REPORT FHWA/NY/SR-04/142
Estimating Fatigue Life of Patroon Island Bridge Using Strain Measurements
RYAN LUND SREENIVAS ALAMPALLI
SPECIAL REPORT 142 TRANSPORTATION RESEARCH AND DEVELOPMENT BUREAU NEW YORK STATE DEPARTMENT OF TRANSPORTATION George E. Pataki, Governor/Joseph H. Boardman, Commissioner
ESTIMATING FATIGUE LIFE OF PATROON ISLAND BRIDGE USING STRAIN MEASUREMENTS
Ryan Lund, Civil Engineer I, Transportation Research & Development Bureau Sreenivas Alampalli, Director, Bridge Program and Evaluation Services Bureau
Special Report 142 November 2004
TRANSPORTATION RESEARCH AND DEVELOPMENT BUREAU New York State Department of Transportation, 50 Wolf Road, New York 12232
ABSTRACT
The design fatigue life of a bridge component is based on the stress spectrum the component experiences and the fatigue durability. Changes in traffic patterns, volume, and any degradation of structural components can influence the fatigue life of the bridge. A fatigue life evaluation, reflecting the actual conditions, has value to bridge owners. This report presents a study where the remaining fatigue life of the Patroon Island Bridge, which carries Interstate 90 over the Hudson River, was estimated as part of a structural integrity evaluation and a larger evaluation of the entire interchange.
The Patroon Island Bridge consists of ten spans. Seven spans are considered the main spans and consist of steel trusses and concrete decks. The other three spans are considered approach spans and consist of plate girders. The overall bridge length is 1,795 feet. Procedures outlined in the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges and strain data from critical structural members were used to estimate the remaining fatigue life of selected bridge components. The results indicate that most of the identified critical details have an infinite remaining safe fatigue life and others have a substantial fatigue life, however the remaining fatigue life has not been determined for damaged and cracked members.
iii
CONTENTS
I. INTRODUCTION ...... 1
A. Background ...... 1 B. Current Status of the Bridge...... 4 C. Study Objectives ...... 6
II. INSTRUMENTATION AND DATA COLLECTION ...... 7
A. Gage Locations ...... 7 B. Instrumentation ...... 11 C. Data Collection and Analysis...... 12
III. EFFECTIVE STRESSES FOR CRITICAL DETAILS ...... 19
IV. FATIGUE LIFE EVALUATION ...... 25
A. Detail Classification ...... 25 B. Infinite Fatigue Life Check...... 26 C. Finite Remaining Fatigue Life ...... 28
V. CONCLUSIONS ...... 31
ACKNOWLEDGMENTS ...... 33
REFERENCES ...... 35
APPENDICES Appendix A: Histograms ...... 37
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I. INTRODUCTION
The remaining fatigue life of bridges can play a major role in making cost-effective decisions regarding rehabilitation versus replacement of existing bridges, especially for those bridges that are part of a busy interchange. The design fatigue life of a bridge is generally based on the truck traffic data, prevalent specifications, and the fatigue durability of the components in the bridge. The actual truck traffic volume that a bridge experiences, and is expected to experience for the design life of the bridge, will influence the fatigue life of the bridge components. An evaluation based on actual conditions can benefit bridge owners.
The remaining safe fatigue life of the Patroon Island Bridge in New York State was evaluated as part of a structural integrity evaluation of a major interchange (1). Procedures outlined in the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges and strain data from critical structural members were used to estimate the remaining fatigue life of selected bridge components (2, 5). A rain-flow algorithm (3,4) and Miner's Rule (2) were used as part of the analysis. The analysis showed that most of the members analyzed have an infinite fatigue life, however the remaining fatigue life has not been determined for damaged and cracked members.
In this report the remaining safe life of structural details is determined as opposed to the remaining mean life. There is a 97.7% and 99.9% probability that the actual fatigue life will exceed the remaining safe life for redundant and non redundant details, respectively (2).
A. BACKGROUND
The Patroon Island Bridge, as illustrated in Figures 1 and 2, is part of a major interchange in Albany, New York. The bridge carries Interstate 90, in the east-west direction, over the Hudson River. The bridge consists of ten spans. Seven spans are considered the main spans and consist of steel trusses and concrete decks. The other three spans are considered approach spans and consist of plate girders. The overall bridge length is 1,795 feet and has been in service since 1968, however some structural repairs were made in 1992. The average daily traffic (ADT) count was 70,787 in 1998 with a 4.5% estimated traffic growth during the life of the bridge.
