Field tests of a strengthened timber trestle railroad bridge

Transcription

1 Field tests of a strengthened timber trestle railroad bridge Gutkowski, Richard M. 1, Shigidi, Abdalla M.T. 2, Peterson, Michael L. 3 ABSTRACT A comprehensive field testing study had been done in 1995 on a typical three span, open deck timber deck railroad bridge. The test program was done in cooperation with the Transportation Technology Center, Inc. of the Association of American Railroads. Analytical studies by elementary modeling methods as well as a "semi-continuous beam" model was done and comparisons made with field results. This three span right bridge was later strengthened by the addition of a stringer ply to each chord. The bridge was retested after the strengthening. An extensive empirical study of the load sharing in the plies of stringers was completed from results of each test. Comparisons of the stiffness of the bridge before and after the strengthening were made. Results of this field study are reported in this paper. The efficiency of the strengthening was also examined and showed it was between 82%-97% effective. INTRODUCTION During the recent 2-3 decades train loads in the United States, have increased significantly and higher required design loads for railroad bridges are under consideration. Numerous railroad bridges are of the open-deck, timber trestle type and have been in existence for many years, typically years or more. In 1994, the Association of American Railroads developed an interest in better understanding field behavior of these older bridges, as well as the effectiveness of strengthening them. In 1995, three open deck timber trestle railroad bridges were load tested in the field by Colorado State University (CSU) researchers working in collaboration with the AAR. The bridges were all located in Colorado. One of these bridges was strengthened in 1996 and tested again in This paper highlights the main observations made during the two tests of the strengthened bridge. STANDARD OPEN DECK TIMBER TRESTLE BRIDGE The design standard for timber railroad bridges (AREA, 1995) details the configuration of a typical open deck, timber trestle bridge (see Fig. 1 herein). Each steel rail is centered above a parallel "chord" consisting of 3 to 5 "plies" of stringers. The rails are supported by wood cross-ties spanning the chords. Pile bents, comprised of round wood piles and a solid sawn timber cap, provide intermediate supports. End piers are similar and have a timber retaining wall. Plies of the chords are either "spaced" with gaps of a 1-4 inches between adjacent plies or "packed" tightly together. Fig. 1. Typical Open-Deck, Timber Trestle Bridge 1 Professor, Dept. of Civil Engineering, Colorado State University, Ft. Collins, CO, USA 2 Graduate Assistant, Dept. of Civil Engineering, Colorado State University, Ft. Collins, CO, USA 3 Professor, Dept. of Mechanical. Engineering, University of Maine, Orono, ME

2 BRIDGE SITE The three bridges tested in the field were described in detail previously (Gutkowski et al. 1998,1999). The bridge pertinent to this paper is located in Pueblo, Colorado and is officially numbered Bridge No It is a 3 span bridge about 40 feet long, with an attached walkway on each side. Its spans are about 13, 14 and 13 feet. The bridge supports a slightly curved rail on 9 foot long wood ties (8.75 inches wide by 8.5 inches deep). Each chord had four packed stringers (6.5 inches wide by 15.5 inches deep). Plies were continuous over two spans but staggered by one span length. In each end span, every other ply was single span and simply supported. Caps were 13.5 inches wide by inches deep, 14 feet in length and supported by five round piles, each about 12 inches in diameter. Caps were lag screwed to the piles and chords were through-bolted to the caps. Plies were through bolted to each other near their ends. Main components were comprised of creosote treated Douglas fir timbers. Modulus of elasticity values were measured ultrasonically and reported previously (Gutkowski et al. 1998,1999). In 1996, the bridge was strengthened by the addition of an additional ply in each chord of each span. Plies were fitted on the outside, inserted between the ties and caps. A one-span ply and a two-span ply were used on each side of the bridge. The plies were the same material and size as the existing plies and placement was consistent with existing staggered ply pattern. The plies were added without re- centering the chord under the steel rails. FIELD LOADING PROCEDURES Track Loading Vehicle For conduct