Field Performance of Timber Bridges

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components force level of stressing bars and behavior of the bridge under static load conditions In addition comprehen sive visual inspections were conducted to assess the overall condition of the structure Based on field evaluations the bridge is performing well Keywords Wood bridge stress laminated T timber performance Acknowledgments We thank the following from the Hinds County


Abstract Contents, The North Siwell Road bridge was constructed during De Page. cember 1994 in Hinds County Mississippi The bridge is a. Introduction 1, single span stress laminated T beam structure that measures. 9 1 m 30 ft long and 8 7 m 28 5 ft wide Performance of Background 1. the bridge was monitored for 24 months beginning at the. Objective and Scope 2, time of installation Monitoring involved gathering and. evaluating data relative to the moisture content of the wood Design Construction and Cost 2. components force level of stressing bars and behavior of the. bridge under static load conditions In addition comprehen. sive visual inspections were conducted to assess the overall Construction 4. condition of the structure Based on field evaluations the. Assembly 4,bridge is performing well,Installation 4. Keywords Wood bridge stress laminated T timber,performance.
Evaluation Methodology 5,Acknowledgments Moisture Content 7. We thank the following from the Hinds County Department Bar Force 7. of Public Works for assistance with field data collection load. tests and visual inspection of the North Siwell Road bridge Load Test Behavior 7. Wayne Williamson Vonda Moore and the bridge crew Load Test 1 8. In addition we express sincere appreciation to the following Load Test 2 9. from the USDA Forest Service Forest Products Laboratory Predicted Deflection Analysis 9. Mike Ritter James Wacker Paula Hilbrich Lee and Lola. Hislop for assisting in the field evaluation data analysis Condition Assessment 9. and report preparation Vyto Malinauskas for fabricating Results and Discussion 9. load cells and the Information Services Team for assistance. in preparing this report Moisture Content 9,Bar Force 10. March 1998 Load Test Behavior 10,Load Test 1 10, Kainz James A 1998 Field performance of timber bridges 16 North. Siwell Road stress laminated bridge Res Pap FPL RP 570 Madison WI Load Test 2 12. U S Department of Agriculture Forest Service Forest Products Labora. tory 17 p Load Test Comparison 12, A limited number of free copies of this publication are available to the. public from the Forest Products Laboratory One Gifford Pinchot Drive Predicted Deflection Analysis 12. Madison WI 53705 2398 Laboratory publications are sent to hundreds of. libraries in the United States and elsewhere Condition Assessment 12. The Forest Products Laboratory is maintained in cooperation with the Bridge Geometry 12. University of Wisconsin, The use of trade or firm names is for information only and does not imply Wood Condition 14.
endorsement by the U S Department of Agriculture of any product or. service Preservative Treatment 14, The United States Department of Agriculture USDA prohibits discrimi Wearing Surface 14. nation in its programs on the basis of race color national origin sex. religion age disability political beliefs and marital or familial status Not Anchorage System 14. all prohibited bases apply to all programs Persons with disabilities who. require alternative means for communication of program information Conclusions 14. braille large print audiotape etc should contact the USDA s TARGET. Center at 202 720 2600 voice and TDD To file a complaint write the References 16. Secretary of Agriculture U S Department of Agriculture Washington. DC 20250 or call 1 800 245 6340 voice or 202 720 1127 TDD Appendix Information Sheet 17. USDA is an equal employment opportunity employer,Field Performance of Timber Bridges. 16 North Siwell Road Stress Laminated Bridge,James A Kainz General Engineer. Forest Products Laboratory Madison Wisconsin, Introduction grant from the NWITIC and matching funds from Hinds. County Mississippi An information sheet on the character. In 1988 the U S Congress passed legislation known as the istics of the North Siwell Road bridge is provided in the. Timber Bridge Initiative TBI USDA 1995 As part of an Appendix. effort to revitalize rural America the legislation established a. national program emphasizing wood as a structural material. for highway bridges Recently the emphasis of this program Background. has shifted to include all timber transportation structures The North Siwell Road bridge is located in Hinds County. such as sound barriers and bridges approximately 7 miles southwest of Jackson Mississippi. Fig 1 The bridge is owned by Hinds County and is, Responsibility for the development implementation and.