Girders span from the west abutment, continuously over pier 1 to pier 2, in the mainline and ramp spans. The floor beams rest on sole plates, which are welded to the top flange of the girder, so the girder and slab are non-composite. The mainline and ramp spans have two girders in each direction. In 1992, the east and west bound main line spans were tied together with diaphragms and floor beam splices. Span 3 consists of rolled and built-up members in each of the four, constant depth trusses. The floor beams are spaced at 11' 7" and rest on top of the girders. The trusses consist of welded plates or rolled I sections and there are no stringers in span 3.
Spans 7, 8, and 9 consist of rolled and built-up members in each of the four, constant depth trusses. The trusses are welded into box shapes. The tension members have welded intermediate stay plates and the compression members have welded perforated plates.
Figure 1. Patroon Island Bridge.
Figure 2. Location of the Patroon Island Bridge.
2 Spans 4, 5, and 6 consist of built up truss members. The compression members consist of two main plates and perforated cover plates. The tension truss members consist of main plates with intermediate stay plates and diaphragms. During construction in 1968, the intermediate stay plates and diaphragms were connected to the inside of the truss plates with fillet welds. In 1992, some of the intermediate stay plates and diaphragms were removed and new plates were reconnected with bolts to increase the fatigue strength. Figure 3 shows a bolted intermediate stay plate.
The cantilever and interior floor beam webs in spans 4, 5 and 6 are connected to the truss node gusset plates using steel angles, but the top flanges are made continuous with a top strap plate that is not connected to the truss (See Figures 4, 5 and 6). Many cracks have been observed in the upper portion of the floor beam web, near the top strap plate. This is a critical connection, but the fatigue life for this detail could not be determined because cracks exist.
Figure 3. Bolted intermediate stay plate.
Figure 4. Floor beam to truss connection.
3 Span 10 originally had independent spans to facilitate the east and west bound lanes. In 1992 the east and west bound spans were tied together with diaphragms, floor beam splices and a closure pour. Span 10 has bolted sole plates on the top flange to act as a spacer for the floor beams. The stringer webs are bolted to the web of the floor beams to allow shear transfer, but limit moment transfer. There are several flange plate transitions that are connected using groove welds.
B. CURRENT STATUS OF THE BRIDGE
Several structural components experienced fatigue cracks. The bridge and two ramp structures have a documented history of cracks in connection welds and floor beams. These include the webs of floor beams (Figures 4-6), sole plates between the floor beams and girders (Figure 9), truss to bearing welds (Figures 7-8), and transverse web stiffeners (Figure 9).
Many of the recent problems have been related to the floor beam webs in spans 4, 5 and 6 (Figures 4-6). The cause of this cracking was attributed to the deformation of the truss in the longitudinal direction with respect to the slab and local stress concentrations in the upper portion of the web, near the truss. The floor beam web is attached to the truss using an angle. A top strap plate creates continuity between the top flanges of the floor beam on each side of the truss, but is not attached to the truss. The truss is exerting an out of plane load on the web of the floor beam. The floor beams are constrained by the stringers which are connected to the slab. The slab is very stiff along the length of the bridge, when compared to the top truss chords in the bridge, but the web of the floor beam must transmit the shear between the truss and the slab. In addition to the out of plane shear stresses in the floor beam web, there are also stresses in the longitudinal axis of the floor beam that are contributing to the fatigue damage. The strap plate connects to the top flanges on each side of the truss and transmits a large portion of the moment since the web is connected, via an angle, to the truss. This creates a large in-plane shear stress in the upper portion of the floor beam’s web, just below the flange and strap plate. Similar damage in floor beams has been observed in the literature (7, 9).
Figure 5. Crack in the upper portion of floor beam.
4 Figure 6. Crack in the upper portion of floor beam with retrofit angle welded to top strap plate and bolted to floor beam.
Figure 7. Crack in weld connecting bearing sole plate (span 4 at pier 3) to truss bottom.
Figure 8. Crack in bearing sole plate.
5
Figure 9. Crack in the weld of a stiffener to the web of floor beam.
C. STUDY OBJECTIVES
The objective of this study is to determine the fatigue life of selected bridge components, using the measured field data, to assist the structural integrity evaluation of the structure and the interchange with Interstate 787. The AASHTO Guide Specifications for "Fatigue Evaluation of Existing Steel Bridges," outlines a method to estimate the remaining fatigue life of selected bridge components using measured strain data from critical structural members. This involves identification of fatigue critical details, obtaining strain time histories under normal traffic for these critical details, and conducting the required analysis to obtain the remaining fatigue life.
6 II. INSTRUMENTATION AND DATA COLLECTION
The bridge details, current status, and the structural problems were evaluated. Based on this evaluation, the fatigue critical details on the Patroon Island Bridge were identified. An instrumentation plan was then developed to collect strain data near critical details. This section gives details related to instrumentation and data acquisition.
