase should Therefore it is important to have practical be conducted to identify appropriate grinding

9-12 Noise and Vibration Control

intervals. A grinding interval equal to the will also raise the pinned-pinned mode exponential growth time of corrugation (time resonance frequency above the anti- for corrugation to grow by 167%) gives a resonance frequency of the Bochum tire rough estimate of the optimum grinding wheel, thus placing the maximum driving interval. Varying the location of the contact frequency of the tire in the stop band region of zone is used by some systems to reduce the rail vibration transmission spectrum This rutting of the wheel tread, and thus reduce design provision should be investigated wear resulting in conformal contact and spin- further. A 600-millimeter (2-foot) rail support slip. spacing is now being considered by one transit system overseas with high volume and strict noise control requirements. 9.2.2.4.3 Rail Support Spacing Rail support stiffness and damping, fastener resonances, and fastener spacing all directly 9.2.2.4.4 Direct Fixation Fastener Design influence high-frequency vibration of the rail. Resilient rail fasteners are effective in One of the most common sources of noise is controlling structure-radiated wheel/rail noise short-pitch rail corrugation. Modification of rail by providing vibration isolation between the support parameters may offer an opportunity rail and structure and eliminating looseness in to influence and possibly control the formation the rail fixation. Resilient elastomeric of rail corrugation, which has been related to fasteners significantly reduce wayside noise the pinned-pinned mode of rail vibration. The from steel elevated structures relative to pinned-pinned mode is, in turn, controlled by levels for conventional timber tie and cut-spike fastener spacing. The pinned-pinned mode track. Softening the fastener further produces resonance frequency is on the order of 800 a marginal reduction of A-weighted noise. and 500 Hz for fastener spacing of 750 to 900 The best performing fasteners would include millimeters (30 and 36 inches), respectively. those that had the lowest static and dynamic Reducing the fastener spacing to 600 stiffness with a top plate bending resonance in millimeters (24 inches) would drive the excess of about 800 Hz. pinned-pinned mode resonance frequency to above 1,000 Hz, possibly high enough to Noise radiated by rail in resilient direct fixation smooth-out short-pitch corrugation at the track is usually greater than for ballasted track contact patch, and thus reduce the due to the high acoustic reflectivity of concrete corrugation rates. A second concern with plinths and inverts. The character of wayside respect to rail fastener spacing is a “singing noise from resilient direct fixation track also rail” phenomenon associated with regularly differs significantly from that produced at spaced (concrete) crossties, rail seat pads, ballasted track, probably due to differing and spring clips. The transmission of dynamic characteristics of the rail support and vibration along the rail is subject to certain rail support separation, as well as the amount stop bands and pass bands in the frequency of trackbed sound absorption. domain, which are closely related to the pinned-pinned mode resonance. Very precise Soft natural rubber fasteners support efficient fastener spacing may contribute to singing rail propagation of bending waves that radiate and pinned-pinned modes, and a slight noise. Incorporation of damped elastomers randomization of crossties or fastener spacing may be desirable to absorb rail vibration may be beneficial. Reduction of concrete energy, thus reducing noise radiation. An crosstie spacing to 600 millimeters (24 inches) attractive elastomer for this purpose is neoprene, which has an added advantage of

9-l 3 Light Rail Track Design Handbook

resistance to ozone and oils, and is common very close to the track are claimed to provide in track construction. However, neoprene a noise reduction of 3 dB when installed on should not be used where vibration isolation is direct fixation track, which is consistent with required to control structure-radiated or that obtained with ballasted track relative to groundborne noise radiation. Where vibration direct fixation track. This treatment has not isolation is needed more than airborne noise been implemented in the US to date. control, such as on steel elevated structures or in subway tunnels, natural rubber is the 9.2.2.4.6 Rail Wbration Absorbers preferred elastomer, providing a dynamic-to- Rail vibration absorbers are resonant static stiffness ratio of less than 1.4. mechanical elements that are attached to the The load vs. deflection curve of the fastener rail base to absorb vibration energy and thus should be reasonably linear within +/- 15% of reduce noise radiation by the rail. Rail the mean static stiffness over the load range vibration absorbers have been employed in to maintain its dynamic properties over the Europe, but have received little attention load range. Specifying this linearity in an within the United States. Rail vibration unambiguous way is critical in the absorbers may be desirable at certain site- procurement process. The fastener should specific locations. However, the size of the provide full 3-degree-of-freedom isolation. absorber may require substantial clearance Fasteners with hard horizontal snubbers can space beneath the rail. The absorbers are exhibit high non-linearity and compromise the usually tuned to frequencies above about vibration isolation that might be otherwise 1,000 Hz, while the maximum noise levels achievable. Fasteners with elastomer in may occur at about 500 to 800 Hz. Absorbers shear are some of the best performing tuned to 500 to 800 Hz may require more fasteners in this regard. mass than those now being offered in Europe. Data provided by certain manufacturers The tendency today in direct fixation track indicate a reduction of about 3 to 5 dB in rail design is to provide fasteners with stiffness on vibration at l/3 octave band frequencies the order of 15 to 20 MN/m, utilizing natural between 300 and 2,000 Hz for 111 km/hr rubber elastomers in addition to neoprene and trains on tangent track. Absorbers were other synthetics. As noted above, while mounted on each rail, one between each rail natural rubber has desirable properties for fastener. The mass of each absorber was 23 vibration isolation, the low damping capacity kilograms (50 pounds). of these materials may promote bending wave propagation and noise radiation by the rail. Absorbers utilizing an elastomer element and optimized for moderate to high temperatures For additional information on direct fixation rail may lose a portion of their effectiveness at low fasteners refer to Section 54.3. frequencies. The leaf vibration absorber might be susceptible to freezing in sub-freezing weather with snow. 9.2.2.4.5 Trackbed Acoustical Absorption Ballasted track is well known to produce about Vibration absorbers may be impractical on 5 dB less wayside noise than direct fixation ballasted track unless they can be positioned track, due to the sound absorption provided clear of the ballast to maintain electrical by the ballast and differences in the track- isolation. Further, the ballasted track with support characteristics. Acoustically timber crossties and cut spikes may provide absorptive concrete and wood blocks placed substantial energy absorption without

9-14 Noise and Vibration Control vibration absorbers, so that the addition of the special trackwork by wheels traversing frog absorber may provide little additional noise gaps and related connections is a special reduction. The absorber is effective where case of impact noise discussed above. the track exhibits little damping, such as at Special trackwork noise may be controlled by concrete crossties and on ballast systems grinding the frog to provide as smooth a with spring clips and resilient rail seat pads. transition as possible for each wheel to pass from one side of the flangeway to the other. Special frogs, including movable point, swing 9.2.2.4.7 Wear-Resistant Hardfacing nose, and spring frogs, have been developed “Hardfacing” is the weld application of a metal to minimize impact forces by eliminating the alloy inlay to the rail head. The procedure fixed gap associated with the frog Because involves cutting or grinding a groove in the rail the frog gap, combined with poorly maintained surface and welding a bead of the alloy into wheels, contributes to the increase in noise the groove. The Riflex welding technique has when a train passes through a turnout, the been used on a limited basis in the United use of special frogs to reduce special States, primarily for wear reduction, but has trackwork noise may be a practical noise been promoted in Europe since the early control provision for many transit systems. 1980s for rail corrugation control and wheel squeal. For additional information on Riflex welding, refer to Section 52.5 in this 9.2.3.1 Frogs handbook. Various frog designs have been used in transit installations: solid manganese, flange bearing, liftover, railbound manganese, 9.2.2.4.8 Low Height Sound Barriers spring, and movable point frogs. For Low height barriers placed very close to the additional information on frog design, refer to rail have been explored in Europe for Section 6.6. The following guidelines are controlling wheel/rail noise, perhaps just provided for frog design selection for noise outside the wheel’s clearance envelope. In control. one case, an aerial structure has been designed to provide a trough in which the vehicle runs, blocking sound transmission to 9.2.3.1.1 Solid Manganese Frog the wayside. Sound absorption is used to Solid manganese frog design with welded toe absorb sound energy before it escapes the and heel joints provides a virtually continuous wayside. The height of the barriers must be running surface except for the open determined by careful analysis. A l- to a-inch flangeway. Proper wheel and frog design thick glass fiber or mineral wool sound along with continuous track maintenance and absorber with perforated protective cover wheel truing should provide adequate low- should be incorporated on the rail side of the noise operation. Hollow worn wheels with barrier. Adding sound absorption to the false flanges will contribute to noise and concrete slab surface of direct fixation track vibration when traversing through the frog. should be considered.

9.2.3.1.2 Flange-Bearing Frog 9.2.3 Special Trackwork Noise Flange-bearing frog design with welded toe and heel joints is similar to the solid Special trackwork includes switches, turnouts, manganese design except the frog provides and crossovers. The noise generated at support to the wheel flange while traversing

9-l 5 Light Rail Track Design Handbook the flangeway opening frog point area. The performance and is the source of wheel batter depth of the flangeway is reduced to a limit to noise and vibrations from the outset of support the wheel in the point area. If the installation They are not as quiet as the frogs wheel and frog are properly maintained, this described above. design reduces impact of the wheel in the open flangeway frog point area. Gradual 9.2.3.1.5 Movable Point Frogs ramping of the flangeway is critical to avoiding Movable point frogs are perhaps the most impact noise. effective way to eliminate the impact noise associated with fixed flangeway gap frogs. 9.2.3.1.3 Liftover Frog The frog flangeway is eliminated by laterally Liftover frog design with welded toe and heel moving the nose of the frog in the direction in joints is similar to the flange-bearing design which the train is traveling. The movable except the frog provides a continuous main point frog generally requires additional line running rail surface and open flangeway. signaling, switch control circuits, and an The lateral move flangeway is omitted in this additional switch machine to move the point of design. the frog. Movable point frogs have been incorporated on people mover systems in When a movement occurs for the diverging Canada and in Australia, but have received route, the frog flangeway and wing rail portion little or no application on light rail transit is ramped up to a level that allows the wheel systems in the United States. to pass over the main line open flangeway and running rail head. If the wheel and frog are properly maintained, this design 9.2.3.1.6 Spring Frogs eliminates impact on the main line moves and Spring frogs also eliminate the impact noise reduces impact of the wheel in the diverging associated with fixed flangeway gap frogs for direction. trains traversing the frog in a normal tangent direction. The spring frog includes a spring- The three frog designs described above are loaded point, which maintains the continuity of recommended for light rail transit installations the rail’s running surface for normal tangent to reduce noise and vibration. The frogs can operations. For diverging movements, the be considered for three track types: ballasted, normally closed frog is pushed open by the direct fixation and embedded special wheel flange. There may be additional noise trackwork. associated with trains making diverging movements, because the train wheels must still pass through the fixed portion of the frog 9.2.3.1.4 Railbound Manganese Frogs Thus, use of these frogs in noise-sensitive Railbound manganese frogs with the running areas where a significant number of diverging rail surrounding the central manganese movements will occur will not significantly portion of the frog introduce interface mitigate the noise impacts associated with openings in the running rail surface in addition standard frogs. to the flangeway openings. Light rail main line track installations should always consider welded joints at the toe and heel of the frog. 9.2.4 Wheel Squeal Noise The manganese-to-rail-steel interface in the frog design introduces a joint in the running Wheel squeal is one of the most serious types surface that severely impacts wheel of noise produced by light rail transit systems

9-16 Noise and Vibration Control and can occur at both short- and long-radius Wheel flange rubbing is due to contact curves. In a central business district, between the flange and high rail and occurs pedestrians and patrons are in close proximity on short-radius curves with significant to embedded track curves of light rail crabbing of the wheel set, such as at gauge systems; consequently, they are subjected to widened curves However, lubrication of the high levels of squeal noise. The high levels of flange does not entirely eliminate wheel noise at discrete squeal frequencies result in squeal and wheel squeal is not limited to the high perceptibility and annoyance high rail, suggesting that flange contact is not necessarily the only significant cause of Wheel squeal may be intermittent, due to squeal. Flange rubbing is also accompanied varying contact surface properties, surface by lateral slip, which may be the primary contaminants, or curving dynamics of the cause of squeal. vehicle and rail. On wet days, wheel squeal may be eliminated when negotiating all or Lateral slip with non-linear lateral oscillation of most of a curve. the tread running surface across the rail head is believed to be the principal source of squeal. Figure 9.3 illustrates the geometry of 9.2.4.1 Causes of Wheel Squeal curve negotiation by a transit vehicle truck. Three assumed types of vibratory motion Lateral slip across the rail head is producing wheel squeal noise are: necessitated by the finite wheel base (6) of 1 Longitudinal slip with non-linear rotational the truck and the radius of curvature of the oscillation of the tire about its axle rail, where no longitudinal flexibility exists in 2 Wheel flange contact with the gauge face the axle suspension. However, Figure 9.4 of the rail illustrates the actual crabbing of a truck. In 3 Lateral slip with non-linear lateral this case, the leading axle of the truck rides oscillation of the tire across the rail head. towards the high rail, limited only by flange contact of the high rail wheel against the Longitudinal slip is due to the different gauge face of the rail. The trailing axle travels translation velocities between the high and between the high and low rail, and the low rail low rail wheels in a direction parallel with the wheel flange may, in fact, be in contact with rail. Longitudinal slip is expected on curves the low rail gauge face. Gauge widening, where the distance traversed at the high rail is common on many transit systems, increases greater than at the low rail. Wheel taper is the actual creep angle (angle of attack) and sufficient to compensate for differential slip on exacerbates the generation of wheel squeal. curves with radii in excess of about 610 For additional information on truck rotation meters (2,000 feet), though shorter radii may refer to Section 4.2.9. be accommodated by profile grinding of the rail head and gauge widening. Further, Rudd The friction between the wheel and rail reports that elastic compression of the inner running surfaces during lateral slip varies non- wheel and extension of the outer wheel tread linearly with the lateral creep function, defined under torque can compensate for the wheel as the lateral relative slip velocity divided by differential velocities, and, further, that trucks the forward rolling velocity. The coefficient of with independently driven wheels also friction initially increases with increasing creep squeal. t19 The consensus of opinion is that function, reaching its maximum at a creep longitudinal slip is not a cause of wheel function of about 0.09, and declining squeal. thereafter. The negative slope results in negative damping that, if sufficient to

9-17 Light Rail Track Design Handbook

to the change in friction characteristics caused by moisture. Wheel squeal may be naturally reduced in areas of high humidity.