of the primary load tests of the original bridge, the AAR provided its track loading vehicle (TLV). The TLV is a train car fitted with hydraulic loading capability. It is accompanied by a locomotive and instrumentation car (IC). Controlled loading is applied to the two railroad tracks by actuators and a bogey located at its mid-point. The actuators lift the TLV and transfers part or all of its weight to the bridge at that location. Exploratory dynamic (sinusoidal) load testing using the actuator was also done and was reported before (Gutkowski et al. 1998,1999). In the 1997 retest of the strengthened Bridge 101, a typical locomotive and 3 car train without the TLV and IC were used. Static Loadings Static load tests were conducted on the original bridge only. The purpose of the static load tests was to study the effect of sequentially placing the static loads along the length of each bridge for comparison with subsequent rolling train tests. Various multi-point static loadings were achieved by positioning the 3 car, 12 axle (TLV bogey having been lifted off the track) test train at different locations. Positions were selected using qualitative influence diagrams for a 3 span continuous beam to determine approximate positions for maxima of response. Including repetitions, 108 static load positions were used. The total static train weight used was kips. Axle loads due to train weight and spacings used have been published (Gutkowski et. al. 1998,1999). Ramp Loadings By positioning the TLV at various locations, a ramp type loading was applied at these locations by loading and unloading its actuators. The existing and expected new design load levels for a single axle are 60 kips and 78 kips, respectively. Thus, load was increased to 78 kips and returned to zero. Data were recorded at 0, 30, 60 and 78 kip load levels. Rolling Train Loads Several pilot rolling train loads were conducted on the original bridge by passing the 3 car TLV test train over the bridge. Train velocity was less than 10 mph and was estimated by use of a stopwatch to time the train passing between two points on the bridge. More extensive and rigorous rolling train tests were done on the strengthened bridge. Tests were done with speeds of 2, 5, 10, 15 and 20 mph. Several passes were made at each velocity. Electronic triggers were used to activate data acquisition and simultaneously signal the location of the moving train at selected train locations. INSTRUMENTATION Static and Ramp Load Tests For the original bridge, displacement transducers were used to measure absolute (relative to the ground) vertical displacement of members and relative (between adjacent bridge components) vertical displacements. Extensometers were used to measure flexural deformation in the plies of stringers.

3 Rolling Load Tests The intent of the rolling train tests was to examine dynamic impact effects. In 1995, instrumentation used for the static and ramp load tests was left in place for the rolling train load tests. In the strengthened bridge, vertical displacement was measured using string potentiometers. Only absolute displacements were measured, but at considerably more locations. These provided for more extensive examination of load sharing among the plies, than had been done in the 1995 tests. ANALYTICAL STUDIES Chords are neither a sequence of simply supported spans nor a fully continuous, multi-span beam assembly. Lapped plies complicate the load path. Analytical work was not part of the work contracted with the AAR. However, some elementary investigations were done via thesis work (Robinson, 1998). These studies consisted of: 1) bounding individual span responses between fully pinned and fully fixed end beams and comparing to fully continuous 2 and 3 span beams, 2) use of a semi-continuous beam model; 3) preliminary modeling as a grid structure including discontinuous ply ends, staggered placement and mid-span links. The semi-continuous beam model condenses the grid model to fewer members by assembling similarly supported, similarly discontinuous plies within a chord into an aggregate member using average MOE and summed ply moments of inertia. Presently, a comprehensive space frame model is being applied to simulate the complete bridge. RESULTS Previously Published Findings A comprehensive technical report (Gutkowski et al. 1998,1999) and an M. S. thesis (Robinson, 1998) were prepared on the entire 1995 field test program. Typical displacement results for the 1995 static, ramp and rolling train tests have been published (Gutkowski, et al. 1997). Aspects