A. GAGE LOCATIONS
A girder, floor beam, and stringer were investigated from a ramp span near pier 1, in the main line of span 2 (see Figure 10), and in span 10 (see Figure 14). One of the rolled tension truss members from span 3 was investigated (see Figure 11). Eight critical built-up truss members were evaluated in spans 4 and 5 (see Figure 13). Span 6 has mirror symmetry with span 4, therefore comparable behavior was assumed. Two floor beams were instrumented near the connection with end trusses, where many fatigue cracks have been observed in the upper portion of the webs of several floor beams (Figure 13). Four rolled truss members were investigated in spans 8 and 9 (Figure 12). Span 7 has mirror symmetry with span 9 and similar behavior was also assumed here.
Horizontal and vertical gages were installed on the webs of several floor beams near their respective connections to truss members in span 5. These floor beams are expected to have large, out of plane shear stresses induced by a relative displacement, in the direction of traffic, between the upper truss nodes and the slab. This causes a strain component that is orthogonal to the strain that was measured and was therefore undetected with the gages installed. These gages were expected to provide the data necessary to calculate and compare the effective in-plane stresses in the floor beam webs for reinforced and unreinforced connections, and were not intended to predict fatigue life. Furthermore, the fatigue life cannot be estimated for damaged or cracked members, using the procedure outlined in Guide Specifications for Fatigue Evaluation (2). The fatigue life of the floor beams in span 4, 5, and 6 was not estimated, but the fatigue life is limited due to excessive cracking. Repair or replacement is recommended. Figure 10. Location of Gages 1-6.
8
Figure 11. Location of gage 7.
Figure 12. Location of gages 20 through 23.
9 Figure 13. Location of gages 8 through 19.
Figure 14. Location of gages 24, 25 and 26.
10 B. INSTRUMENTATION
Once the fatigue critical locations were identified, strain gages were installed and data was collected. The strain transducers were generally located on the bottom flanges of the beams in positive moment regions, on the top flanges of the negative moment regions, and on the main load carrying plates of the truss members. Traffic count data revealed that Monday to Thursday can be considered normal traffic days and hence all data were collected during this part of the week. Forty-eight continuous hours of strain data was collected during three instrumentation phases due to the cable limitations and data storage capabilities (see Table 1).
Table 1: Test summary. Test Phase Test Name * Gages Start Date 1 West 1 1-9 September 04, 2001 2 West 2 9-19 September 17, 2001 3 East 20-26 October 23, 2001 * Test name indicates the data acquisition system location with respect to the Hudson River.
Strain transducers from Bridge Diagnostics, Inc. were used in the testing (6) and placed to capture high strains near critical details. These transducers have a full Wheatstone bridge, with four active, 350 ohm gages imbedded in an aluminum housing, and an operating temperature range of -50/C to 1200/C. They were attached to the bridge components with threaded mounting tabs using quick setting adhesive. A typical strain time history is shown in Figure 15. A limited number of conventional foil gages were used in the field testing to verify the strain data from the BDI gages.
Gage locations are given in Table 2. Most of the gages were located and oriented to capture high tensile strains near critical details. In the beams (i.e. girders, floor beams and stringers) most gages were located on the bottom flanges of the positive moment regions and on the top flanges of the negative moment regions. Gages were located on the main load carrying plates of truss members. Gages 16 through 19 provide horizontal and vertical strain on the web of the floor beam near the connection to the truss. One of the instrumented floor beams was reinforced with a transverse stiffener as shown in Figures 4, 5 and 6. These gages are expected to provide the data necessary to compare the stresses in the reinforced connection to the stresses in the unreinforced connection. The instrumentation in the floor beam does not provide the out of plane strain in the web of the floor beam. Gages 24, 25, and 26 provide the primary strain from the bottom flanges of floor beam 8, a stringer, and girder 1, respectively.
Type T (copper/constantan) thermocouples were used to measure the temperature fluctuation. These thermocouples have the ability to measure temperatures ranging from -270°C to +400°C.