9.2.4.2 Treatments There are a number of mitigation measures available for controlling wheel squeal. The most effective of these are resilient and 6= WHEEL SET (AXLE) BASE damped wheels. Resilient wheels are not a R= CURM RADIUS component of track design, but their use Figure 9.3 Geometry of Curve Negotiation greatly reduces the need for track or wayside and Lateral Slip mitigation . Again, wheel squeal control is a

DIRECTION OF, system problem rather than simply a vehicle IL1 TRAML or track design problem. Other treatments 1 L may be considered for application directly to _._____.------. -.______46 the track. ? 9 1 i I gs f u-J2 5 9.2.4.2.1 Dry-Stick Friction Modifiers gi; - TANGENT 2% Modification of the friction-creep curve is an A= 2 + 5= B/2R attractive approach to controlling wheel B= WHEEL SET (AXLE) BASE R= CURM RADIUS squeal. Dry-stick friction modifiers applied to 6= ACTUAL CREEP ANGLE the wheel tread, and thus the rail running Figure 9.4 Truck Crabbing Under Actual surface, improve adhesion and flatten the Conditions friction-creep curve, thereby reducing or eliminating the negative damping effect. overcome the internal damping of the system, Friction modifiers are being offered as an will produce regenerative oscillation or squeal. on-board treatment for wheel squeal. The treatment has also be& applied directly to the For a wheel base of 2280 millimeters (7.5 rail head with moderate success. Manual feet), squeal would not be expected for curve application of wayside friction modifiers can radii greater than 125 to 253 meters (410 to be considered for controlling squeal on 830 feet), the lower limit being achieved when curves, but no fixed automatic applicators are there is no gauge widening. As illustrated commercially available at this time. above, gauge widening increases the creep angle for the same radius of curvature. A typical assumption is that squeal does not 9.2.4.2.2 Lubrication occur for curves with radii greater than about Wayside lubricators can be used to lubricate 200 meters (700 feet), corresponding to a the rail gauge face, restraining rail, and wheel dimensionless creep rate equal to 0.7 B/R, flange. However, this leads to an undesirable where B is the wheel base and R is the curve situation; the lubrication tends to migrate to radius. the running rail head, reducing wheel squeal due to lateral slip at the expense of loss of Meteorological conditions affect the traction. The effectiveness of this type of generation of squeal. In wet weather, for lubrication in reducing noise can be example, wheel squeal is greatly reduced due substantial. Without lubrication, maximum

9-l 8 Noise and Vibration Control wheel squeal noise levels may exceed 100 native rail steel. Refer to Section 5.2.5 for dBA. With lubrication, wheel squeal noise additional information concerning rail head levels have been reduced by approximately treatments. 15to25dB.

Wheel tread and rail running surfaces cannot 9.2.4.2.5 Rail Head Damping Inlays be lubricated without loss of adhesion and Rail head damping, consisting of a synthetic braking effectiveness. Loss of braking resin glued to a groove in the rail head, has effectiveness will result in wheel flatting, which been offered as a treatment to control wheel produces excessive rolling noise, a squeal. This procedure has been applied for counterproductive result of improper at least a year on German rapid transit lubrication. Loss of wheel-to-rail electrical systems, and can be applied to all grades of contact from the use of uncontrolled wayside steel. The vulcanization process is used with lubricants is also a concern. Environmental all types of rails and is applied so that the degradation by lubricants is a serious wheel does not come into contact with the consideration; thus lubricants should be resin-based filler material. The manufacturer biodegradable to the maximum extent claims that noise is reduced by the material possible. damping provided by the resin inlay. No performance data have been provided and there are significant questions regarding 9.2.4.2.3 Wafer Sprays actual performance, wear, and squeal noise Water spray by wayside applicators on curved reduction. This approach should be track can be used to control wheel squeal, rail thoroughly checked and tested before corrugation and wear. Both the high and low applying it as a general noise reduction rails can be treated. Water spray has been treatment. reported to reduce wheel squeal by 18 dB on short-radius curves. Water spray cannot be used during freezing weather. Water sprays 9.2,4.2.6 Track Gauge may induce corrosion that is not conducive to Gauge narrowing is an attractive approach to electrical contact, and might not be advisable promoting curving and reducing crab angle for lightly used track or where signaling may and creep, and thus squeal. However, the be affected. Water sprays would likely pose wheel and rail gauges used on trolley systems less of an environmental problem than grease typically vary by 3 millimeters (l/8 inch), and or oil. this slight variation in gauge may dictate against gauge narrowing in curves to prevent the flanges from binding when axle spacing is 9.2.4.2.4 Rail Head Inlays taken into consideration. Refer to Section 4.2 The friction versus creep curve can be for additional information concerning track and modified by treatment of the rail heads with a wheel gauge. babbit-like (soft malleable metal) material. This treatment has been successful in Gauge widening has been incorporated in eliminating wheel squeal, reducing passby track design to control squeal and promote noise levels by approximately 20 dB. curving, but has produced the opposite effect. However, after several months of service, Gauge widening appears to be a holdover “chronic squeal reappeared.” The loss of from steam locomotive days when performance is likely due to wear of the locomotives with three-axle trucks were in material, allowing wheel tread contact with the use, and is not specifically necessary to

9-19 Light Rail Track Design Handbook prevent excessive flange wear for two-axle 9.2.4.2.9 Rail Vibration Absorbers trucks. Quite the opposite; gauge widening Rail vibration absorbers are resonant promotes crabbing because the natural mechanical elements that are attached to the tendency of a truck is to crab its way through rail to absorb vibration energy. Rail vibration a curve, with the high rail wheel of the leading absorbers are reputed to control wheel squeal axle riding against the high rail, as illustrated and also reduce rolling noise. This in Figure 9.4. technology has not been tried in the United States as of this writing. The most attractive design at present incorporates a series of 9.2.4.2.7 Asymmetrical Rail Profile tuned dampers that bear against both the rail Asymmetrical rail head profiles are designed foot and the rail web. Thus, vibration energy to increase the wheel rolling radius differential is absorbed from both these elements of the and promote self-steering of the truck through rail. The absorbers are clamped to the rail the curve, which requires a longitudinally with bolts, and a plate extends beneath the flexible truck. In this case, the contact zone of base of the rail. These systems have been the high rail is moved toward the gauge used in Europe, but not in North America. comer and the larger diameter of the tapered wheel, while the contact zone at the low rail is moved to the field side and the smaller 9.2.4.2.10 Double Restrained Curves diameter of the taper. The wheel taper thus Double restraining rails are designed to allows the high rail wheel to travel a greater reduce the angle of attack and promote distance than the low rail wheel per revolution. steering of the truck without flange contact on In so doing, the axles tend to line up with the gauge widened curves. In this case, the high curve radius, thus reducing the lateral slip rail wheel flange can be brought away from squeal. While this approach is attractive, it is the high rail by the low rail restraining rail and effective for curve radii of the order of 200 the low rail wheel flange can be moved away meters (700 feet) or more. This process has from the gauge face by the high rail been used in Los Angeles and Vancouver. restraining rail. The restraining rail flangeway width would have to be controlled to prevent binding of the wheel set or climbing of the 9.2.4.2.8 Rail Vibration Dampers flange onto the restraining rail. Further, the A rail vibration damper is a visco-elastic restraining rails may be liberally lubricated to constrained layer damping system applied to reduce squeal and wear due to friction the rail web to retard wheel squeal. In one between the wheel and restraining rail. design, the constrained layer damper is held However, no successful installations have against the rail web with a steel plate and been found that completely eliminate wheel spring clip under and about the base of the squeal. Although this approach is rail. The treatment can be applied with theoretically attractive in reducing crab angle, minimal disturbance of track, provided that it mixed results may be achieved. Curving may may be made short enough to fit between the be promoted most by maintaining gauge track supports. A second design includes a through the curve or possibly narrowing the damping compound that is bonded to the rail gauge. Refer to Section 4.2.8 for additional web and constraining steel plate, without the information concerning guarded track and use of a steel spring clip. restraining rail.

9-20 Noise and Vibration Control

9.2.5 Groundborne Noise and Vibration task of predicting groundborne noise and Mitigation vibration has advanced to a highly developed state, relying on downhole shear wave Groundborne noise and vibration is a velocity data, seismic refraction data, phenomenon of all rail transit systems and, if borehole impulse testing, and detailed finite not controlled, can cause significant impact on element modeling of structures and residences, hospitals, concert hails, surrounding soils. As a result, vibration museums, recording studios, and other predictions can be reasonably accurate, sensitive land uses. New light rail transit though still less precise than noise alignments include abandoned railroad rights- predictions. Special track design is now of-way passing through adjacent residential regularly considered as a means to control developments. Residences located within 1 perceptible ground vibration in addition to meter (3 feet) of right-of-way limits are not audible groundborne noise. uncommon, and there are instances where apartment buildings are built directly over light rail systems with little provision for vibration 9.2.5.1 Vibration Generation isolation. Vibration impacts on hospitals, Ground vibration from rail transit vehicles is sensitive “high-tech” manufacturers, or produced by wheel/rail interaction, driven by research facilities may occur. roughness in the wheels and rail running surfaces, discrete track structures, track Groundborne noise is heard as a low level irregularities, and imbalanced conditions of rumble, and may adversely impact rotating components such as wheels and residences, hospitals, concert halls, and other axles. Vibration forces are imparted to the areas or land uses where quiet is either track invert or soil surface through embedded desirable or required. Groundborne vibration track, direct fixation fasteners, or ballast. in buildings may be felt as a low frequency These forces cause the transit structure and floor motion, or detected as secondary noise soil to vibrate, radiating vibration energy away such as rattling windows or dishes. Building from the track in the form of body and surface owners often claim that groundborne vibration waves. Body waves are shear and is responsible for building settlement and compression waves, with respective shear damage, though there have been no and compression wave propagation velocities. demonstrated cases of this occurring. Body waves attenuate (or lose amplitude) at a rate of 6 dB (50% in amplitude) as distance Literature concerning rail transit groundborne from the source doubles without material noise and vibration control is rich with damping (energy absorption) in the soil. Of empirical and quantitative studies conducted these two wave forms, the shear wave is the in North America, Europe, Australia, the Far most important. For surface track, the ground East, and South America. A substantial vibration includes Rayleigh surface waves, review of the state-of-the-art in groundborne which attenuate at a rate of 3 dB (30% in noise and vibration prediction and control was amplitude) as distance from the source conducted in 1984 for the U.S. Department of doubles without material damping or reflection Transportation. Recent research includes losses. Rayleigh surface waves are the major studies on the nature of subway/soil carrier of vibration energy from the surface interaction, surface track vibration generation, track, but inhomogeneities in the soil may and extensive downhole testing to assess convert significant portions of the Rayleigh vibration propagation in soils. Indeed, the surface wave energy into body waves. Within

9-21 Light Rail Track Design Handbook

one wavelength of the track, the distinction both analytical and finite element modeling between surface and body waves is methods, and multiple-degree-of-freedom immaterial, as near-field effects dominate the modeling of transit vehicles and track.n61 response. These methods are very powerful for analyzing changes in structure design, Structure/soil interaction significantly affects structure depth, and vehicle designs. the radiation of vibration energy into the surrounding soil. Heavy tunnel structures produce lower levels of ground vibration than 9.2.5.3 Vibration Control Provisions lightweight tunnels. However, the opposite Numerous methods for controlling has been observed for large cut-and-cover groundborne noise and vibration include box structures very close to the ground continuous floating slab track, resiliently surface relative to circular tunnels. Near- supported two-block ties, ballast mats, surface subway structures produce vibration resilient direct fixation fasteners, precision rail, more easily than deep structures. alignment modification, low stiffness vehicle primary suspension systems, and Ground vibration excites building foundations transmission path modification.n71 Achieving and structures. Vibrating surfaces of the the most practical solution at reasonable cost rooms then radiate noise into the room as is of great importance in vibration mitigation groundborne noise. The interior sound level design. Factors to consider include is then controlled by the degree of acoustical maintainability, inspectability, and cleanliness. absorption contained in the room. Secondary noise, such as rattling windows, might be observed in extreme cases. 9.2.5.3.1 Floating Slab Track Floating slab track is a special type of track structure that is beyond the normal designs 9.2.5.2 Groundborne Noise and Vibration discussed in Chapter 4. The floating slab Prediction concept would be an additional requirement to The procedure for predicting groundborne normal track structure. Track structure design noise and vibration is an empirical approach must allow for floating slabs where they are involving transfer function testing of soils and needed, as the floating slab may require buildings. The procedure has recently been additional invert depth. adopted by the FTA for use in assessing groundborne noise and vibration impacts by Floating slab systems consist of two basic rail transit projects. The predictions of ground types: vibration and groundborne noise are l Continuous cast-in-place floating slabs described in detail in the FTA guidelines for are constructed by placing a permanent rail transit noise and vibration impact sheet metal form on elastomer isolators assessment.[141 Screening procedures and and filling the form with concrete. The detailed prediction techniques are also floating slabs measure approximately 6 described. meters (20 feet) along the track and 3 meters (10 feet) transverse to the track. The state-of-the-art in predicting ground The depth of the slab is generally 300 to vibration has recently advanced significantly 450 millimeters (12 to 18 inches). to include detailed finite element modeling of l Discontinuous double-tie pre-cast floating soil/structure interactiorP1, numerical analysis slabs measure about 1.5 meters (5 feet) of vibration propagation in layered soils using

