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Crossties for Transit Track

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Crossties for Transit Track 30 Crossties for Transit Track ROBERT E. CLEMONS and PHILIP J. McQUEEN ... ABSTRACT anchors is an important element of the installed cost of new at-grade transit track, as indicated by the data in Table 1. The total cost related to The objective of the paper is to define the crossties exceeds 20 percent because the labor and design requirements for rapid transit equipment to distribute and install the crossties is crossties as compared with crossties for included in item 4. Thus the' importance of the railroad service. In one section the im­ crosstie on installed cost of track is established. portance of the transit crosstie is identi­ The maintenance of crossties and rail fasteners fied, the significant differences between and anchors is a significant element of the annual the transit and railroad versions are com­ expense to maintain at-grade track, as indicated by pared, and the materials used for transit the data in Table 2. All ta8ki; art! 1Jt!rfurmt!tl 111 unt! crossties are reviewed. In the second sec­ pass and scheduled on an 18-month cycle. Actual tion the application of concrete ties to working time averages 3 hr per night during system transit tracks is addressed. The specific shutdown. Equipment maintenance and amortization requirements of tie design parameters, the are not included. rail fastening system, and other hardware were considered. In addition, installation methods and equipment, special applica­ TABLE 1 Construction Cost per Single Track Mile tions, and factors that influence first cost and life-cycle costs were identified. Cost per Mile Item Description ($000s) Percent 1 Furnish rail 47.4 31 2 Furnish concrete Other papers in this Record have generally concen­ crossties 30.9 20 trated on crossties in railroad applications. Al­ Furnish and install ballast 30.5 20 thouqh railroad and transit crossties are similar in 4 1n3ta11-trnck 20.1 13 form and function, there are significant differences Miscellaneous 25.0 16 that are identified and discussed in the two major Total 153.9 sections of this paper. In the first section the importance of the tran­ Note: Data are from San Francisco Bay Area Rapid Transit (BART) average sit crosstie is identified, the significant differ­ bid costs, in January I 9"70 dollars. ences between the transit and railroad versions are compared, and the materials used for transit cross­ t ies are reviewed. In the second section the appli­ TABLE 2 Routine Maintenance Expense per Single cation of concrete ties to transit tracks is Track Mile addressed. The specific requirements of tie design parameters, the rail fastening system, and other Expense per hardware are considered. In addition, installation Item Task Cycle• ($) Percent methods and equipment, special applications, and 3,240 30 factors that influence first costs and life-cycle 1 Line and surface 2 Tamp ballast 2,880 26 costs are identified. 3 Torque and lubricate bolts 2,160 20 4 Grind rail 1,620 15 5 Materials 890 8 GENERAL CONSIDERATIONS 6 Safety ........l1.Q_ 1 Total 10,960 The functions of the crosstie in track have been de­ fined by other papers in this Record. However, for Note: Data are from BART 1983 budget planning. transit application, the functions can be divided 3 1983 dollars. into the following categories: 1. Primary--tie the rails together to hold track Transit and Railroad crosstie Compariso n gauge and distribute the various three-dimensional Crosstie differences can be categorized into those loads to the ballast: and related to the pr ima.ry function, secondary fnnr.­ 2. Secondary--provide resilience and electrical tions, maintenance, and procurement. insulation, support the third rail and other elec­ trical equipment, and support other track devices such as guardrails and restraining rails. Primary Function Significant differences between transit and railroad crossties occur in the primary function because of The primary function differences between transit and lighter transit loading, and in the secondary func­ railroad crossties are as follows. tion because of additional electrical and structural requirements for transit applications. l. Loading conditions: The maximum wheel load for transit vehicles is less than half that of freight railroad locomotives and cars. The transit Importance of Transit Cross tie impact factor (percentage of static load) is less because of the more sophisticated vehicle suspen­ The procurement of crossties and rail fasteners and sions and tighter maintenance tolerances for vehicle Clemons and McQueen 31 and track. However, there are five factors that, if Maintenance applied to equal wheel loads, would result in more severe conditions for transit than railroads. The The maintenance differences between transit and first is the uniformity of transit wheel loads railroad crossties are a matter of degree. In gener­ caused by a smaller difference between empty and al, the maintenance of transit tracks is more diff i­ loaded vehicles. The second is the more common oc­ . cult than railroad tracks because of the following currence of flat wheels and the resultant high fre­ factors. quency impact loads. The third is the higher speed. The fourth is the higher acceleration and braking 1. Limited access: Workers, equipment, and ma­ rates and resultant longitudinal loads on the track. terials must move on the tracks to the work site. The fifth is the stiffer truck, which tends to in­ All work must be done within the right-of-way or crease lateral load on the railhead. structural limits. 