11 Table 2. Strain gage locations. Gage Test Span Member Gage Location on Member 1 1 2 (BF) FB 8 midspan, bottom flange 2 1 2 (BF) stringer (FB 8 - FB 9) midspan, bottom flange 3 1 2 (BF) girder 42" east of FB 8 centerline, bottom flange 4 1 2(ML) FB 6 midspan, bottom flange 5 1 2(ML) stringer (FB 9 - FB 10) midspan, bottom flange 6 1 2(ML) girder midspan, top flange 7 1 3 L4 - L6 truss 8 1 4 U0 - L1 truss 9 2 4 L3 - L4 truss 10 2 4 U4 - U5 truss 11 2 4 U8 - U9 truss 12 2 5 U9 - L10 truss 13 2 5 U13 - L14 truss 14 2 5 U14 - U15 truss 15 2 5 L14 - L15 truss 16 2 5 FB 16 not retrofitted, cantilever, FB, horizontal 16V 2 5 FB 16 not retrofitted, cantilever FB, vertical 17V 2 5 FB 16 not retrofitted, interior FB, horizontal 17 2 5 FB 16 not retrofitted, interior FB, horizontal 18 2 5 FB 17 retrofitted, cantilever, FB, horizontal 18V 2 5 FB 17 retrofitted, cantilever, FB, vertical 19V 2 5 FB 17 retrofitted, interior FB, horizontal 19 2 5 FB 17 retrofitted, interior FB, horizontal 20 3 8 L19 - U20 truss 21 3 9 U20 - U21 truss 22 3 9 L25 - L26 truss 23 3 9 L29 - U30 truss 24 3 10 FB 8 8' south of midspan, bottom flange 25 3 10 stringer midspan, bottom flange 26 3 10 girder midspan, bottom flange Notes : BF = Ramp from 787 north bound to I90 east bound. ML = Main line of bridge. FB = Floor Beam.
C. DATA COLLECTION AND ANALYSIS
A typical time history segment is illustrated in Figure 15. All the time histories were reviewed carefully to eliminate instrument related errors. In some cases unreasonable spikes, with magnitudes of 100 times the expected strain, were noticed in the data. These were attributed to sporadic radio signals. If a small portion of the data was affected by these spikes, then that portion of the data was neglected in the analysis.
After the strain/time history data was collected, turning points were extracted from the strain/time history (see Figure 16). Turning points are the peaks and valleys of the strain/time history.
12 Figure 15. Typical time history.
Figure 16. Time history with turning points.
13 Strain Histograms
Once the peaks and valleys have been extracted, the data is reorganized so that the highest peak is the first turning point in the data set. The rainflow algorithm, which is outlined in the ASTM Specifications (4) and given as a FORTRAN program in Downing and Socie (3), extracts strain cycles from the reorganized turning point data.
The next step is to obtain the strain histograms. The strain cycles were categorized into approximately 20 equal width strain bins, that were chosen so that each bin boundary was an integer number of microstrains. The bridge is constantly oscillating at its natural frequency, so there is a great number of strain cycles in the lowest bin (see Figure 17).
Figure 17. Typical strain histogram.
Establishing Threshold Strains
The large number of low magnitude strains will reduce the calculated effective stress, so the low amplitude cycles will be removed if they are below a threshold level. Figure 18 shows a typical histogram with an amplitude filter. A study conducted by Dr. Fisher for the American Institute of Steel Construction (7) indicates that a truck with a gross vehicle weights less than 20 kip has a very
14 low probability of causing fatigue damage, hence the strain ranges corresponding to a 20-kip truck were determined with a load test and then used as a strain cycle threshold. The strain ranges below the strain threshold were then ignored for the calculation of effective stress. Low amplitude filtering gives a higher and more conservative estimate of the effective stress. Figure 19 shows the strain time history for a rolling truck test. The strain ranges were extracted from this data. The actual truck weight was 37 kips, therefore strain ranges were scaled linearly with 0.5405 ( 20/37). Three rolling truck tests were conducted during the West 2 phase of the testing and an average value was chosen to reduce random errors. Rolling truck tests were performed only during the West 2 testing phase, so a correlation was found between the threshold strain and the effective stress, calculated without amplitude filtering . This correlation is illustrated in Figure 20. The correlation equation from Figure 20 and the effective strain, calculated without amplitude filtering, were used to estimate the strain caused by a 20 kip truck. Appendix A shows the strain histogram for each gage, while neglecting strains below each respective threshold strain.
Figure 18. Histogram with threshold strain of 20 micro-strain.
15 Figure 19. Typical strain time history from rolling truck test.
y = 0.1046x - 1.9819 R2 = 0.9491 35
30
25
20
15
10
5
0 Maximum Strain Range Due to 20 kip Truck 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Effective Stress [psi] with No Threshold Strain
Figure 20. Correlation between the zero threshold effective stress and the strain ranges resulting from a 20 kip truck.
16 Estimation of Effective Stress
The effective stress was calculated using the histograms in appendix A and miner’s rule (2).
n 1 3 3 SfSriri= ∑() i=1
where:
Sr = effective stress amplitude
fi = fraction of stress ranges within interval i
Sri = average stress range for interval i
Fs1 = 0.85 (factor related to procedure for calculating effective stress) n = number of histogram bins
The effective stress range can be calculated from the effective strain for a linear elastic material:
SEri= ε ri
and
n 1 3 SE= fε 3 . riri∑() i=1
Threshold strains, effective stress, and other gage results are given in Table 3.