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along the track and 3 meters (10 feet) The main support pads of all discontinuous transverse to the track. The depth, and floating slabs used in the United States are thus the mass, of the slab may vary from manufactured from natural rubber. Synthetic about 200 to 600 millimeters (8 to 24 rubber formulations exhibit higher creep rates inches). The mass of the slab may range than natural rubber formulations and should from 2,000 to 7,000 kilograms (4,409 to be avoided. Natural rubber formulations 15,430 pounds.) The most common exhibit low creep over time, high reliability, configuration is with a 2,000-kilogram and dimensional stability. Natural rubber (4,409-pound) slab 200 millimeters (8 pads are not subject to corrosion and provide inches) thick. The slabs are referred to as natural material damping that controls the double ties because they support each amplification of vibration at resonance. rail with two direct fixation fasteners, Natural rubber pads installed beneath floating giving a total of four direct fixation slabs have survived subway fires without fasteners per slab. needing to be replaced and their use results in a virtually maintenance-free isolation system. The design resonance frequency of a floating There have been concerns over debris slab system is the resonance frequency for accumulating beneath floating slabs, as well the combined floating slab and vehicle truck as providing methods for removal of such mass distributed over the length of the debris. Another concern is the possibility of vehicle. The design resonance frequency of the gaps between discontinuous floating the continuous floating slab and vehicle slabs, which could trap the feet of persons combination is typically on the order of 16 Hz, escaping down a tunnel during an emergency. while that of the discontinuous precast Both of these concerns may be avoided by double-tie floating slab and vehicle providing flexible seals, but care must be combination ranges from 8 to 16 Hz, taken to avoid increasing the overall stiffness depending on isolation needs. Wtth a of the floating slabs by using the seals. continuous floating slab, the entrained air stiffness must be included with the isolator spring stiffness when computing the 9.2.5.3.2 Resiliently Supported Two-Block Ties resonance frequency. Resiliently supported two-block tie designs The normal configuration for the discrete are referred to as encased direct fixation track double-tie design includes four natural rubber in Section 4.5.3.4. In resiliently supported isolators. Additional isolators are incorporated two-block tie designs, each rail is supported to increase the isolation stiffness at transition on individual concrete blocks set in an regions between non-isolated and isolated elastomer boot encased by the concrete slab track. The main support pad shape was or invert. A stiff elastomer or plastic rail seat selected to provide low shear strain and pad protects the concrete block at the rail control lateral slip between the bearing base, which is retained by a spring clip or surface of the pad and concrete surfaces. other fastening system. The design used for Lateral slip is further reduced by gluing the light rail transit vibration isolation must provide pads to the concrete surfaces. The pad is a low rail support modulus, achieved by about 100 millimeters (4 inches) thick, with an including a closed-cell elastomer foam (or overall diameter of 400 millimeters (16 micro-cellular pad) between the bottom of the inches). concrete block and invert inside the elastomer boot. Static stiffnesses of the order of 8.9 to

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17.8 MN/meter (50,000 to 100,000 reduction relative to standard ballasted track. pounds/inch) can be obtained, though the The ballast mat is, therefore, not a substitute dynamic stiffness is likely to be much higher. for floating slab track. There may be some The design constitutes a two-degree-of- amplification of vibration at the ballast mat freedom vibration isolation system, though the resonance frequency in the range of 16 to 30 vibration isolation at low frequencies is Hz. controlled by the elastomer boot surrounding the concrete block. Three configurations of ballast mats have recently been recommended for surface track. The vibration isolation provided by resiliently The first includes a concrete base with a mat supported two-block ties is believed to be consisting of inverted natural rubber cone higher than that of very stiff direct fixation springs placed on a concrete base beneath fasteners. The vibration isolation provided by the ballast. The second includes the mat the two-block tie should be comparable to that placed in a concrete “bath tub” slab with the provided by soft fasteners, with stiffness of 8.9 track slab consisting of a second pour MN/meter (50,000 pounds/inch) and dynamic concrete slab supporting the rails. The third, stiffness of 11 6 MN/meter (65,000 and potentially less effective design, pounds/inch). Damping has been postulated incorporates a uniforrn ballast mat placed as a cause for the low-frequency vibration directly on tamped soil or compacted sub- isolation provided by some of the two-block ballast. systems. The two-degree-of-freedom isolation of the two-block system may provide Conventional installations of ballast mats in greater vibration isolation at frequencies Europe have been in subways with concrete above 200 Hz than that provided by soft bases, for which vibration insertion losses fasteners. have been predicted to be higher than observed in practice. Surface track There have been cases of rail corrugation application presents challenges that limit the associated with the resiliently supported tie effectiveness of ballast mat installations. The system, though this appears to be related to shear modulus of the soil at or near the the interaction of the rail with the concrete surface may be low and can result in a block through the rail seat pad. Reducing the support modulus comparable to that of the rail seat pad stiffness appears to defer the ballast mat, thus rendering the ballast mat onset of rail corrugation. less effective than if it were employed in tunnel track.

9.2.5.3.3 Ballast Mats The vibration reductions are limited to the Ballast mats are employed to control frequency range in excess of about 30 Hz. groundborne noise and vibration from For ballast mats on compacted subgrade, the ballasted track and have been incorporated as insertion loss would likely be on the order of 5 the principal isolator in certain floating slab to 8 dB at 40 Hz. For ballast mats on a track installations. The effectiveness of a concrete base or concrete invert, the insertion ballast mat is limited to frequencies above loss at 40 Hz would be between 7 and 10 dB. approximately 25 to 30 Hz. The maximum The most effective ballast mat is a profiled vibration isolation that has been measured mat with a natural rubber elastomer on a from trains with ballast mat is about 10 dB at concrete base or trough. This type of 40 Hz. At lower frequencies, the ballast mat is too stiff to provide sufficient vibration

9-24 Noise and Vibration Control

installation provides the greatest vibration fastener is now being replaced. Modern isolation, about IO dB at 40 to 50 Hz. designs include vulcanize-bonded fasteners with rolled steel top and bottom plates. More The selection of a ballast mat should favor low recent designs include cast top plates and static and dynamic stiffness, low creep, good either rolled steel or cast base plates. drainage, and ease of installation. There are considerable disparities between the dynamic Very soft fasteners provide a modest measure stiffnesses of various ballast mats, even of groundborne noise reduction. Certain though their static stiffnesses may be similar. fasteners use elastomer in shear to provide The most desirable material is natural rubber, good rail head control. Soft fasteners have which exhibits a low dynamic-to-static been designed for use in reducing ground stiffness ratio of about 1.4 or less. These vibration and groundborne noise at high-performance natural rubber mats may frequencies above about 30 Hz. The cost more than synthetic elastomer mats, but elastomers shear design provides a vertical may be the only choice in critically sensitive stiffness of about 10 MN/meter (55,000 locations. Specifications for ballast mats pounds/inch). A unique aspect of this type of should include dynamic stiffness requirements fastener is that it must pass a qualification for the intended frequency range over which test, which includes a measure of the dynamic vibration isolation is desired. If this is not stiffness over a frequency range of 10 to 500 done, much less isolation than expected may Hz. The fastener employs elastomer in shear actually be achieved, rendering the vibration and provides a reasonably high lateral isolation provision ineffective. There is a very stiffness to maintain rail position. The high distinct possibility that providing a ballast mat lateral stiffness and captive design of the top may increase low frequency vibration in the plate also help to reduce rail rotation under 16- to 25Hz region. If this is the range of the lateral load in spite of its low vertical stiffness. most significant vibration, the ballast mat may This is, perhaps, one of its most important actually create or exacerbate a vibration design features. For additional information on impact. Thus, great care must be exercised in direct fixation fasteners, refer to Section 54.3 design, specification, and installation of the in this handbook. ballast mat. One feature of a low stiffness fastener is that A further consideration is ballast pulverization the rail static deflection will be distributed over and penetration into the mat. Ballast mats more fasteners; thus the rail will appear to be have been incorporated in the track structure more uniformly supported. Low rail support to reduce pulverization. stiffness is advantageous in reducing the pinned-pinned mode resonance frequency due to discrete rail supports, as well as the 9.2.5.3.4 Resilient Direct Fixation vertical resonance frequency for the rail on Fasteners the fastener stiffness. Resilient direct fixation fasteners are used for concrete slab aerial deck or subway invert The ratio of vertical dynamic-to-static stiffness track. In some instances, resilient direct describes the quality of the elastomer; a low fixation fasteners have been incorporated into ratio is very important for vibration isolation. embedded track. One of the earliest direct The ratio is obtained by dividing the dynamic fixation fastener designs was the Toronto stiffness (measured with a servo-actuated Transit Commission unbonded fastener with hydraulic ram) by the static stiffness natural rubber pad. This relatively stiff

9-25 Light Rail Track Design Handbook determined over the majority of the load 9.2.5.3.6 Rail Straightness range. A desirable upper limit is 1.4, easily Though often overlooked or not considered obtained with fasteners manufactured with a during track design, rail straightness is natural rubber elastomer or derivative thereof. fundamentally important in controlling low Dynamic-to-static stiffness ratios of 1.3 are frequency ground vibration in critically not uncommon with natural rubber elastomer sensitive areas. Roller straightened rails have in shear. As a rule, elastomers capable of produced ground vibration frequency meeting the limit of 1.4 are high quality and components that can be related to the generally exhibit low creep. Neoprene straightener’s roller diameter. More recently, elastomers provide a dynamic-to-static substantial vibration was generated at stiffness ratio greater than 1.4 and can be as residential structures located adjacent to a high as 4. (Note: A neoprene elastomer may main line freight railroad alignment after be desirable for controlling rail noise radiated replacing “gag-press” straightened rail with from at-grade or aerial structure track due to roller straightened rail with excessive vertical the material damping of the elastomer, which undulation. Narrowband analyses of the absorbs rail vibration energy. Thus, the wayside ground vibration data identified a choice of elastomer may depend on whether linear relation between frequency peaks and groundborne vibration isolation or airborne train speed that related directly with the roller noise reduction is desired.) diameter of the straightening machine. Subsequent field measurements of rail profile High lateral restraint is often incompatible with with a laser interferometer corroborated the vibration isolation design requirements. vibration data. The roller straightened rail was Therefore, a stiffness range is desirable for replaced with new rail that was also roller the lateral restraint to ensure both an straightened, but to British standards. Repeat adequate degree of horizontal position control measurements indicated a substantial and sufficient lateral compliance to provide reduction of ground vibration, even though the vibration isolation. Hard snubbers are effects of the roller straightener pitch diameter undesirable in fasteners, because they limit were still identifiable in the wayside ground vibration isolation to the vertical direction only. vibration spectra.[‘8*‘g1 The design principle is to provide a three- degree-of-freedom isolation. This experience leads to the recommendation of “super-straight” rail for sensitive areas where a low-frequency vibration impact is 9.2.5.3.5 Rail Grinding predicted and unwanted. Examples include Rail grinding to eliminate checks, spalls, and alignments in very close proximity to sensitive undulation of the rail head reduces receivers of all types in areas with very soft groundborne noise and vibration, provided soil. Controlling low-frequency vibration due that the vehicle wheels are new or recently to rail undulation by controlling rail trued. This applies especially to corrugated straightness is far less costly than the rail track. Rail grinding to reduce ground installation of a floating slab track structure. vibration at low frequencies must remove long Soft fasteners would provide no positive wavelength roughness and corrugation, which benefit, and may even exacerbate low- may require special grinders with long frequency vibration. Corrective rail grinding is grinding bars or special controls. incapable of removing rail height undulation over long wavelengths of 2 meters (6 feet) or more. U.S. steel suppliers have not produced