of some exploratory dynamic excitation tests were also included. Load sharing in plies and preliminary analytical modeling were described in subsequent papers (Gutkowski, et al. 1998,1999). This paper focuses on additional load share aspects and dynamic impact effects related to the consequences of the strengthening of the bridge. Results of the analytical modeling have been published (Gutkowski, et al. 1999) and are not addressed herein. A technical report on the 1997 is in progress (Gutkowski, et al. 1999). Quasi-Static Response For the 1997 tests, the differences between the transient responses for the 2 mph and 20 mph train velocities were small, or non-existent. For example, Figs. 2 and 3 show a typical difference in transient response at a selected location. Quantitative dynamic impact effects are described subsequently, but were predominantly negligible. Thus 2 mph data was taken as a basis for extracting "quasi-static" results for purpose of comparison with the 1995 data for static loads. Bridge Section AD - Position 3 Northerly Train Direction - Velocity = 2 mph Stringers 1,10 - Continuous Span Bridge Section AD - Position 3 Northerly Train Direction - Velocity = 20 mph Stringers 1,10 - Continuous Span Vertical Stringer Displacement, (inches) Vertical Stringer Displacement, (inches) Time, (seconds) Stringer #1 Stringer # Time, (seconds) Stringer #1 Stringer #10 Fig. 2. Transient Response at 2 mph Fig. 3. Transient Response at 20 mph Specifically, the response for any particular instant in time for the train rolling at 2 mph was considered as essentially a static load response. Four positions of the 2 mph train are presented herein. Fig. 4 shows the measured absolute displacement patterns for the loadings. The loadings are termed Load Case 1@2 through Load Case 4@2 ("@2" inferring "at 2 mph"). The

4 results for three passes of the train were averaged. The mean value of the plies in each span are plotted. The axles present on the bridge corresponding to each loading are identified by labels LC1, LC2, LC3 and LC4, respectively. One potentiometer gave suspect data, so plots are shown both with and without that data. Fig. 4 provides a visual sense of the changing response. It is also evident that measurable support motion occurred at the intermediate piers, as was the case in the original bridge. Mean Displacements 1997 Test Load Cases 1@2, 2@2, 3@2, & 4@2 mph LC3 LC3 LC4 LC4 LC3 Vertical deflection relative to ground, (in) A B C D E F G H I J Test - LC 1@2 mph LC 1@2 mph - excluding suspected data 1997 Test - LC 2@2 mph LC 2@2 mph - excluding suspected data 1997 Test - LC 3@2 mph LC 3@2 mph - excluding suspected data 1997 Test - LC 4@2 mph LC 4@2 mph - excluding suspected data LC2 Fig. 4. Response to Pseudo Static Loadings Fig. 5 (6) shows the plotted results from Fig. 4 for Load Case 1@2 mph (Load Case 3@ 2mph) compared to the corresponding Load Case 1@ 20 mph (Load Case 3@20 mph). In the former case, the results are coincident. In the latter case, they are nearly so. This further exemplifies the evident absence of noticeable dynamic impact effect for the train speeds investigated. LC2 LC1 LC1 Vertical deflection relative to ground, (in) Test - Load Case 1 (Load 2mph vs. 20mph A B C D E F G H I J LC 1@2 mph Test LC 1@20 mph Test LC 1@2 mph Test - excluding suspected data 59,677 lbs 59,677 lbs Vertical deflection relative to ground, (in Test - Load Case 3 (Load 2mph vs. 20mph A B C D E F G H I J 59,677 lbs 59,677 lbs 60,438 lbs LC 3@2 mph Test LC 3@20 mph Test LC 3@2 mph Test - excluding suspected data Fig. 5. Load Case 2 mph vs. 20 mph Fig. 6. Load Case 2 mph vs. 20 mph Efficiency of the Retrofit To make an equivalent comparison between the 1995 results and the 1997 results, static load positions reasonably similar to the 1997 quasi-static load positions were identified. The 1995 displacements for the counterpart static loadings were modified. First, the displacements resulting from the August 1995 static load test were scaled by the ratio of the two different train loadings (sum of axle loads on the span of interest) in order to compare the displacements of the bridge as if it were under the same load before and after retrofit. These are referred to as the load-adjusted 1995 values. The load-adjusted 1995 values were then decreased by a factor of 4/5 to account for the additional stringer ply added in each chord. The resulting "adjusted" 1995 measured values gave an estimate of what the 1997 measured values would be after the retrofit, if the added ply was essentially fully effective.