17 Table 3. Strain gage result summary for the 48 hour monitoring period.
Gage* Threshold Effective Number of Maximum Maximum Maximum Minimum No. cycle** Stress cycles used*** cycle cycle (:,) (ksi) (psi) (:,) (:,) (:,) 1 18 1.55 2214 3799 131 116 -29 2 20 1.50 2176 2813 97 87 -27 3 13 0.97 1486 2465 85 49 -46 4 15 1.10 3272 3016 104 111 -32 5 18 1.25 3339 2581 89 93 -27 6 19 1.20 2214 2813 97 32 -71 7 20 1.16 3832 6931 239 85 -195 8 18 1.03 2521 2813 97 88 -26 9 16 1.23 2747 2929 101 75 -71 10 5 0.36 2225 841 29 16 -49 11 5 0.29 2651 928 32 27 -38 12 12 0.83 2206 2059 71 73 -38 13 13 0.95 2132 2378 82 67 -36 14 5 0.33 2309 899 31 11 -51 15 6 0.49 2924 1276 44 41 -41 16 32 1.76 2196 6206 214 129 -97 16V 12 0.70 1622 1740 60 27 -39 17V 13 0.82 2138 3016 104 266 -42 17 33 1.72 2063 4524 156 99 -73 18 5 0.26 1102 667 23 15 -44 18V 5 0.25 809 551 19 18 -11 19V 4 0.23 1457 754 26 15 -26 19 8 0.46 1679 1044 36 41 -20 20 17 1.09 2909 2668 92 84 -26 21 11 0.67 3070 1479 51 41 -21 22 19 1.17 3092 2407 83 79 -41 23 19 1.14 2720 2407 83 91 -24 24 6 0.31 8635 1015 35 31 -27 25 20 1.26 4735 3045 105 94 -16 26 14 0.75 2650 2233 77 63 -28 * Gage locations are in Table 2. ** Strain cycle caused by a 20 kip truck. *** Number of cycles in a 48 hour period, which were above the threshold strain range.
18 III. EFFECTIVE STRESSES FOR CRITICAL DETAILS
The gages were placed on accessible structural components, but some of the gage locations did not correspond with a fatigue critical location. In several cases, interpolation, based on structural analysis, was required to estimate the effective stress at locations that did not have instrumentation. This section describes the relationship between the effective stress at critical location and the effective stress at gage locations.
Girders
Girders are continuous over pier 1 for the mainline span and the Y span. There are several points in the girders that could be sensitive to fatigue, however the stress was only measured at one point in selected girders. The effective stress at critical locations was interpolated using the effective stress at gage locations.
Influence lines for the moment at several critical locations were generated for a unit load moving along the length of the girder in spans 1 and 2, using the following assumptions for simplifying the analysis:
• There is no moment transfer at the east abutment or pier 2 ( i.e. bearings cannot transmit moment).
• The girders are continuous over pier 1, but there is no moment transfer to pier 1.
• The girder deforms like an Euler beam, independently from the slab and stringers.
• The loads that produce the maximum stresses in the girders are taken as point loads instead of axle loads and they are applied at points on the girder that may not correspond to floor beam locations. Figure 21. Moment influence lines for points in span 2.
The most vulnerable detail is the weld between the sole plate and the girder (E’), but this detail is only on the top flange. The top flange may experience tension, compression or both, depending on the location, since the girders in span 1 and 2 are continuous over pier 1. Influence lines for the moment and the resulting moment cycles at points in span 2 are given in Figures 21 and 23, respectively. A top flange stress cycle will be proportional to a moment cycle. Reference 2 states that a detail has an infinite fatigue life if:
2RSSt< S C (Equation 3.1 b from (2)) where:
RS = reliability factor
St = tension portion of stress range
SC = compression portion of stress range The reliability factor for the girder is 1.5 so there is an infinite remaining safe fatigue life, when: S C > 3 St The maximum stress cycle occurs in the top flange at midspan (Figure 23), but this location experiences large compression and small tension stresses as illustrated in Figure 22. Figure 22 illustrates that equation 3.1 b from specifications (2) is satisfied in the top flange at midspan, so there is an infinite fatigue life here. Equation 3.1 b from specifications (2) becomes satisfied at a
20 point that is approximately 152 feet from the west abutment, and the nearest sole plate weld towards the pier is at point that is 147' from the west abutment. This is the sole plate that will have the lowest remaining safe fatigue life. Normalized moments at points in span 2 are given in Table 4.
Figure 22. Ratio of maximum to the minimum moment for points in span 2.
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Figure 23. Moment cycles for point in span 2.