2. Track alignment and tolerances: Transit cri­ 2. Limited time: Train frequency, which is usu­ teria allow sharper curves and steeper grades than ally in minutes, requires either late night work or feasible with railroad, as the data in Table 3 indi­ complex operating and work procedures under traffic. cate. In general, transit follows railroad practice for maximum superelevation and unbalance. Therefore, These factors tend to stimulate interest among tran­ the lateral acceleration acting on transit and rail­ sit engineers in crosstie and fastener designs that road vehicles is similar. Transit track maintenance allow for quick change-out of worn components. tolerances are generally set in accordance with pas­ senger comfort and therefore are tighter than freight railroads. Procurement 3. Track configuration: Typical transit design practice uses the freight ra.ilroad ballast section, The procurement differences between transit and as recommended by the American Railway Engineering railroad crossties include the following. Association (AREA). This includes 12 in. under the crosstie and 9 to 12 in. of shoulder width. Transit 1. Capital financing: Transit projects are crosstie spacing is increased based on the maximum funded with public money and controlled by govern­ allowable pressure on the ballast and subgrade, as ment regulations that cover competitive bidding, recommended by AREA or as required by local con­ bonding, minority participation, minimum wages, and ditions. so forth. Railroad projects are privately funded subject to accounting practices and income tax regu­ lations. TABLE 3 Transit and Railroad Alignment Comparison 2. Procurement quantities: Transit projects re­ quire smaller quantities of crossties than railroad Requirement Unit Transit Railroad projects. Maximum horjzontal Feet 500 1,146 curve radius Degree 11-28 5 Maximum vertical Transit Crosstie Materials curve rate Sag Feet per station 1.17 0,05 Transit crossties, like their railroad counterpart, Summit Feet per station 0.58 0.10 Maximum grades Percent 4 2 are fabricated from a full range of materials, in­ cluding timber, steel, and concrete. Historically, No ,.:i; Data are typical for new construction oF main tracks for heavy rail transH and timber ~rossties and standard railroad fasteners and frels;ht railroads. anchors were selected by most transit properties be­ cause of availability and lowest first cost. Even today, the majority of transit crossties are of this Secondary Functions type. Timber crossties are cut, seasoned, and pressure The secondary function differences between transl t treated in accordance with specifications, such as and railroad crossties include the following. those by the American Wood Preservers Association. Wood is selected by the locale i however, oaks and 1. Electrical insulation: A resistance or im­ mixed hardwoods are the most common. The widespread pedance of 20, 000 ohms between running rails is a use of timber has not hampered the development of common requirement for transit and railroad cross­ new technology. Laminated and remahufactured timber ties to be installed in tracks with automatic sig­ ties have been tested, and new resilient rail fas­ nals. However, transit crossties and rail fasteners teners are in service to reduce maintenance expenses and anchors to be installed on direct current (de) and improve performance. traction systems must provide higher resistance to Steel crossties are used where transit tracks are minimize electrical leakage and the resultant elec­ set in pavement. There are two types: the tie bar trolytic corrosion of adjacent pipelines and struc­ bolted through the rail webs, and the flat type tures. Resistance values of up to 1,000,000 ohms be­ clipped to the rail bases. Both types hold track tween each rail seat and ground have been specified gauge and anchor the track in the pavement. for transit crossties. Concrete crossties are designed and fabricated in 2. Electrical equipment: Transit crossties must accordance with the performance requirements of each support and restrain an electrified third rail. transit property. A number of new transit projects, Loads transmitted to the crossties include gravity, and extensions to ex'isting systems, have selected wind, thermal expansion, and electromagnetic attrac­ concrete crossties during the past 2 decades. These tion. In addition, other electrical equipment (such decisions to use concrete were based on an analysis as impedance bonds, trip stops, and signal loops) of total purchase cost plus annual expenses, and may be mounted on the crosstie. clearly set a new trend for transit crossties.
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