5 For the 1995 tests only the mid-span displacements for the end spans were measured for the four counterpart static load positions. Fig. 7 shows the mean displaced shape for 1997 Load Case 1@2 mph. The plot is done both including and excluding the suspect data for ply 4 in Span 2, with no noticeable difference evident. The actual mean displacements for the counterpart 1995 static load case (termed Load 1@H in the report on that work (Gutkowski, et al. 1998)) were available at two midspan locations. These values are plotted: a) after adjusting for the difference load level for the two tests and b) after further adjusting for the additional ply. Fig. 8 shows the same information for Load Case 3@2 mph vs. the corresponding 1995 static load case Load 1@B. In this case, excluding the suspect ply 4 data in Span 2 makes a noticeable difference. Indeed, the results suggest the suspect data is erroneous as the adjusted shape has curvature that better matches the sense of the loads. The efficiency of the bridge retrofit was calculated by dividing the 1995 fully adjusted mean mid-span displacements by the absolute 1997 measured mean mid-span displacement. The calculated efficiencies for the north end span 1 were, 92.7% and 96.8% for Load Cases 1@2 mph and 3@2 mph, respectively. The efficiency of the south end span for Load Case 3@2 mph was calculated as 82.4%. Based on these computations, the average efficiency of the bridge retrofit is 91%. Vertical deflection relative to ground, (in) Test vs Test Load Case 1 (Load 1@H) A B C D E F G H I J Mean Displacements - 2 mph Test Mean Displacements Test (original) Mean Displacements Test (only load-adjusted) Mean Displacements Test (load and ply -adjusted) Mean Displacement - 2 mph Test - excluding suspected data 59,677 lbs 59,677 lbs Fig. 7. Load Case 1- Efficiency of Retrofit Vertical deflection relative to ground, (in) Test vs Test Load Case 3 (Load 1@B) A B C D E F G H I J 59,677 lbs 59,677 lbs 60,438 lbs Mean Displacement 2 mph Test Mean Displacements Test (original) Mean Displacements Test (only load-adjusted) Mean Displacements Test (load and ply-adjusted) Mean Displacement 2 mph Test - excluding suspected data Fig. 8. Load Case 3- Efficiency of Retrofit Empirical computation of load share between plies For the displacement data it was assumed that the load share (LS) in a ply was related to the measured (relative to the displaced chord) mid-span displacement, D, according to LS = f(c x D x E x I / L 3 ) (1) where E is the measured MOE, I is the moment of inertia, and L is the span length. C is constant depending on the loading and end support conditions. For a simply supported ply under a single midspan load, C would be 48. For a two-span continuous ply, C is about 67, i.e. 40% stiffer than a single span ply. If all plies are the same size and span and loaded and supported identically, C and I and L are identical and load share is proportional only to D and E. For interior spans all plies are so identical. As L was the same for all spans and I was constant, empirical ply load shares in a span were calculated by weighting each measured ply displacement by the corresponding measured E value and the relative C value (1.0 vs. 1.4) and proportioning the resulting values to their sum. Using flexural strain data, it can be shown that LS = f(c' x FS x E x S / L) (2) where FS is the measured flexural strain and S is the section modulus. C' is a constant depending on loading and end support conditions. For a simply supported ply under a single midspan load, C would be 4. For identical ply geometry and load

6 conditions, load share is proportional only to FS and E. Thus, a similar proportioning of weighted strain data was done to determine approximate empirical ply load shares. Load shares for the 1995 TLV loading For TLV actuator at midspan of the middle span (loading 11@F in the reports (Gutkowski, et al. 1998,1999), the resulting load shares for the middle span are shown in Fig. 9. Ideally, the ply load shares would be 12.5% of the total span load (25% of the total load for one chord). Based on the empirical procedure for deflection data measured relative to the displaced chord, they ranged from 8.8% to 17.4%. Within reasonable expectation, the strain data gave similar findings (range of 8.4%-18.1%). For end spans, some plies are simply supported and others are two span continuous, so C and C' differ for these two ply conditions. Treatment of that situation has been reported based on deflection data from Bridge No. 101 (2,3). Results showed load shares were between 12.0% and 15.5% of the total span load. Calculated Percentage of Total Load Based on Measured Deflection and Strain Ply Contribution (%) Deflection Strain Ply From East Fig. 9. Example of Load Sharing Based on Deflection vs. Strain Data Load Sharing for Quasi-Static Loads-1997 Tests In the 1997 tests, all displacements were measured relative to the ground and the effect of end support displacements is embedded in the results. Hence, the empirical load share calculation is not strictly applicable. This complexity must be neglected in applying the empirical procedure to that data. However, for the 1995 tests, the calculated load share for data for the two references ("relative to the ground" and "relative to the displaced chord") were somewhat similar, as shown subsequently. The displacements for four quasi-static load cases from the 1997 tests were used to calculate the empirical load share values. Only the middle span had all plies instrumented in both chords. Thus, for that span the load share values were calculated for both chords. In the end