Table 4. Normalized moments for critical or gage location points. Point Location in mainline or BF ramp girders Normalized moment A 113' 6" from west abutment, gage 3 0.72 B 147' from west abutment, fatigue critical 0.90 at sole plate weld C center of span 2, gage 6, fatigue critical 1 at toe of transverse stiffener, bottom flange
The ratio of the normalized moments (Table 4) will be used to estimate the stresses at other critical locations. The effective stress at the top of the girder at point B can be estimated using the following equations:
SBtoprr(, )= 090 . * SCtop (, )c
090. S(, B top )==*(,) S A bot125 .*(,) S A bot rrr072.
22 The effective stress in the bottom flange of the girder at point C can be estimated using the effective stress in the top flange at point A. 1 S(, C bot )==*(,) S A bot139 .*(,) S A bot rrr072. where:
SAbotr (, ) : Effective stress in the bottom flange of the girder at point A.
SBtopr (, ) : Effective stress in the top flange of the girder at point B.
SCtopr (, ) : Effective stress in the bottom flange of the girder at point C.
These equations are used to determine the effective stress in the girder top flange, at point B and in the toe of a transverse stiffener at point C.
Midspan of Stringer to Bolted Connection
The level of moment stiffness in a connection can effect the stresses observed in that connection. The stringer web is connected to the web of the floor beam, so the moment stiffness of this connection is low. The gages on the stringers are located in the bottom midspan of the stringers but it is desirable to check the fatigue life at the connection with the floor beam, since there is some moment stiffness in this connection. A stringer and floor beam model is shown in Figures 24.
Figure 24. Floor beam and stringer model.
23 The ratio of the effective stress between the bolted connection, point B, and the bottom of the midspan of a stringer (point A) has been determined with the following assumptions:
• Torsional rotation at the floor beam is zero at neighboring girders.
• There is displacement and rotation continuity at the connection between the stringer and floor beam.
• The effective stress at B is factored using the effective stress at A and the ratio of maximum tensile stress at points A and B.
• The maximum stress at both locations is caused by a point load at midspan of the stringer.
• The stringer deforms symmetrically about its midspan point.
• The moment is only transferred to one adjacent stringer. After one adjacent stringer there is a fixed connection to, conservatively, simplify the model.
Based on these assumptions, the effective stress at B was found to be 1.86 times the effective stress at point A.
Simply Supported Floor Beam
Gage 24 was mounted on the bottom of a floor beam, at a distance 4 ft. North of the Girder G1. The maximum stress should occur at the center of the floor beam between Girders G1 and G2. The distance between Girders G1 and G2 is 24 feet. The spacing of the stringers is 8'. The floor beam is assumed to be simply supported. With a load applied to the midspan of the floor beam, the bottom flange stress at midspan is twice the bottom flange stress at gage 24, so the following equation was used to estimate the effective stress in the bottom flange at the midspan of the instrumented floor beam:
Srr(,)2(4',) midspan bottom= S bottom
whereSr (,) midspan bottom is the effective stress at the midspan of the floor beam, and
Sr (4', bottom ) is the effective stress at the location of gage 24.
24 IV. FATIGUE LIFE EVALUATION
Once the effective stress is determined, it is then compared with the limiting stress range for infinite fatigue life (SFL). If the limiting stress range, SFL, is greater than the factored effective stress, then the corresponding fatigue detail has an infinite remaining life. Otherwise, the remaining fatigue life is calculated. The limiting stress ranges from the Guide Specifications for Fatigue Evaluation of Existing Steel Bridges (2) are about a third of the constant amplitude fatigue thresholds found in the Standard Specification for Highway Bridges (8). A detail’s limiting stress range is dependent on its fatigue durability.
A. DETAIL CLASSIFICATION
The detail categories, limiting stress range, SFL, and detail constant, K are given in Table 5 (2). For example, the longitudinal welds between the flange and web of a plate girder are classified as a B detail, so the limiting stress range is 5.9 ksi. All structural members, considered in this analysis, were taken as non-redundant, except for the stringers.
Plate Girders
Welds between the sole plate and the top flange of the girder are classified as E’, because the length of the weld in the direction of stress is greater than 4". The plate girders in spans 1 and 2 have transverse stiffening plates with category C fillet welds at diaphragm weld toes. In span 10, the groove welds, between the flange transitions, are classified as detail category B.
Floor Beams
The fillet welds in the floor beam stiffeners are classified as detail category C. The continuous floor beams have bolted connections with the stringers, which are classified as type B. The floor beams in spans 4, 5 and 6 have bolted connections over the outside truss that are detail category B.