9-26 Noise and Vibration Control rail with an adequate straightness 9.2.5.3.8 Resilient Wheels and Rail Head specification. However, such rail is available Ball Radius from European manufacturers, where high- Resilient wheels may provide some degree of speed rail systems require strict adherence to vibration isolation above 20 to 50 Hz, straightness limits. depending on elastomer stiffness. However, light rail systems have experienced substantial ground vibration from urethane 9.2.5.3.7 Vehicle Primary Suspension embedded track due to corrugation with Design vehicles using resilient wheels mounted on Vehicle primary suspension design is not part mono-motor trucks. Numerical modeling of track design, but has a direct bearing on suggested that a vertical resonance exists in wayside ground vibration amplitudes. the wheel and track system at a frequency Selection of trackwork vibration isolation coincident with the corrugation frequency. provisions should ideally be based on the type Other factors are likely relevant. More of vehicle involved. In general, vehicles with research is required to further define the soft primary suspensions produce lower levels cause of this type of corrugation and of vibration than vehicles with stiff determine which, if any, track design suspensions. Differences in suspension parameters may influence its generation. characteristics may be sufficient to eliminate the need for floating slab isolation at otherwise critically sensitive locations. 9.2.5.3.9 Subgrade Treatment Introduction of vehicles with stiff primary The vibration amplitude response of soil is, suspensions relative to existing vehicles with roughly, inversely proportional to the stiffness soft suspensions may introduce vibrations in of the soil. Therefore, stiff soils tend to vibrate the lo- to 25Hz frequency region. less than soft soils. Grouting of soils or soil Unfortunately, the track design is often stabilization with lime or cement is attractive blamed. where very soft soils are encountered, such as soft clays or sands. Unfortunately, large The selection of chevron-type suspension volumes of soil would have to be treated; this systems in lieu of stiff rubber journal bushing would probably not be attractive for vibration suspension systems may provide sufficient control unless such treatment were necessary vibration reduction to reduce the need for for structural support. Test data have not other vibration isolation provisions in the been developed for predicting the frequency range of about 16 to 31.5 Hz. Most performance of soil cement or lime modern light rail transit vehicles in the U.S. stabilization of track subgrades. Grouting is incorporate chevron primary suspension expected to have a significant though possibly systems with low vertical stiffness, thus mixed effect on ground vibration. Grouting reducing the demand on vibration isolation should increase the efficiency of vibration elements in the track. However, a chevron propagation at high frequencies between track suspension design is no guarantee of low and building structures, but reduce the stiffness. If the vehicles have stiff primary vibration energy input into the soil at low suspension systems, particular attention frequencies. Tests at one site indicated low should be paid to low-frequency vibration levels of vibration for alluvial soils that had control in track at the primary suspension been pressure grouted to prevent building resonance frequency. settlement. Additional testing and evaluation are necessary.

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9.2.5.3.10 Special Trackwork 9.2.5.3.13 Pile-Supported Track Turnouts and crossovers are sources of Piling used to reinforce a track support system vibration. As the wheels traverse the frogs can be effective in reducing ground vibration and joints, impact forces are produced that over a broad range of frequencies. An cause vibration. Grinding the frog to maintain example would be a concrete slab track contact with a properly profiled wheel can supported by piles or ballasted track on a minimize impact forces at frogs. Spring frogs concrete trough supported by piles. and movable point frogs are designed to Performance improvement is likely to be maintain a continuous running surface. substantial if the piles can be extended to rock Spring frogs are practical for low speed layers within about 20 meters (65 feet). turnouts, while movable point frogs are more Standing wave resonances may occur in long suited to high-speed turnouts. Refer to piles, so that there is a limit on the Chapter 6 for additional discussion on frog effectiveness of piles in controlling audible types. groundborne noise. Unfortunately, piles may interfere with utilities and the cost of piling is substantial. Piling may be attractive for civil 9.2.5.3.11 Distance reasons, however, and the added benefits of The track should be located as far from vibration control can be realized with sensitive structures as possible within a right- appropriate attention directed to design. of-way. Where wide rights-of-way exist, there may be some latitude in locating the track. A shift of as little as 3 meters (10 feet) away 9.3 REFERENCES from a sensitive structure may produce a beneficial reduction of vibration. Avoid [ I] Journal of Sound and Vibration, locating track close to sensitive structures Academic Press, Ltd., Published by where sufficient right-of-way width exists to Harcourt Brace Jovanovich, London alter the alignment. [2] Nelson, J. T., Wheel/Rail Noise Control Manual, TCRP Report 23, Wilson, 9.2.5.3.12 Trenching and Barriers lhrig & Associates, Inc., for TRB Open trenches have been considered for National Research Council. vibration reduction, but are of limited effectiveness below 30 Hz for a depth of 7 [3] Nelson, J. T., H. J. Saurenman, G. P. meters (20 feet) and even less for shallower Wilson, State-of-the-Art Review: trenches. At higher frequencies, the vibration Prediction and Control of Ground- reduction of a trench filled with Styrofoam may Borne Noise and Vibration from Rail be as little as 3 to 6 dB. Concrete barriers Transit Trains, Final Report, Wilson, embedded in the soil have also been lhrig 8 Associates, Inc., for US considered. While they may interrupt surface Department of Transportation, Urban wave propagation, their mass must be Mass Transit Administration, UMTA- substantial to provide sufficient vibration MA-06-0049-83-4. reduction. Detailed finite element modeling is necessary in this case to predict performance.

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[4] Saurenman, H. J., G. P. W&on, J. T. [I I] Remington, P. J., Wheel/Rail Roiling Nelson, Handbook of Urban Rail Noise, What Do We Know, What Noise and Vibration Control, Wilson, Don’t We Know, Where do We Go lhrig & Associates, inc., for from Here, Journal of Sound and USDOT/TSC, 1982, UMTA-MA-06 Vibration, Vol 120, No. 2, ~~203-226. 0099-82-I. [12] Ver, I. L., C. S Ventres, and M M. Miles, [5] Transit Noise and Vibration impact Wheel/Rail Noise - Part ill: impact Assessment, for the U.S. Noise Generation by Wheel and Rail Department of Transportation, Discontinuities, Journal of Sound Federal Transit Administration, April and Vibration, Vol 46, No. 3, 1976, 1995. pp 395-417.

[6] IS0 2631, Mechanical Vibration and [I 31 Rudd, M J., Wheel/Rail Noise, -Part Ii: Shock-Evaluation of Human Wheel Squeal, Journal of Sound and Exposure to Whole-Body Vibration, Vibration, 46(3),1976. pg385 2”d Ed., International Organization for Standardization (ISO), 1997 [14] Transit Noise and Vibration impact Assessment, Harris, Miller, Miller & [7] ANSI S3.29-1983, Guide to the Hanson, Inc., for the Federal Transit Evaluation of Human Exposure to Administration, U.S. Department of Vibration in Buildings, American Transportation, Washington, DC. National Standards Institute, 1983 April 1995 DOT-T-95-16. (See subsequent revisions) [15] Crockett, A. R., and R. A. Carman, Finite [8] ANSI S3.29-1983, Guide to the Element Analysis of Vibration Levels Evaluation of Human Exposure to in Layered Soils Adjacent to Vibration in Buildings, American Proposed Transit Tunnel Alignments, National Standards Institute, 4 April Proceedings of Internoise 97, 1983. (This standard has been Budapest, Hungary, 25-27 August recently revised.) 1997. Institute of Noise Control Engineering. [9] Remington, P. J., The Estimation and Control of Rolling Noise Due to [16] Nelson, J. T., Prediction of Ground Roughness, BBN Report No. 8801, Vibration Using Seismic Reflectivity for ERRI Committee C 163,1994. Mefhods for a Porous Soil, Proceedings of the IWRN 1998 [IO] Kalousek, J., and K. L. Johnson, An Conference, Isle de Embiez, investigation of Short Pitch Wheel November 1998. and Rail Corrugation on the Vancouver Skytrain Mass Transit [17] Nelson, J. T., Recent Developments in System, Proc. Institute Mechanical Ground-Borne Noise and Vibration Engineers, Part F, Vol. 206 (F2), Control, Journal of Sound and 1992, pp. 127-135. Vibration, 193(l), pp.367-376, (1996)

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1181 Nelson, J. T., and S. L. Wolfe, Kamloops [19] Nelson, J. T., Recent Developmen& in Railroad Ground Vibration Data Ground-Borne Noise and Vibration Analysis and Recommendations for Control, Journal of Sound and Control, Technical Report, Wilson, Vibration, 193(l), 1996, p.373. lhrig & Associates, Inc., for CN Rail.

9-30 Chapter I O-Transit Signal Work Table of Contents

10.1 TRANSIT SIGNAL 10-I 10.1.1 General 1 o-1 10.1.2 Transit Signal System Design 1 o-2 10.2 SIGNAL EQUIPMENT 1 o-2 10.2.1 Switch Machines 1 o-2 10.2.1 .I General 10-2 10.2.1.2 Trackwork Requirements 1 o-2 10.2.1.3 Types of Switch Machines 1 o-3 10.2.1.3.1 Electric 1 o-3 10.2.1.3.2 Electra-pneumatic I o-3 10.2.1.3.3 Hand-Operated 1 o-3 10.2.1.3.4 Yard 1 o-3 10.2.1.3.5 Embedded (Surface) IO-4 10.2.2 Impedance Bonds 10-4 10.2.2.1 General 10-4 10.2.2.2 Trackwork Requirements 10-4 10.2.2.3 Types of Impedance Bonds 10-4 10.2.3.3.1 Audio Frequency 10-4 10.2.3.3.2 Power Frequency 10-5 10.2.3 Loops and Transponders IO-5 10.2.3.1 General 1 o-5 10.2.3.2 Trackwork Requirements 1 o-5 10.2.3.3 Types of Loops and Transponders IO-5 10.2.3.3.1 Speed Command 1 o-5 10.2.3.3.2 Train Location 1 o-5 10.2.3.3.3 Traffic interface 1 o-5 10.2.3.3.4 Continuous Train Control Loop 1 o-5 10.2.3.3.5 Transponders 1 O-6 10.2.4 Wheel Detectors/Axle Counters IO-6 10.2.4.1 General IO-6 10.2.4.2 Trackwork Requirements 1 O-6 10.2.4.3 Types of Wheel Detectors/Axle Counters 10-6 10.25 Train Stops 10-6 10.251 General IO-6 10.2.5.2 Trackwork Requirements 10-6 10.2.5.3 Types Of Train Stops 1 o-7 10.2.5.3.1 Inductive 1 o-7 10.2.5.3.2 Electric IO-7 10.2.6 Switch Circuit Controller/Electric Lock 1 o-7 10.2.6.1 General 1 o-7 10.2.6.2 Trackwork Requirements 1 o-7 10.2.6.3 Types of Switch Circuit Controller/Electric Lock 1 o-7

1 O-i Light Rail Track Design Handbook

10.2.6.3.1 Switch Circuit Controller 1 o-7 10.2.6.3.2 Electric Lock 1 o-7 10.2.7 Signals 1 O-8 10.2.7.1 General 1 O-8 10.2.7.2 Trackwork Requirements 1 O-8 10.2.7.3 Types of Signals -lO-8 10 2.8 Bootleg Risers/Junction Boxes 1 O-8 10.2.8.1 General 1 O-8 10.2.8.2 Trackwork Requirements 1 O-8 10.2.8.3 Types of Bootleg Risers/ Junction Boxes 1 o-9 10.2.8.3.1 Junction Boxes 1 o-9 10.2.8.3.2 Bootleg Risers 1 o-9 10.2.9 Switch and Train Stop Heaters/Snow Melters IO-9 10.2.9.1 General 1 o-9 10.2.9.2 Trackwork Requirements 1 o-9 10.2.9.3 Types of Switch/Train Stop Snow Melters IO-9 10.2.10 Highway Crossing Warning Systems IO-10 10.2.10.1 General 10-10 10.2.10.2 Trackwork Requirements 10-10 10.2.10.3 Types of Highway Crossing Warning System IO-10 10.2.11 Signal and Power Bonding IO-10 10.2.11 .I General IO-IO 10.2.11.2 Trackwork Requirements 10-11 10.2.11.3 Types of Signal and Power Bonding IO-11 10.3 EXTERNAL WIRE AND CABLE IO-12 10.3.1 General IO-12 10.3.2 Trackwork Requirement 10-12 10.3.3 Types of External Wire and Cable Installations 10-12 10.3.3.1 Cable Trough 10-12 10.3.3.2 Duct Bank 10-12 10.3.3.3 Conduit 10-13 10.3.3.4 Direct Burial 10-13

10.4 SIGNAL INTERFACE IO-13 10.4.1 Signal-Trackwork Interface IO-13 10.4.2 Signal-Station Interface 1 o-14 10.4.3 Signal-Turnout/Interlocking Interface 10-14

10.5 CORROSION CONTROL IO-14

10.6 SIGNAL TESTS IO-15 10.6.1 Switch Machine Wring and Adjustment Tests IO-15 10.6.2 Switch Machine Appurtenance Test IO-15 10.6.3 Insulated Joint Test 10-15 10.6.4 Impedance Bonding Resistance Test 10-15 10.65 Negative Return Bonding Test IO-15

10.7 SUMMARY IO-15

1 O-ii CHAPTER IO-TRANSIT SIGNAL WORK

10.1 TRANSIT SIGNAL detectors, induction couplers, or other non-vital devices to improve speed by 10.1 .I General eliminating intersection delays