spans, only the East chord was monitored, so results exist for that chord only. The overall results produced appear reasonable, in that individual plies shared between 5.5% and 17.5% of the span loading. If all plies share load equally, they all would be at 10%. If the suspect ply 4 data are excluded in the middle span, the actual load share cannot be determined for the East chord. Instead "pseudo-load share values" were calculated for the other plies. These were determined by ignoring ply 4 and calculating the load share based on weighting the resistances of the other 4 plies, relative to the total of their resistances. These values were then multiplied by 4/5. The ply load shares ranged between 5.5% and 16%, including pseudo-values the East chord. Comparison between 1995 and 1997 Results In 1995, for the East chord of the south end span, load share values were based on displacements measured relative to the ground. The 1995 values for the TLV loading and the 1997 load shares for the 2 mph speed were consistent. As expected, considering all plies, the 1997 values (7.5%-13%) were about 4/5 of the 1995 values (10.5%-17%). The new plies carried

7 somewhat less load share then the old plies. The 1995 values for the West chord (10%-17.5%) were essentially the same as those for the East chord (10.5%-17%). For the middle span, based on data measured relative to the displaced chord, the 1995 load share values for the East chord were essentially the same for both the static train loads (10%-15.5%) and the TLV loads (9.5%-15.5%). For the West chord, the corresponding ranges of values (8.5%-17% vs. 9.5%-16%) were close and the average values are the same. For the middle span, based on data referenced to the displaced chord, the 1995 TLV load share values (9.5%-15.5% East chord, 9.5%-16% West chord) were slightly less than the 1995 values for the north span (10%-17% East chord, 10%-17.5% West chord) based on data referenced to the ground. The difference between the load shares for two measurement references is modest. The 1997 load share values are all based on data referenced to the ground. For the middle span, the pseudo-load share values (5%-16%) for the East chord were similar to the actual empirical values (6%-13.5%) for the West chord. In the East Chord, the range is not close to being 4/5 of the 1995 range. The lower (upper) extreme is about 1/2 (1/1) of the 1995 value. For the West chord, the upper extreme is about 4/5 of the 1995 value. The lower extreme is only about 3/5 of the 1995 value. This suggests the pseudo load shares for the East chord may not be dependable. The caveat is that the data reference differs for each set of test data. For the south end span, the 1995 load share ranges for the TLV load differed for each chord (12%-14% East chord, 8.5%-15.5% West chord), but the average results were close. The 1997 load share ranges for the East chord (5.5%-14.5%) had a much wider range than for the 1995 TLV load values (12%-14%). On average, the 1997 load share values were about 3% lower than the 1995 results. Comparing the 1995 values for the south end span (12%-14% East chord, 8.5%-15.5% West chord) the north end span (10.5%-17% East chord, 10%-17.5% West chord), the latter had a much wider range of values for the East chord. For the West chord, the ranges were similar for the two spans. Based on displacement data measured relative to the ground in the 1995 TLV tests, the empirically calculated load share for individual plies in the 4 ply chords was between 8.5%-17%. This is stated without regard to ply continuity and location. Thus, the ideal value of 12.5% per ply was exceeded by as much as 4.5%. Based on quasi-static loads and displacement data measured relative to the ground reference in the 1997 tests, the empirically calculated load shares for the 5 ply chords ranged between 5-16%. This is stated without regard to ply continuity and location. Ignoring, the pseudo-load shares calculated for some plies, the range was %. These latter values are reasonably consistent with an expectation they would be about 4/5 of the 4 ply chord values in Thus, excluding the pseudo load share values, the ideal value of 10% was exceeded by as much as 4.5%. For the 1995 TLV loadings tests, the load shares in the end spans for ground referenced ranged between 8.5%-17.5% for a four ply configuration. The load shares for the displaced chord referenced data in the middle span ranged between 9.5%-16%. The two are close in range and in average. The load shares in the middle span for the static train loads, for displaced chord referenced data, ranged between 8.5-%17%. It is surmised that support motions was sufficiently small so as to not affect the empirical calculation of load shares. In the 1997 tests, the load shares for ground referenced data in the end spans for the 2 mph train ranged between % for a 5 ply configuration. The load shares for the ground referenced data for middle span for the TLV loadings ranged between 6%-13.5%, ignoring the pseudo load share for the East chord. The two are close in range and in average. They were close to being 4/5 of the ranges observed for the original bridge. This furthers the observation that support motion was sufficiently small as to not affect the empirical calculation of load shares. Based on the 1997 empirical results, it appeared a ply in a 5 ply chord can have a maximum load share of about 16% vs. the nominal equal share of 10%. Based on the 1997 data, single span plies carried less load share (maximum of 11%, typically less than 10%) than two-span continuous plies (max of 16%, typically greater than 10%).