Stingers
The stringers are rolled sections, but there are welded shear studs on the top of the stringer that are classified as detail category C. The bolted connections at the end of the stringers are classified as detail category B. Truss
In spans 4, 5, and 6, some tensile truss members still have welded intermediate stay plates ( detail category E), although others were retrofitted with bolted (detail B) stays (Figure 3) and diaphragms. The compression members generally have welded perforated cover plates (detail B) and welded diaphragms (detail C).
In spans 3, 7, 8 and 9 there are some rolled sections and built up sections. Most of the tension members have welded intermediate stays that are 8" long and the thickness of the stays are less than an inch (detail E). Compression members generally have perforated cover plates with longitudinal fillet welds which are classified as type B, but the fillet welds to the transverse diaphragms are in detail category C.
Table 5. Detail Constants (2). Detail SFL (ksi) K A 8.8 68 B 5.9 33 B' 4.4 17 C 3.7* 12 D 2.6 6 E 1.6 2.9 E' 0.9 1.1 F 2.9 2.9 * Use 4.4 ksi for stiffeners
B. INFINITE FATIGUE LIFE CHECK
The infinite fatigue life check was performed in accordance with the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges (2) and results are given in Table 6. Most details that were checked showed an infinite fatigue life, so no further calculations were necessary for these details.
26 Table 6. Infinite fatigue life check.
Span Member Critical location Detail Gage Stress Rs*Sr Remaining Factor (ksi) Safe Life 2 BF FB, 24WF84 bolted connection with stringer B 1 1 2.30 infinite 2 BF stringer, 21WF55 shear studs, midspan C 2 1 1.73 infinite 2 BF stringer, 21WF55 shear studs, end C 2 1.86 3.21 infinite 2 BF girder sole plate to girder fillet weld E’ 3 1.25 1.80 finite (L> 4"), point B 2 BF girder toe of transverse stiffener, C 3 1.4 2.02 Infinite bottom, center of span 2 2 Main FB, 24WF84 midspan, bolted connection B 4 1 1.64 infinite with stringer 2 Main stringer, 21WF55 shear studs, midspan C 5 1 1.43 infinite 2 Main stringer, 21WF55 shear studs C 5 1.86 2.66 infinite 2 Main girder midspan of span 2, transverse C 6 1 1.78 infinite toe stiffener 2 Main girder sole plate to girder fillet weld E’ 6 0.9 1.60 finite (L> 4"), point B 3 truss L4-L6 (T) longitudinal fillet weld B 7 1 1.72 infinite 4 truss U0-L1 (T) bolted diaphragms and B 8 1 1.53 infinite intermediate stays 4 truss L3-L4 (T) welded intermediate stay E 9 1 1.83 finite plates 4 truss U4-U5 ( C ) diaphragm welds C 10 1 0.53 infinite 4 truss U8-U9 (T) bolted diaphragms and B 11 1 0.42 infinite intermediate stays 5 truss U9-L10 (T) bolted diaphragms and B 12 1 1.24 infinite intermediate stays 5 truss U13-L14 (T) bolted diaphragms and B 13 1 1.41 infinite intermediate stays 5 truss U14-U15 © ) diaphragm welds C 14 1 0.49 infinite 5 truss L14-L15 (T) bolted diaphragms and B 15 1 0.72 infinite intermediate stays 8 truss L19-U20 (T) longitudinal fillet weld between B 20 1 1.61 infinite top flange and web 9 truss U20-U21 (T) welded intermediate stays E 21 1 1.00 infinite 9 truss L25-L26 (T) welded intermediate stays E 22 1 1.74 finite 9 truss L29-U30 (T) rolled member with bolted end B 23 1 1.70 infinite 12WF92 connection 10 FB, 30WF99 bolted connection with B 24 2 0.93 infinite stringers 10 stringer, 21WF55 shear studs, midspan C 25 1 1.44 infinite 10 stringer, 21W55 shear studs, end C 25 1.86 2.68 infinite 10 girder midspan, toe of transverse C 26 1 1.12 infinite stiffener Notes : BF = Ramp from 787 north bound to I90 east bound. ML = Main line of bridge. FB = Floor Beam.
Rs = Reliability Factor Sr = Effective Stress in ksi
27 C. FINITE REMAINING FATIGUE LIFE
Four of the details have a factored effective stress greater than the limiting stress range, SFL, and the stresses exceeded tension limits, so the fatigue life is finite. The following equation was used to estimate the remaining safe fatigue life (2):
fK ×106 Yaf =−3 TCasr() RS where,
Yf = remaining fatigue life in years f = 1.0 for calculating remaining safe life K = detail constant found in (2) or in table 5
Ta = estimated lifetime average daily truck volume in the outer lane C = stress cycles per truck passage = 1 for the girder and truss spans
Rs = reliability factor
Sr = effective stress a = present age of the bridge in years = 36 years in 2004
The estimated lifetime average daily truck volume in the outer lane is:
TADTFFaTL= () = (70,787)(.094)(.45) = 3000 where:
ADT = average daily traffic volume in 1998 = .094 (Fraction of Trucks excluding panel, pickup, and other 2-axle/4-wheel trucks) FT = .45 (fraction of trucks in the outer lane ) FL
The remaining safe fatigue life in the weld between a girder and a sole plate for a floor beam was found to be 27 years (see Table 7). The remaining safe fatigue life for tensile truss members L3-L4 and L25-L26 was found to be 122 and 146 years, respectively. Neither of these two truss member has been retrofitted with bolted stays or diaphragms to replace the welded connections.