Street-running light rail systems can be Power operation of track switch facing operated without signals only at low speeds. points: power on/off switches, time Train operators must obey the local traffic sequences, induction couplers, or other laws and yield the right-of-way (ROW) to non-vital devices are used to improve traffic on the tracks. In higher speed LRV speed by eliminating stops to throw operations on exclusive rights-of-way, trains switches, thereby allowing trains to keep use signal systems to avoid collisions with moving other trains and with street vehicles crossing Block supervision (single track, low-speed the tracks. operation): similar to preemptive devices, allows an opposing train to advance The principles of light rail transit signaling are without incurring schedule delay if similar to railroad main line signaling in possible providing for the safe movement of trains. The track is divided into segments called Block and switch protection: basic blocks. Signals keep two trains from railroad signaling technology employing occupying the same block at the same time wayside signals, sometimes in and generally keep an empty block between conjunction with mechanical or inductive trains that are travelling at the posted speed. train stops, to provide safe operation Track circuits detect trains in a block. Block (newer light transit systems have systems ensure train separation with safe employed cab signals with or without train stopping distance. Interlocked switches and stops for continuous speed control) crossovers protect against conflicting routes Grade crossing warning: based on and improper switch operation. Transit railroad signaling technology, gates and signaling also provides block supervision as flashers eliminate slow downs to required for street operation, warning of determine if grade crossings are clear; approaching trains at grade crossings and generally recognized as the most effective supervising coordination with proximate type of crossing warning system, allowing vehicle traffic schemes as required for system improved LRV operating speed performance and safety. The choice of which system is most Typically, there are six light rail transit appropriate for a specific section of track is signaling designs: based on operational and political l No signaling at all: the system operates considerations. A light rail system may utilize with fixed-guideway vehicles in a free- different signal technologies at different wheeled community with no resultant locations based on these concerns. A street- speed advantage over bus operation running operation with slow speed requires different controls than a high-speed operation l No signaling except to provide preferential access over cross traffic: the LRV uses on an exclusive ROW. signal preemption devices such as overhead wire contractors, wheel

1 o-1 Light Rail Track Design Handbook

The appropriate level of signal automation 10.2 SIGNAL EQUIPMENT varies by transit systems. The optimum cost/benefit ratio depends on local 10.2.1 Switch Machines circumstances and is determined by the authority responsible for providing the service. 10.2.1 .I General These various types of signaling have little Track switches can be operated by hand or by impact on the track designer, but the power. When time and convenience are interfaces are important. Design differences important, automated switch machines are in light rail systems are primarily related to advantageous. Switch machines may be their operating environments. controlled from a central control facility or by the vehicle operator.

10.1.2 Transit Signal System Design Switch machines are used on main lines, interlockings, and yards. Switch machines The system designer is obliged to consider can operate a switch, derail, or wheel stop. the signaling technology available to provide The type of switch machine selected is the desired system operating performance at dependent on operating parameters, the least total cost. Within the scope of light clearances, and the type of track installation- rail transit applications, a well-established timber or concrete switch ties or direct fixation catalogue of proven technology is available. track.

Transit signal system design must consider not only what technology is available, but also 10.2.1.2 Trackwork Requirements the most rational combination of equipment for Switch machines in ballasted track rest on a particular application. Signal systems are headblock switch ties and interface with customized or specified by each transit turnouts through operating and switch rods. system to provide safe operation at an This interface is often complicated, particularly enhanced speed. The location of signal block in direct fixation (DF) or embedded track, boundaries is based on headway where blockouts in the concrete must be requirements and other considerations such provided for proper clearance. The following as locations of station stops, highway elements associated with track and structure crossings, and special interlocking operating design should be considered when designing requirements. turnout switch machines:

l Size of turnout or crossover

Selection and spacing of track circuits for ac l Number of head ties and dc propulsion systems are influenced by l Size, height, width, and length of head tie many factors. These include: the degree of l Type of number one rod-vertical or defective insulated rail joint detection or horizontal broken rail protection required, the likelihood l Thickness of number one rod of stray current, the frequency of interfering l Type of basket on number one rod sources of power (propulsion and cab l Distance from centerline of switch signaling), and the inherent advantages of machine to gauge line of the nearest rail various types of track circuits. l Types of tie plate for number one and two ties

1 o-2 Transit Signal Work

Tie or mounting spacing between switch switch machines are usually installed adjacent machine rods to the normally closed point of the switch. Type of derail or wheel stop Location of mounting of switch machine to 70.2.7.3.2 Nectro-pneumatic ties or surface Electra-pneumatic switch machines require a insulation of trackwork switch, basket, and tie plate reliable source of compressed air. While this Distance to throw of the switch machine is economical for heavy rail transit, which features short block lengths and frequent Location of extension plate mounting interlockings, the economics on light rail lines holes and interface plate usually make air power switches too costly. Lubrication of switch plate and track layout 10.2.1.3.3 Hand-Operated Hand-operated switch machines are typically 10.2.1.3 Types of Switch Machines used where facing-point lock protection is 10.2.1.3.1 Electric required to help safeguard the movement of Electric switch machines are common for light high-speed main line traffic over a switch. rail operations because of the ready These switch machines contain a locking bar availability of electric power throughout the that, with the switch in the normal position, system. Electric switch machines are rugged, enters a notch in the lock rod. This reliable units designed for any installation arrangement locks the switch points in their where electric power is available. Electric normal position to provide facing-point lock switch machines may be used in main line, protection. interlocking, and yard service. For installations in which extra vertical clearance 10.2.1.3.4 Yard is needed for a third-rail shoe, a low-profile Yard electric switch machines are simple and electric switch machine can be used. Electric compact machines designed for transit yard switch machines are available in a variety of application. For installation in tight spaces, operating speeds and motor voltages. the low-profile yard electric switch is available with external switch indicator lights. Unlike Switch machines are usually specified to meet many main line switch machines, some yard the requirements of AAR Load Curve 14511, electric switch machines can handle trailing providing ample thrust to operate the heaviest moves at maximum yard speeds up to 32.2 of switches. Electric switch machines are kilometers per hour (20 miles per hour). The normally provided with one throw rod, one yard switch machine can be used in either lock rod and one point detector rod connected horizontal or vertical No. 1 rod switch layouts. to the rails. They are also available with two If point detection is required, an additional lock rods and two detector rods. The track circuit controller can be installed. Built to fit designer and signal designer must coordinate practically any yard switch, this machine can to ensure the specifications provide these be adjusted for throw, from 114 millimeters critical elements. Gauge plate extensions can (4.5 inches) up to a full 140 millimeters (5.5 be supplied that attach the switch machine to inches). the track switch to aid in holding the adjustments of the switch machine. Electric

1 o-3 Light Rail Track Design Handbook

Electra-pneumatic switch machines are also flow around the insulated joints while inhibiting available for yard application, and a the flow of signal current between adjacent compressed air plant at the yard or track circuits. Audio frequency track circuits maintenance facility may make them are separated from each other by using a economical. different frequency in each circuit; as such they do not normally require insulated joints to isolate the track circuits. Insulated joints are 10.2.1.3.5 Embedded (S&ace) used with audio frequency track circuits when Embedded (surface) switch machines are a true definition is needed, such as at signal designed to throw all tongue and mate, locations. The stagger between insulated double-tongue, or flexible switches with a joints should be 610 millimeters (2 feet) or maximum switch throw of 114 millimeters (4.5 less for transit signaling to reduce the amount inches). The embedded switch machine is of cable needed as well as the unbalance in installed between the rails (preferred) or on the current in the rails associated with the outside of the switch tongue on a paved impedance bonds. street. Embedded switch machines can be powered from available 600 to 750 Vdc or from an ac source through a transformer and 10.2.2.2 Trackwork Requirements bridge rectifier unit. The switch tongue can be The following elements associated with track trailed without damage to the embedded and structure design should be considered switch machine and can be thrown manually when designing impedance bonds: in an emergency. l Tie spacing for signal equipment

l Location of tie or direct fixation mounting Drainage of switches and switch machines is holes for signal equipment critical. The embedded switch machine track l Location of impedance bond, either box should be drained to a nearby storm pipe, between or outside the rails because an undrained box collects a mixture l Location of guard and restraining rails of sand, water, salt, etc., that increases wear l Location and spacing of insulated joints on moving parts and prevents their proper l Space for cables and conduit to pass lubrication. Normally a copper bond wire is beneath the rail installed between the box and rail to complete l Conduit and cable location for signal the circuit. This can be omitted if the power equipment source is a rectifier. Where circuit controllers are used, either one or two conduits are required to accommodate the cables. A 10.2.2.3 Types of Impedance Bonds cleanout box is installed to provide access to connecting rod adjusting nuts if they extend 10.2.3.3.1 Audio Frequency beyond the switch. Audio frequency impedance bonds are designed to terminate each end of audio frequency track circuits in transit installations. 10.2.2 Impedance Bonds The impedance bonds provide: 10.2.2.1 General 0 Low resistance for equalizing propulsion Impedance bonds are necessary when current in the rails

insulated track joints are used to electrically l Means of cross bonding between tracks isolate track circuits from each other. The 0 Connection for negative return impedance bonds permit propulsion current to

IO-4 Transit Signal Work

l Means of coupling the track circuit l Attaching the loop or transponder to the transmitter and receiver to the rails rail l Means of coupling cab signal energy to l Tie spacing and mounting method for loop the rails or transponder l Means of inhibiting the transmission of l Cable and conduit location for signal other frequencies along the rail equipment

l Block out area for loop or transponder and junction box 10.2.3.3.2 Power Frequency Power frequency bonds are designed for use in ac or dc propulsion systems that use 10.2.3.3 Types of Loops and Transponders insulated joints to isolate track circuit signaling current from signaling currents of adjacent 10.2.3.3.1 Speed Command circuits, but permit propulsion current to flow Speed command loops are used to provide a around the joints to or from adjacent track means for coupling cab signal energy to the circuits. AC impedance bonds are usually rails. Typically, speed command inductive rated for 300 amps per rail and dc impedance loops are installed with or without rubber bonds are usually rated for between 1,000 hoses within the turnout diverging track. They and 2,500 amps per rail. Typically, power may be attached to the tie or concrete, or frequency impedance bonds are installed in clipped to the rail. The rubber hose with wire pairs at insulated joint locations and mounted inside is installed near the inside of the rail at between the rails across two adjacent ties. interlockings and turnout switches. These loops provide isolation from the track circuits.

10.2.3 Loops and Transponders 10.2.3.3.2 Train Location 10.2.3.1 General Train location loops are designed to provide Loops and transponders are used to transmit more precise definition of a train’s location information to the train independent of track and two-way train/wayside communication. A circuits. They may be found in all types of wire loop installed between the rails and on trackwork and can be used for intermittent ties links the train to the rails. The horizontal transmission or continuous control systems. loop of the wire is directly mounted or placed In determining the type or location of loops or in a heavy polyvinyl chloride (PVC), epoxy, or transponders to be used for a light rail transit fiberglass (FRE) conduit that may also be system, consideration should be given to the encased in pavement. operation plan, type of track circuits, propulsion system, and train control system 10.2.3.3.3 Traffic Interface that is installed. Loops or transponders can be used to pre- empt traffic signals or provide phasing 10.2.3.2 Trackwork Requirements command and release of traffic control The following elements associated with track devices. and structure design should be considered when designing loops or transponders: 10.2.3.3.4 Continuous Train Control Loop l Location of loop or transponder inside or Typically loops between stations are outside the rail transposed at regular intervals. This provides a signal to the on-board equipment that can

1 o-5 Light Rail Track Design Handbook be used to recalibrate an on-board odometer. 10.2.4.3 Types of Wheel Detectors/Axle In station areas, short loops may be provided Counters for accurate station stopping purposes. The wheel detector/axle counter unit consists of a detector head or mechanical detector arm, mounting hardware, logic board, and 10.2.3.3.5 Transponders interconnecting cabling. Wheel detectors and Transponders are designed to transfer data to axle counters are mounted with clamps that wayside equipment. the vehicle or attach to the base of the rail or are bolted Transponders or antennae may be mounted directly to the web. The wheel detectors/axle overhead, on the wayside, or embedded counters are activated when a vehicle passes. between the rails. The magnetic wheel detector/axle counter is independent of the wheel load and subjected 10.2.4 Wheel Detectors/Axle Counters to almost no wear, since there is no mechanical interaction between the detector 10.2.4.1 General and vehicle wheels. Wheel detectors and axle counters are used to detect trains without relying on a track 10.2.5 Train Stops circuit. Since they do not require insulated joints, they cause less interference with 10.2.5.1 General traction return current than detection devices Train stops trip the train’s braking mechanism that depend on electrical signals in the rails. if a restrictive cab signal aspect or signal is When used without track circuits or cab ignored. They can be inductive units or signaling within the rails, they eliminate the electrically-driven mechanical units. In However, need for insulating switch rods. designing train stops, consideration should be they are unable to detect broken rails. In given to the location of vehicle equipment, selecting the type and model of wheel type of trackbed, operation (directional or bi- detector/axle counter, consideration should be directional), relationship to wayside signal given to the operation and mounting method layouts, and location of the train stop used. elements. Train stops are used in exclusive ROWS and are not conducive to street- 10.2.4.2 Trackwork Requirements running applications. The following elements associated with track and structure design should be considered 10.252 Trackwork Requirements when designing wheel detectors/axle The following elements associated with track counters: and structure design should be considered l Type and size of rail when designing train stops: l Mounting hole size l Type of track-ballasted, direct fixation, or l Conduit and cable location dual block l Rail grinding l Tie spacing l Maintenance l Type of tie-timber or concrete . Block out requirements or box l Location of train stop requirement l Conduit and cable location

l Relationship to signals, insulated joints, and impedance bonds

1 O-6 Transit Signal Work

10.253 Types Of Train Stops protects vehicles by ensuring that the switch points are closed. 10.2.5.3.1 Inductive Inductive train stops are designed with a magnetic system that interacts with carborne 10.2.6.2 Trackwork Requirements vehicle control equipment. Both the vehicle The following elements associated with track magnet and the track magnet need to be and structure design should be considered strategically mounted on the vehicle and track, when designing wheel switch circuit respectively. controller/electric locks: Type of track bed-ballasted, direct fixation, or dual block 10.2.5.3.2 Electric Type of tie-timber or concrete The key component of the electric train stop is Length of tie the driving arm, which is pulled to the clear Left or right hand layout position 12 millimeters (0.5 inches) below the Type of hand-operated switch machine or top of the running rail by the electric motor derail and returned to its tripping position by a Number and location of connection lugs spring. Electric train stops are usually on derail mounted on plates midway between two rails. Location of conduit and cable