8 CONCLUSIONS The empirical procedure for calculating ply load share produced rational results. Cautiously, based on the 1995 load share data vs. the 1997 load share data, it appears that support motions were not significant enough to make the measurement reference a factor in the observations about load share values. An individual ply in a 4 ply (5 ply chord) chord can carry a maximum of 17.5% (14.5%) share of the chord loading. Dynamic transient responses in 1997 for 2 mph and 20 mph velocities were predominantly the same. The additional plies performed at an efficiency level between 82-97%. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support received from the U. S. Department of Transportation via the Mountain Plains Consortium which is federally sponsored through the University Transportation Centers Program. Funding was also provided by the Association of American Railroads and the Union Pacific Railroad. Dr. Duane Otter, Ms. Diana Oliva and other personnel of the Transportation Test Center in Pueblo, Colorado were directly involved in the planning and conduct of the field tests on. Cost share funds provided via CSU are also appreciated. REFERENCES American Railway Engineering Association 1995 Manual for Railway Engineering, Chapter 7 Timber Structures, AREA, Washington, D.C. Gutkowski, R. M., Robinson, G. C. and Peterson, M. L Field testing of old timber railroad bridges, Proc. of 7th Int. Conf. and Exhibition, Structural Faults and Repair 1997, ed. M. C. Forde, (1) Engineering Technics Press, Edinburgh Gutkowski, R.M., Peterson, M. L. and Robinson, G.C., Field studies of timber railroad bridges - Contract Number TRAC , technical report submitted to the Association of American Railroads, Transportation Technology Center, Inc., Pueblo, CO. Gutkowski, R. M., Robinson, G. C. and Peterson, M. L Field load tests of timber railroad bridges under static and ramp loads, Proceedings of the World Conference on Timber Engineering, Montreux-Lausanne, Switzerland, (2), (Presse Polytechnique. Universitaires Romande-EPFL, Lausanne, Robinson, G.C Field testing open-deck timber trestle railroad bridges, M. S. thesis, Dept. of Civil Engineering, Colorado State University, Ft. Collins, CO. Gutkowski, R. M., M. L. Peterson, G. C. Robinson, S. Uppal, D. Oliva-Maal and D. Otter Field load testing and modeling of a strengthened timber trestle railroad bridges, Proceedings, CMEM 99, 9th International Conference on Computational Methods and Experimental Measurements, April, 1999, Sorrento, Italy. Wessex Institute of Technology, Southampton, Gutkowski, R, Brown, K., Doyle, K. and Peterson, M Load Testing of Rehabilitated Timber Railroad Bridges. Proc. of 8 th Int. Conf. and Exhibition, Structural Faults and Repair (London), ed. M.C. Forde, Engineering Technics Press, London. Gutkowski, R., Robinson, G., Peterson, M. and Tran, A Field testing of open-deck timber trestle railroad bridges. Proceedings of 1 st RILEM Symposium on Timber Engineering (Stockholm), RILEM Publications s.a.r.l, Cachan Ceder, France, pp Gutkowski, R.M., M. L. Peterson, G. C. Robinson, S. Uppal, D. Oliva-Maal, and D. Otter., Field studies of timber railroad bridges, R-933, Association of American Railroads, Transportation Technology Center, Inc. Pueblo, CO. Gutkowski, R., Shigidi, A., Tran, V-A. and Peterson, M Field studies of timber railroad bridges II contract No. TRAC , technical report submitted to the Association of American Railroads Transportation Technology Center, Inc., Pueblo, CO. (under preparation).