28 Table 7. Remaining safe fatigue life for critical details
Span Member Location Rs*Sr Detail K span Yf (ksi) (ft) (yrs) 2 BF girder sole plate to girder fillet weld ( L> 1.80 E’ 1.1 110 27 4"), point B 2 Main girder sole plate to girder fillet weld ( L> 1.60 E’ 1.1 110 53 4"), point B 4 truss L3-L4 welded intermediate stay plates 1.83 E 2.9 225 122 (T) 9 truss L25-L26 welded intermediate stay plates 1.74 E 2.9 170 146 (T) Note: BF = Ramp from 787 north bound to I90 east bound.
29
V. CONCLUSIONS
Infrastructure owners often need the estimated remaining fatigue life of bridges to make appropriate, cost-effective decisions regarding repair, rehabilitation, and replacement of existing bridges. This report describes a study, where the remaining safe fatigue life of the Patroon Island Bridge in New York State was estimated based on actual traffic conditions through continuous monitoring of the bridge during normal traffic loads.
The test data showed that most of the identified critical details had an infinite remaining fatigue life, except for sole plates on top of the girders in spans 1 and 2, and truss member in spans 4 and 9. The minimum remaining safe fatigue life was calculated to be 27 years at the weld between the sole plate and the girder top flange in span 2 , and the fatigue life of this detail can be increased with a bolted connection between the girder and sole plates. Both truss members have welded interior stay plates, which are detail category E, and the fatigue life of these welded truss members can be increased with bolted interior stays.
The fatigue life procedure used here has limitations related to strain measurement restrictions and the inability to estimate the fatigue life of damaged members. This analysis only applies to the undamaged components outlined in table 6. In some cases it was difficult to measure strain in critical regions, so effective stress interpolation was required. The floor beam connections to truss gusset plates in spans 4, 5 and 6 were not assessed due to excessive fatigue cracking, and the difficulties in measuring out of plane stresses. This detail needs to be repaired or replaced.
ACKNOWLEDGMENTS
George Schongar and Harry Greenberg of the Transportation Research and Development Bureau installed the gages and collected the data with assistance from other Department personnel. Arthur P. Yannotti and Dr. Mengisteab Debessay of the Structures Design and Construction Division provided required guidance and logistical support. Department personnel from Region 1 (Albany) provided the traffic crews, and snooper truck operators for the instrumentation and data acquisition. Arthur P. Yannotti, Rajesh Taneja, Francois Ghanem, and George Christian of the Structures Design and Construction Division reviewed the report.
REFERENCES
1. “Structural Integrity Evaluation of Patroon Island Bridge,” Interim Report, February 2002.
2. “Guide Specifications for Fatigue Evaluation of Existing Steel Bridges,” American Association of State Highway and Transportation Officials, Washington, D.C., 1990.
3. Downing, S. D., and Socie, D. F. “Simple Rain Flow Counting Algorithms,” International Journal of Fatigue, 4(1), January 1982, pp. 31-40.
4. “Standard Practices for Cycle Counting in Fatigue Analysis,” E1049-85, American Society for Testing and Materials, West Conshohocken, PA, 2004.
5. Moses, F., Schilling, C. G., and Raju, K. S. “Fatigue Evaluation Procedures for Steel Bridges,” Report 299, National Cooperative Highway Research Program, Transportation Research Board, Washington, D.C., November 1987.
6. “BDI Strain Transducer Specifications,” Bridge Diagnostics, Inc., Boulder, CO, 2001.
7. Fisher, J. “Bridge Fatigue Guide: Design and Detail,” American Institute of Steel Construction, New York, 1977.
8. “Standard Specification for Highway Bridges,” Sixteenth Edition (with 1999 Interim Revisions), American Association of State Highway and Transportation Officials, Washington, D.C., 1999.
9. Yen, B.T., Huang, T., Lai, L-Y, and Fisher, J. “Manual for Inspecting Bridges for Fatigue Damage Conditions,” Report No. FHWA-PA-89-022, Lehigh University, Bethlehem, PA, 1989.
Appendix A Histograms
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