10.2.6 Switch Circuit Controller/Electric 10.2.6.3 Types of Switch Circuit Lock Controller/Electric Lock

10.2.6.1 General 10.2.6.3.1 Switch Circuit Controller A switch circuit controller is a mechanism that A switch circuit controller is a ruggedly provides an open or closed circuit indication constructed unit commonly used with switches for a two-position track appliance, such as a to detect the position of switch point rails. The switch point. A mechanical linkage to the switch circuit controller has a low clearance crank arm of the controller actuates its profile and is mounted on a single tie. normal/reverse contacts. The switch circuit controller provides break-before-make contacts that allow separate adjustments at 10.2.6.3.2 Electric Lock each end of the stroke. Commonly used to An electric switch lock operates by a means of detect switch positions, the switch circuit a plunger that is lowered into a hole in the lock controller can be used to detect positions of rod connected to switch points, derails, or derails, bridge locks, slide detectors, and other devices. Some electric switch locks are tunnel doors. They can shunt track circuits as designed for low-profile application to locking well as control relay circuits. Electric switch levers located between the rails at the middle locks prevent unauthorized operation of switch of hand-operated crossovers, where stands, hand-throw switch machines, derails, clearance is limited to 280 millimeters (11 and other devices. In determining the rods inches). Another electric switch lock secures and type of switch circuit controller/electric the hand-throw lever on a switch stand or locks, consideration should be given to switch machine in the normal position. operation, type of switch or derail, mounting, and clearances. The switch circuit also

1 o-7 Light Rail Track Design Handbook

10.2.7 Signals is typically used in subway installations where space is limited, while a 162-millimeter (6.4- 70.2.7.1 General inch) signal is used for outdoor service. Wayside track signals are usually light fixtures Transit signals are supplied with brackets for mounted on poles or at ground level (dwarf mounting on subway walls, ceilings, or poles. signals) next to switches. One installation even uses airport runway lights mounted Signals are normally installed on the train between the rails. Several variations of color- operators side of the tracks with adequate light signals with various indications are horizontal/vehicle clearance from gauge of currently in use on light rail systems. In rail. Where insulated joints are used, the determining the type and configuration of signal is typically located between the two wayside signals to be used, consideration insulated joints in double-rail territory. The should be given to operation, clearances, signal can be moved ahead of the insulated signal layout, track layout, right-of-way, and joints to a distance no greater than the insulated joint locations. overhang of the vehicle.

10.2.7.2 Trackwork Requirements 10.2.8 Bootleg Risers/Junction Boxes The following elements associated with track and structure design should be considered 10.2.8.1 General when designing signal mast installations: Bootleg risers/junction boxes provide a central l Insulated joint locations termination point for signal cables. Bootleg l Right-of-way clearances risers/junction boxes come in a variety of l Conduit and cable location sizes, with or without pedestals, and are l Vehicle clearances constructed of cast iron or steel. Based on l Stopping distances the application of the bootleg risers/junction boxes, the location can be in the center of tracks, outside or inside the gauge side of the 10.2.7.3 Types of Signals running rail, outside the end of tie, outside the Long-range color-light signals consist of one toe of ballast, or next to the switch machine or or more light units with a 213-millimeter (8.4- other signal appliance. In selecting the type inch) outer lens for high signals and a 162- and size of bootleg risers/junction boxes, millimeter (6.4-inch) lens for dwarf (low) consideration should be given to the type of signals. These high and dwarf signals have trackbed, cable, signal equipment, and lenses for both tangent and curved tracks. mounting method used. The dwarf signals are designed for direct mounting on a ground-level pad such as a 10.2.8.2 Trackwork Requirements concrete foundation. The main line high When designing bootleg risers/junction boxes, signals have backgrounds, hoods, pipe posts, the following elements associated with track ladders, pole mounting brackets, and and structure design should be considered: foundations. l Conduit and cable location 0 Type of trackbed-ballast, direct fixation, Transit color-light signals are compact units or dual block designed for lines where clearances are very l Tie spacing limited. A 127-millimeter @-inch) lens signal l Maintenance

1 O-8 Transit Sitmal Work

10.2.8.3 Types of Bootleg Risers/Junction l Type of rail brace with notch, if required

Boxes l Conduit and cable location

l Junction box(es) location(s) 10.2X3.1 Junction Boxes l Length of switch point Pedestal-mounted junction boxes are typically l Number of switch rods used in ballasted track at switch machines, l Type of trackbed-ballasted, direct switch circuit controllers, track circuit fixation, or dual block locations, etc. as a central termination point for underground cables. A variety of adapter plates allow the junction box to be used with 10.2.9.3 Types of Switch/Train Stop Snow air hose adapters and connectors. Melters There are several snow melter systems commonly used in the transit industry. The 70.2.8.3.2 Bootleg Risers most popular system features tubular resistor Bootleg risers are designed as a termination electric snow melters that can be installed on point between the underground cable and the either the field side or gauge side and either at track wire to the rail or signal device. They the underside of the rail head or at the base of are available with a bottom outlet, as well as the rail. For gauge side installation, holes are side and bottom cable outlets. A typical drilled in the neutral axis of the rail using a bootleg riser installation would locate the riser clearance drill for heater support clips with lo- box in the center of the track with the top millimeter (0.4-inch) bolts. For field side slightly below the top of ties. installation, snap-on clamps are used (no drilling is necessary). Tubular electric snow 10.2.9 Switch and Train Stop melters mounted on the field side and base of Heaters/Snow Melters the rail require the special trackwork rail brace to be notched for passage of the snow melter. 10.2.9.1 General Switch and train stop heating systems are The rail web heater can also be used to designed to keep rail switches, switch rods prevent switches from freezing. The rail web and tongues, and train stop arms free of ice heater is a low-density panel that spans the and snow in a predictable and reliable fashion. rail web. It consumes 20 to 40 percent less In designing the heating system, consideration power than a tubular heater installation Rail should be given to the type of power available, web heaters are interconnected to provide type of trackwork, type of track bed, operation, more heat to the point and snapped into place type of train stop, type of switch machine, and using rugged clips and a special clip tool. No mounting method used. braces need to be loosened or grooved to allow installation, which provides for easy removal in the spring prior to track 10.2.9.2 Trackwork Requirements maintenance or repair. When designing switch heaters and snow melters, the following elements associated Power is supplied to electric snow melters with track and structure design should be from the overhead catenary through a snow considered: melter control cabinet or case. 0 Size of turnout or crossover Switch rod heaters are used to melt snow and l Type of switch point -curved or straight ice away from switch rods. These switch rod l Maintenance

1 o-9 Light Rail Track Design Handbook heaters are installed in the bottom of the crib 10.2.10.2 Trackwork Requirements where the switch rods are located. They When designing highway crossing warning consist of a steel channel or panel with tubular systems, the following elements associated electric heaters or a series of heating with track and structure design should be elements attached. The tubular electric considered:

heater can be mounted on a swing bracket l Location of insulated joints (if required) that clamps to the base of the rail on the field l Location of crossing slabs side and is adjustable for all sizes of rails. l Minimum ballast resistance

l Tie spacing

Train stop mechanisms can be furnished with l Right-of-way clearance to highway hairpin-shaped heaters or heating panels. crossing equipment

l Conduit and cable location Other types of snow melting systems include: l Insulation of running rails from each other oil, natural gas, or an electric high-pressure if a track circuit is used for the warning heating unit that forces hot air throughout the system switch area via ducts and nozzles. An alternate snow blower arrangement uses ambient non-heated air to blow snow clear of 10.2.10.3 Types of Highway Crossing the switch point areas. Warning System A typical highway crossing may consist of flashing light units, gate mechanisms with 10.2.10 Highway Crossing Warning arms up to 12 meters (40 feet) long, poles, Systems foundations, cantilever assemblies, cables, case or signal houses, junction boxes, and 10.2.10.1 General track circuits with island circuits. Highway crossing warning systems provide indications to motorists that a light rail vehicle is approaching the crossing. In determining 10.2.11 Signal and Power Bonding the type and configuration of the highway crossing warning system consideration should 10.2.11 .I General be given to LRV operations, type of track Signal and power bonding is used to establish circuit, roadway layout and posted speeds, electrical continuity and conductive capacity traffic signal(s) location, right-of-way, and for traction power return and signal track clearances. The challenge of fail-safe circuits. It prevents the accumulation of static crossing protection is to protect the LRV and charges that could produce electromagnetic highway traffic without closing the crossing interference or constitute a shock hazard to gates for extended periods of time. The track maintenance personnel. It also provides federal Manual of Uniform Traffic Control a homogeneous and stable ground plane, as Devices is being updated to include well as a fault current return path. recommendations for light rail vehicle operations. Power bonding is typically installed at all non- insulated rail joints, frogs, restraining rails, Crossing gate installations should be guard rails, and special trackwork locations. interconnected with the traffic signals within Power bonding of the restraining rails requires 60 meters (200 feet) of the highway grade special attention to avoiding run around paths crossing. that can falsely energize the track circuit.

IO-10 Transit Sicmal Work

There are basically two types of rail types of bonds of the same length and connections used in the transit industry: cable stranding. Resistance will not mechanical and exothermic welding. In change throughout the life of the bond. determining the type and the amount of signal There is no corrosion between an and power bonding, consideration should be exothermic weld bond and the rail. given to type of track circuits, capacity of the Intermittent signal failures due to the traction power equipment, type of rail, vehicle varying resistance of a corroded rail joint wheels, and the amount of broken rail will be eliminated. detected. Bond losses caused by dragging equipment, reballasting, and snowplows 10.2.11.2 Trackwork Requirements are reduced. The following interface elements associated Vehicular traffic will not loosen a properly with track and structure design should be installed exothermic weld bond. considered when designing signal and power bonding: Rail head signal bonds that are applied Type and size of rail within 125 millimeters (5 inches) of the Spaces for bonding to be installed end of rail (per AAR Part 8.1.20. E.2.c) Space for signal and power bond passing provide better detection of broken rail than beneath the rail plug bonds that are applied outside of the Type of track bed-ballasted, direct splice bars. fixation, or dual block Rail web bonds from 14 to 250 square Location of rail joints, insulated or non- millimeters (0.2 to 0.4 square inches) insulated provide a convenient means of connecting Location of guard and restraining rail all cable outside the confines of the splice Signal cable connection to rail in special bar, including special trackwork. Located trackwork at the neutral axis, the connection is less susceptible to vibration fatigue and is kept 10.2.11.3 Types of Signal and Power clear of dragging equipment and Bonding maintenance machinery. Impedance bond leads are factory made to The exothermic weld process provides an system specifications and impedance bond efficient field method for any electrical type for ease of installation, eliminating a connection from signal and power to typically cumbersome field application. One ground. method of connecting cables to rails is via The exothermic weld normally outlives the plug bonds. This method involves drilling a conductor itself. hole in the rail and hammering the plug into the hole. Exothermic welding, on the other Advantages of plug bonds vs. exothermic hand, generates molten copper to create a welding for connecting signal and power solid bond between the cable and rail or bonding include: between cables. Advantages of exothermic l The rail connector clamp can connect welding vs. plug bonds for connecting signal cables from 250 to 1000 square and power bonding include: millimeters (0.4 to 1.6 square inches) to l The installation resistance of a length of the running rails. exothermic weld bond is less than other

IO-11 Light Rail Track Design Handbook

Mechanical connectors such as plug 10.3.3 Types of External Wire and Cable bonds provide a rail connection without Installations the risk of overheating the rail steel. 10.3.3.1 Cable Trough Rail connection can be easily relocated or A cable trough system is a surface trench that temporarily removed without grinding the protects and provides continuous accessibility rail or chopping the connection. to the signal cables. When installed within the Splice bar to rail web bonds may be used track gauge between two ties, care must be to detect a break in the splice bar itself. taken in track tamping. Signal cables can exit and enter the cable trough system either from Where signal bonds cannot be installed the bottom or sides. from the field side due to tight areas, such as frogs and switches, a multi-purpose The typical cable trough installation requires a bond can be used by drilling through the trench of minimum width to provide free rail web. access to both sides of the trough while maintaining 200 millimeters (8 inches) of 10.3 EXTERNAL WIRE AND CABLE ballast and sub-ballast below the trough. The maximum particle size should not exceed 19 10.3.1 General millimeters (0.75 inches). Fill material should not be placed on frozen ground and should be Various types of cable and methods of tamped. The cable trough should be placed installation are required for transit signal so that the uppermost part is 25 millimeters (1 systems. Main cables are those cables that inch) higher than the surrounding ground or run between housings or that contain ballast surface. conductors for more than one system function. Local distribution cables are those cables The cable trough should be capable of running between a housing and an individual supporting an H-20 load at any point. unit of equipment. In selecting the method of installation of external wire and cable, 10.3.3.2 Duct Bank consideration should be given to cost, The underground duct system should be maintenance, and type of right-of-way. completely encased in concrete with a minimum clearance of 50 millimeters (2 IO.32 Trackwork Requirement inches) between conduits and the outside edge and a minimum cover of 300 millimeters When determining the location of external wire (12 inches) for non-metallic conduits and 150 and cable the following should be considered: millimeters (6 inches) for rigid metal conduits. l Conduit and cable location If a non-metallic conduit is not encased in l Maintenance of trackwork concrete, allow 460 millimeters (18 inches) of l Drainage separation for signal cables carrying 0 to 600 l Locations of pull boxes, handholes, volts. For cables carrying over 600 volts, non- manholes, duct banks, etc. shielded cables should be installed in rigid l Compaction of soil and subballast metal conduits with a minimum cover of 150 l Location of cable trough millimeters (6 inches) . For cables carrying l Visual impact over 600 volts in rigid non-metallic conduits, the conduit should be encased in no less than

10-12 Transit Signal Work

75 millimeters (3 inches) of concrete, or have Where direct burial signal wires cross the 450 millimeters (18 inches) of cover if not tracks, it is beneficial to install the wiring prior encased in concrete. Cables are connected to the tracks. This improves the integrity of to the duct bank systems using handholes, the track structure, but complicates signal pull boxes, and manholes for proper pulling installation. points or cable routing. A minimum cover of 760 millimeters (30 inches) is recommended Signal cables can be plowed in at a depth of for protection (per AAR Part 10.4.40.D.2) 760 millimeters (30 inches) and 300 when signal cables pass under tracks, ballast, millimeters (12 inches) beyond the toe of sub- or a roadway. ballast. Avoiding the track ballast and sub- ballast is important to maintain the structural One of the common problems in constructing integrity of the track. light rail systems is the protection of duct banks while the track is being installed. It is important that the responsibility for the care of 10.4 SIGNAL INTERFACE duct bank risers be assigned in the contract 10.4.1 Signal-Trackwork Interface documents. Signaling and trackwork interface issues 10.3.3.3 Conduit include: Encased or direct burlal conduit should be Location of insulation joints installed as outlined above or as required by Location and mounting requirements for the National Electric Code, Article 300-5 and impedance bonds, train stops, track 1110-4(b). transformers, junction boxes, and bootleg risers

10.3.3.4 Direct Burial Physical connection of impedance bond Signal cable and wire should be buried to a track cables and track circuit wiring uniform depth where practicable, but not less Location and mounting layout of track than 760 millimeters (30 inches) below switch operating mechanisms, switch finished grade. Where signal cable and wire machine surface and subsurface areas is installed within 3 meters (10 feet) of the (ballast, direct fixation, and embedded) centerline of any track, the top of the cable should be a minimum of 760 millimeters (30 Cable and conduit requirements for inches) below the sub-ballast grade. interconnection of signal apparatus at track Signal cables and wires should be laid loosely Location and installation of train stops, in the trench on a sand bed a minimum of 100 inductive loops, transponders, wheel millimeters (4 inches) thick and covered with a detectors, and axle counters minimum of 100 millimeters (4 inches) of sand before backfilling. Backfill should be Interface pick-up with the traffic signal compacted to not less than 95% of the system maximum dry density of the respective Location of block outs for wayside signal materials as determined by AASHTO Test equipment Designation T-99 or to the original density of Electromagnetic interference/ electromag- compaction of the area, whichever is greater. netic compatibility (EMVEMC)

IO-13 Light Rail Track Design Handbook

. Track alignments with cab speeds 10.4.2 Signal-Station interface . Grounding The following signal equipment is typically . Yard signaling installed at station locations: impedance bonds, inductive loops, bootleg risers, junction . Grade crossing warning systems boxes, and transponders. If the station is . Wayside equipment housings and cases located near an interlocking or highway . Corrosion control crossing, there should be sufficient room from the end of platform to the signal equipment . Tie spacing for signal control equipment, (impedance bonds and signals) and insulated impedance bonds, train stops, and joints if required. switches . Tie size and length requirements for 10.4.3 Signal-Turnout/Interlocking switches and derails Interface . Signal cable connection to rail at special trackwork The following signal equipment is typically located at turnouts and interlockings: switch . Suitable air gap between vehicle machines; impedance bonds; inductive loops antenna/transponder and rails including speed command loops; train stops; . Physical connection of switch machines to bootleg risers; junction boxes; switch special trackwork controllers; electric locks; transponders; wire . Loop or transponder mounting on track for and cables; signal and power bonding; train-to-wayside communication cases/signal equipment houses; signals; and snow melter systems. The design of the track . Location of insulated joints circuit and fouling protection used will . Spaces for cables and conduit passing determine the location of insulated joints in the beneath the rail special trackwork. Typically in transit applications, the insulated joint should be . Location of guard and restraining rail located approximately 7 to 7.6 meters (23 to . Horizontal clearance between track and 25 feet) ahead of the switch points to allow for wayside signals and equipment the use of track. The size of the turnouts and crossovers determines the speed at which the . Vertical clearance between track and train can operate. This speed should be one signal equipment of the available cab speeds. The insulted joint . Space and drainage for switch machine in for the turnout must be located with a direct fixation or embedded track minimum of clearance taking into account the . Provision for installation of snow melters longest overhang of any equipment that may operate on the track. . Location of switch indicators for embedded track 10.5 CORROSION CONTROL . Location of cross bonding and negative return cables Leakage of stray currents into the ballast bed * Location of speed limits and earth can be a significant problem if the cables running from the rails are electrically . Ballast resistance connected to the impedance bond housing

IO-14 Transit Signal Work

case and the case is in contact with the earth. l Center insulation of the front rod

This can occur if the cases are mounted on l Front rod to switch point reinforced concrete where the mounting bolts l No. 1 vertical or horizontal switch rod contact the re-bar, if the bottom of the case is center insulation resting on concrete, or if dirt and debris l Throw rod insulated from No. 1 switch rod accumulate between the bottom of the case l Point detector piece insulated from switch and the concrete. An accumulation of ballast, point dirt, or other debris around the locations l Lock rod insulated from front rod where the cases are installed along the right- l Other vertical rods as required per layout of-way can also provide a path for current l Switch machine insulated from the leakage. This type of installation can result in running rails a continuous maintenance problem if an effectively high rail-to-earth resistance is to be 10.6.3 insulated Joint Test achieved. Insulated joint tests measure the resistance Some impedance bonds are located outside between two ends of the rail separated by the tracks on timber ties to eliminate points of insulating material. An insulated joint checker possible contact with earth. The center taps requires the traction power system to be of the impedance bonds should be insulated disconnected. Any reading under 30 ohms from the mounting case. should be evaluated. Measurements for a set of insulated joints should be within 30 percent Yard tracks should be isolated from the main of each other or they should be rechecked. line tracks to reduce corrosion. For additional Insulated rail joint tests for ac track circuits information on corrosion control, refer to can be performed using a volt-ohmmeter. Chapter 8.

10.6.4 impedance Bonding Resistance 10.6 SIGNAL TESTS Test

10.6.1 Switch Machine Wiring and Impedance bonding resistance tests ensure Adjustment Tests that a proper connection has been made using a low-resistance ohmmeter. Switch machine wiring and adjustment tests verify the wiring and adjustment of the switch machine. They should preferably be carried 10.6.5 Negative Return Bonding Test out, in conjunction with the track installer, to confirm throw rod capability, ensure point Negative return bonding tests verify the closure, and ensure proper nesting of the resistance of each mechanical or welded switch point rail to stock rail. power bond using a low-resistance ohmmeter.

10.6.2 Switch Machine Appurtenance Test 10.7 SUMMARY

Switch machine appurtenance tests verify the Communication-based signaling systems are integrity of switch machine layout by taking replacing traditional track circuits. They resistance measurements across the following eliminate the need for impedance bonds, assemblies: signal bonding, and bootleg risers and greatly

10-15 Light Rail Track Design Handbook reduce the number of signal wires and cables. Track designers need to coordinate closely Transit system designers are challenged to with signal designers to determine the types of find the correct level of transit signaling for signal equipment that will be installed on the each segment of a light rail transit line. The trackway. Once the equipment is identified, different needs for signals are indicated by the the interfaces with the track must be defined wide variety of right-of-way types and so a coordinated system can be constructed. operating conditions, coupled with the broad Construction phasing is an important part of catalogue of proven, available transit signal this coordination. equipment. This should encourage designers to seek the technical solution that will both respond to conditions and minimize total costs.

IO-16 Chapter I l-Transit Traction Power

Table of Contents

11.1 GENERAL 11-I 11.1.1 Interface II-1 11.2 SUBSTATION LOCATIONS II-I

11.3 WAYSIDE DISTRIBUTION 11-2

11.4 CATENARY ALTERNATIVES 11-3

11.5 CATENARY DESIGN 11-4 11.51 Introduction II-4 11.52 Conceptual Stage 11-4 11.53 Application of the Catenary System to the Track Layout 11-4 11.5.3.1 Track Centers 11-5 11.5.3.2 Horizontal Curves 11-5 11.5.3.3 Vertical Curves 11-5 11.5.3.4 lnterlockings 11-5 11.5.3.5 Track Adjacent to Stations 11-6

11.6 TRACTION POWER RETURN SYSTEM 11-6 11.6.1 Territory with Two-Rail Track Circuits for Signaling 11-6 1 ‘l.6.2 Territory with Single-Rail Track Circuits for Signaling 11-6 11.6.3 Territory Without Signaling Track Circuits 11-6

11.7 CORROSION CONTROL MEASURES II-6 11.8 MAINTENANCE FACILITY YARD AND SHOP BUILDING 11-7

1 l-i CHAPTER 1 I-TRANSIT TRACTION POWER

11.1 GENERAL Traction power positive supply system, including substation locations Light rail systems, by definition, use electrical Wayside catenary distribution positive power from overhead wires to provide traction system, providing power to the vehicles power to the light rail vehicles. Light rail systems use the rails, in conjunction with Traction power negative return through negative cables, as the return conductor to the the rails negative terminal of the rectifiers. Therefore, Corrosion control measures to mitigate the electrical properties of the rails and tracks the effects of stray direct currents passing require special consideration. through adjacent conduits, pipes and cables Theoretically, the traction current flows along the overhead contact system to the train from 11.2 SUBSTATION LOCATIONS the substation and back to the substation through the running rails. To obtain good The design of the track structure interface with conductivity for the track as a whole, a rail the traction power system must consider the system must have a low resistance not only cable and conduit access that will pass under for reasons of economy but also for safety. the track at substation locations to provide This requires a low voltage drop in the rails. power to the catenary pole. Cables and conduits for the return current to the The traction power system consists of the: substation will also pass under the track. l Traction power substation l Cables connecting this substation to the The location of traction power substations is distribution system developed using a computerized train performance program that simulates proposed l Wayside distribution system (catenary or peak operations along an accurate contact wire) providing adequate voltage geometrical and geographical depiction of the levels throughout the alignment planned route. Therefore, in the early stages l Return system cables connecting the of any light rail transit project, track and running rails to the substation traction power designers must interface to integrate the traction power system into the l Corrosion control drainage system overall system design. directing stray return current back to the appropriate substation The final selection of substation sites is an iterative process with repeated simulations to 11 .I .I Interface confirm the capability of the traction power system to sustain peak-hour operations. The There are four elements in the traction power sequence of events to develop substation system that affect, or are affected by, sites is as follows: trackwork design, construction, and l The traction power designer, using the maintenance: simulation program, selects theoretically ideal positions along the route, taking into

11-l Light Rail Track Design Handbook

account the distribution system’s voltage Placement of a substation at, or near, a drop and the lowest voltage acceptable to crossover is often desired to sectionalize the vehicle without degrading electrical supply for each travel direction and performance. The normal, single to optimize the operational flexibility of the contingency criteria for determining crossover. traction power requirements is to test the system with alternating substations out of operation. 11.3 WAYSIDE DISTRIBUTION l The designer discusses these proposed The trackwork element of the traction power locations with the local power utility to supply system design should allow adequate determine any impacts of the proposed space for the conduit to interface with the power demand on their network. The wayside distribution system. The electrical utility then evaluates the availability of sectionalization of the distribution system power circuits and the potential impacts usually takes place at the substation for all on its other customers. travel directions. Adequate space is required for conduit systems, including terminations, l An agreement is eventually reached, if necessary, by moving the substation to conduit risers, and manholes. Wayside enable it to be supplied from lightly loaded distribution systems can be subdivided into power circuits or by building spur cables the overhead contact wire system and to the substation location. It is also supplemental cabling systems. important, for reliability, that the power In systems utilizing overhead contact wire, company avoid supplying two adjacent substations from the same circuit. wayside connections are made to the overhead catenary system (OCS) from After an agreement is reached with the power trackside at substation supply points, company, the traction power designer can switching station locations, crossovers, finalize the substation design. Newer junctions, and wayside feed points. The substations for light rail systems are generally connection of the power supply to the modular, factory assembled units, that are overhead suspension network impacts track delivered to site complete. They are erected design since the cables are routed in on a prepared base that incorporates an underground conduits and must include riser extensive grounding network below the transitions at the appropriate height for concrete. termination. The riser transitions can be located at the sides of the OCS poles or within Substations are located along the track route the poles, either of which requires an as close to the wayside as possible within the appreciable foundation at trackside. Once the constraints of available real estate. However, power supply is terminated to the overhead the final placement must also consider wire, the power supply distribution usually interfaces and underground cable duct routes remains on aerial structures and does not for both the power distribution supply and interface further with the track. return systems; access roadways; and security requirements. The impact of this However, in visually sensitive areas where the construction on trackwork design is limited to community insists that only a single trolley the interfaces with the supply and return wire be utilized, additional cabling is required power distribution system. to support electrical loading. This

11-2 Transit Traction Power supplementary distribution is routed system height at the support is reduced to underground and conduit risers are required approximately 457 millimeters (1.5 feet). This quite frequently (every third or fourth pole) to style is applied in aesthetically sensitive areas make the transition from the underground where a lower profile and simple single-wire system to the overhead wire. This situation cross spans are more desirable The trade- requires enlarged pole foundations, possibly off, however, is that the span length between stanchion foundations, for switches at each supporting poles is reduced to approximately riser. At the power supply feed points to the 46 meters (150 feet). overhead wire, it is common practice to utilize poles situated on the field side of the tracks The traditional single-wire systems are instead of center poles to minimize impacts to considered by some to be much less obtrusive the track design. This also limits the amount in the urban environment. It provides power of underground conduit between and beneath through a single trolley wire that must be the tracks. supported at least every 30 meters (100 feet). The span length is limited by the sag of the The style of catenary and most of the basic unsupported trolley wire which, in high design parameters can be developed prior to temperatures, could encroach on vehicular finalization of the track configuration. traffic as well as the ability of the supporting However the application of a catenary design hardware to carry the weight of a whole span to suit the track layout can only proceed after of wire. It also requires the wire to be the track alignment has been finalized. supported electrically by parallel feeders that must be bonded frequently to the trolley wire to achieve adequate conductivity. These 11.4 CATENARY ALTERNATIVES feeders may run underground through a series of ducts and manholes, which are catenafy There are generally three styles of expensive, or hung from poles, which are used on LRT systems: simple catenary, low- unsightly. This system, therefore, has twice profile catenaty and the traditional single the number of poles than the equivalent trolley wire system. Simple and low-profile simple catenary system. catenary systems may have fixed terminations that cause the conductors to rise and fall as As mentioned above, modern, lightweight the temperature varies or balanced weight catenary systems adopt balance-weight tensioned to maintain constant tension and tensioning to limit the load applied, therefore height under all weather conditions. affecting the size of the poles, foundations, and hardware. However, this type of A simple catenary system uses a messenger construction requires the system to be wire to support the horizontal trolley wire. separated into l-mile segments with weights Both conductors are used to transmit power applied at each end to maintain constant from the substation to the vehicle. The tension in the conductors. Therefore, the system height at the support-the distance design requires overlaps to ensure smooth between the contact or trolley wire and the passage of the vehicle pantograph from one messenger-is approximately 1.2 meters (4 segment to the other. feet). This allows spans between poles of up to 73 meters (240 feet).

The low-profile catenary system is similar to the simple catenary design, except that the

11-3 Light Rail Track Design Handbook

11.5 CATENARY DESIGN The catenary system is the most conspicuous and possibly the most visually undesirable 11 S.1 introduction element of a light rail transit system. TCRP Report No. 7 discusses “visual pollution,” to Generally, technical papers have not the extent that it cited a case where a addressed rail/catenary interface issues, since community refused to introduce an electric- transit catenary design has developed from powered transit system because of the operating railway systems where the track is expected visual impact. Unfortunately wires already in place and the catenary must allow are needed to distribute power to vehicles. for the existing .layout. In many new transit Therefore, poles are needed to support and systems, the track alignment has been register them over the pantograph under all selected prior to the catenary designers adverse conditions. However, if the track involvement in the project. The results of this designer considers the catenary constraints, lack of coordination are chronicled in TCRP then the size and number of poles can be Report No. 7 Reducing fhe Visual Impact of minimized. Overhead Contact Systems. Involving the catenary designer in the track The catenary distribution system interfaces design/alignment selection process can be with trackwork in the following manner: cost-effective and reduce the visual impact of On single-wire catenary systems, the the catenary system. track designer must coordinate the longitudinal and transverse track feeder Horizontal and vertical track alignment, conduits that support the electrical trackwork, passenger station locations, distribution system. substation sites, etc., must all be determined before the preliminary catenary design can The track designer must also provide proceed. However, the locations and design adequate clearance between tracks for of these components can greatly influence the foundations, poles, catenaty balance catenary design and its visual impact on the weights, and down guys. environment. Track design and maintenance standards must be coordinated so that the vehicle pantograph remains beneath the catenary 11.5.2 Conceptual Stage wires under all adverse operating and climatic conditions. The catenary engineers task is to develop a conductor configuration to supply power to the vehicle from a position over the track that will 11.5.3 Application of the Catenary System allow good current collection under all to the Track Layout adverse weather, operating, and maintenance conditions. The engineer must develop the Since the wire runs in straight lines between most economic solution, considering the support points and the track is curved, pole aesthetic constraints set by the community. layout is a compromise between the number This task involves resolving the number of of poles and the requirement that the contact wires in the air with the number of poles, wire remain on the pantograph under all supports, and foundations to achieve an adverse climatic, operating, and maintenance efficient and environmentally acceptable conditions. Even though the pantograph is design. usually 1,980 millimeters (6.5 feet) wide, only

11-4 Transit Traction Power

460 to 610 millimeters (18 to 24 inches) are components, avoidance of superfluous and available for the wire to sweep the pantograph extremely tight curves is most desirable in head after allowing for track alignment, gauge, catenary system design. cross-level tolerances, vehicle displacement, roll, pantograph sway, and pole deflection. At the midpoint between supports, this distance 1153.3 Vertical Curves is reduced to zero due to deflection of the Vertical curves become critical when in the wires under maximum wind and ice loading vicinity of reduced-clearance overhead conditions. bridges. The rise and fall of the catenary messenger is governed by the formula: The allocation of pole positions must take into WL2 account the limitations of the catenary style, 2T the profile of the contact wire necessary to where: W is the weight of the catenary accommodate overhead bridges and grade L is the distance between supports crossings, track curvature, crossovers and T is the tension in the messenger turnouts, underground utilities, etc. Therefore, if the track is designed with the catenary Therefore, if there is a change in vertical constraints in mind, economies can be grade near an overhead bridge, as is required achieved. The following paragraphs identify when track undercutting is programmed to parameters that should be considered by the achieve increased vertical clearance, then the track designer. catenary designer should consult with the track designer to ensure that the wire can negotiate the vertical curvature. 11.5.3.1 Track Centers The clearance between poles and the track is defined by the system’s dynamic clearance 11.5.3.4 lnterlockings envelope, which comprises three elements: The catenary/pantograph interface is a the vehicle dynamic envelope, construction dynamic system. There are certain and maintenance tolerances, and running constraints applied to ensure that the system clearances, Therefore, if center poles with operates efficiently under all speed and supporting cantilevers on each side are weather conditions. The pole positions at desired to reduce cost and visual intrusion, turnouts are tied to the point of intersection then the distance between tracks should allow (PI). It is desirable for the distance between for this envelope from each track plus at least the inner crossover of a universal interlocking 305 millimeters (12 inches) to permit to be approximately the same length as the installation of standard-sized poles. crossover (PI to PI).

Scissor crossovers can be wired; however 1153.2 Horizontal Curves they present many difficulties for the catenaty If the track is tangent, there will be no track- designer. Usually, for maintenance purposes, related constraints, other than right-of-way the inbound and outbound tracks are boundaries, when placing the poles along separated into different electrical sections. track However, as the wire negotiates curves With tracks crossing within 2 meters (6 feet), using a series of chords, the number of very limited space is available to insert an supports is very dependent on the curvature. insulator and avoid the horns of the Therefore, as with other light rail system pantograph . This is particularly difficult in higher speed sections where constant tension

11-5 Light Rail Track Design Handbook catenary design has been adopted, since the return currents in the rails. At these locations, movement of wires along track due to conduit stub-ups will be installed beneath the temperature change can aggravate the tracks connecting the two track directions. problem. Also since wires serving two Impedance bonds are also required by the separate crossovers in a universal interlocking signal system at the end of each signal block. is much less costly, scissor interlockings should be avoided when catenary is employed. 11.6.2 Territory with Single-Rail Track Circuits for Signaling

11.5.3.5 Track Adjacent to Stations Although most track circuits for signaling in Architecturally the introduction of the catenary new light rail systems are of the two-rail type, system is obtrusive. Architectural design single-rail signaling track circuits do exist in tends to dictate the position of poles to suit the older systems. In such systems, one rail is architectural theme within the station area. used for traction return and the other is This impacts catenary pole positions adjacent designated the signal rail. This type of to station area requiring close coordination installation requires insulated joints separating between the architect, track and catenary the track circuits. With single-rail track designers to ensure adequate space for poles circuits, the impedance bonds described in at stations and approaches. Section 11.6.1 are not required. The cross bonding provided between the traction return rails of separate tracks uses cables without I I .6 TRACTION POWER RETURN impedance bonds for this purpose. Except for SYSTEM these differences, the same cabling is required between the traction return rail and substations as described in Section 11.6.1. 11.6.1 Territory with Two-Rail Track Circuits for Signaling 11.6.3 Territory Without Signaling Track The traction power return system directly Circuits impacts track design. The traction power return system uses the running rails as an The requirements for traction return in this electrical conductor to “return” the traction type of territory are similar to the those power to the substation from which it was described in Section 11.6.1, except that no generated. Traction power supplied to the impedance bonds are required. Instead, train enters the running rail through the cables are installed directly to the rails for vehicle wheels and is extracted from the rail both traction return at the substation and for through impedance bonds in cables installed cross bonding between the rails. at each substation. Therefore, track designers must allow for impedance bond installation, along with the associated conduit 11.7 CORROSION CONTROL MEASURES stub-ups and negative cabling, at each substation. Where there is more than one In designing dc traction power systems, it is track, in addition to the impedance bonds at common and desirable to isolate and insulate each substation, impedance cross bonds are the running rails from ground as much as also located along the track every 610 meters possible. These issues are discussed at (2,000 feet) or less to equalize the traction length in Chapters 4 and 8.

11-6 Transit Traction Power

The traction power return system interfaces Since the traction power return current can be with trackwork in the following manner: more easily controlled in a yard by increasing The siting of impedance bond positions the quantity and locations of return cables, the and cross bonds to adjacent tracks must insulation system provided for the yard tracks be coordinated. may be somewhat less effective than the main line track system described herein. Yard The selection of rail insulation for tie tracks are most commonly placed directly on plates and fastening clips suitable for the ties without insulation. The grounding track and traction power requirements systems for the yard and main line must be must be agreed to by all parties. electrically separate. This is achieved by Continuity bonds on jointed rails must also inserting insulated rail joints in the yard entry be coordinated. track at each arrival and departure The track designer and construction connection. inspector should ensure that ballast is Yard track designers must still consider and clear of rails so that return currents do not account for the many conduit risers necessary stray into the ground and cause corrosion to feed the numerous electrical sections in the problems in underground pipes and overhead contact system. Extra coordination cables. in yard areas should take place due to the Special consideration must be taken when additional users and electrical connections in selecting the insulation of the rails at the complex track layout. grade crossing and embedded track sections to ensure minimum leakage to In the maintenance facility building, the rails ground. are installed directly into the shop floor system and are rigorously electrically grounded for safety of the personnel working on the 11.8 MAINTENANCE FACILITY YARD AND vehicles. The return system is designed for SHOP BUILDING current to return directly to the substation through cables to ensure there is no potential The traction power return system in the difference between the vehicle and the maintenance facility yard and shop area is ground. Space for the conduit and cables usually different from that adopted for the connecting each track section to the building main line. The yard and shop area is usually substation must be coordinated. The shop designed and constructed along with the light floor tracks also contain insulated joints that rail system; therefore, adverse effects of stray electrically separate these totally grounded currents can be allowed for in its design. tracks from the yard track system.

11-7 The Transportation Research Board is a unit of the National Research Council, which serves the National Academy of Sciences and the National Academy of Engineering. The Board's mission is to promote innovation and progress in transportation by stimulating and conducting research, facili- tating the dissemination of information, and encouraging the implementation of research results. The Board's varied activities annually draw on approximately 4,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transporta- tion departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of trans- portation. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distin- guished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the fed- eral government on scientific and technical matters. Dr. Bruce M. Alberts is president of the Na- tional Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the Na- tional Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sci- ences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to se- cure the services of eminent members of appropriate professions in the examination of policy mat- ters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal govern- ment and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purpose of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is admin- istered jointly by both the Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

Abbreviations used without definitions in TRB publications: AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials FAA Federal Aviation Administration FHWA Federal Highway Administration FRA Federal Railroad Administration FTA Federal Transit Administration IEEE Institute of Electrical and Electronics Engineers ITE Institute of Transportation Engineers NCHRP National Cooperative Highway Research Program NCTRP National Cooperative Transit Research and Development Program NHTSA National Highway Traffic Safety Administration SAE Society of Automotive Engineers TCRP Transit Cooperative Research Program TRB Transportation Research Board U.S.DOT United States Department of Transportation