located on a secondary road that provides access to local. administration of the TBI program was assigned to the. residences and schools In 1994 the average daily traffic over. USDA Forest Service The Forest Service established three. this section of the road was 6 520 vehicles Estimates by. primary program areas under the TBI demonstration struc. Hinds County Department of Public Works indicate that the. tures technology transfer and research As part of the dem. average daily traffic is expected to increase to approximately. onstration bridge program the National Wood in Transpor. 9 300 vehicles by the year 2012 The majority of this traffic. tation Information Center NWITIC in Morgantown West. consists of passenger vehicles and school buses, Virginia awards annual grants for demonstration bridges on. a competitive basis Funds are awarded for design and con. The original North Siwell Road bridge was 7 m 23 ft wide. struction of demonstration timber bridges with innovative. and 6 1 m 20 ft long and consisted of prefabricated concrete. designs and those that utilize locally available underutilized. deck panels on timber caps and piles Based on an annual. wood species The NWITIC also maintains a technology. inspection in 1992 it was determined that the wood compo. transfer program to provide assistance and state of the art. nents were deteriorating and the concrete deck was in need of. information related to all aspects of timber bridges. repair Therefore replacement of the bridge was deemed. Responsibility for the research portion of the TBI program necessary to provide adequate access for school traffic emer. was assigned to the USDA Forest Service Forest Products gency vehicles and commuters. Laboratory FPL The FPL has established a broad research. program to conduct a variety of timber bridge studies under In December 1992 the Hinds County Department of Public. laboratory and field conditions including a nationwide Works submitted a proposal for partial funding of the North. bridge monitoring program Through the bridge monitoring Siwell Road bridge replacement as a timber bridge under the. program FPL is able to collect analyze and distribute NWITIC demonstration program As part of this proposal a. information on the field performance of timber bridges to preliminary bridge design was developed for a timber stress. provide a basis for validating or revising design criteria and laminated T beam bridge Southern Pine was selected as the. further improving efficiency and economy of timber bridge primary material for the bridge because of economics and as a. design fabrication and construction locally grown species in Mississippi After review by a. selection panel funds were awarded and final design of the. This report is 16th in a series of reports that documents the replacement bridge was initiated Hinds County had little. field performance of timber bridges It describes the develop experience working with timber bridges therefore FPL was. ment design construction and field performance of the contacted during the final design phase of the project to. North Siwell Road bridge located in Hinds County Missis evaluate the performance of the new North Siwell Road. sippi The structure is a 9 1 m 30 ft long double lane timber bridge As a result personnel from FPL and Hinds. stress laminated T beam bridge Built in 1994 the North County Department of Public Works developed a perform. Siwell Road bridge was funded jointly by a competitive ance monitoring plan that was initiated at installation. moisture content stressing bar force bridge behavior under. static truck loading and general structure performance The. results of this project will be considered with the results of. similar monitoring projects to improve design and construc. tion for future stress laminated T beam bridges,Design Construction and Cost. Design and construction of the North Siwell Road bridge. were completed by several agencies and individuals An. overview of the design construction and cost of the bridge. superstructure follows, A typical stress laminated T beam bridge consists of glued. laminated timber glulam webs and sawn lumber flanges. that are connected by tensioned high strength steel stressing. elements Fig 2 On the North Siwell Road bridge an, American Association of State Highway and Transportation. Officials AASHTO approved method for designing a stress. laminated T beam structure was not available so design. criteria for the stress laminated T beam bridge was based on. guidelines developed from research conducted at West. Virginia University Davalos and Salim 1993 All other. aspects of design including loading wood treatment and. wood strength were designed in accordance with the. AASHTO Standard Specifications for Highway Bridges. for two lanes of HS20 44 truck loading AASHTO 1992. Design and fabrication of the North Siwell Road bridge were. completed by contract The design geometry provided for a. single span simply supported structure 9 1 m 30 ft long. and 8 7 m 28 5 ft wide Fig 3 The new bridge was de. signed to be 3 1 m 10 ft longer than the original bridge. because the increased span length allows additional hydraulic. flow capacity that is required during periods of high water. The North Siwell Road bridge configuration consists of. Southern Pine glulam lumber webs and sawn lumber flanges. The glulam webs were 171 5 mm 6 3 4 in wide and, 593 7 mm 23 3 8 in deep Glulam design was based on.
material properties for combination 24F V3 SP SP,AASHTO 1992 Tabulated design values were 16 5 MPa. 2 400 lb in2 for bending strength 12 4 GPa, Figure 1 Location maps for the North Siwell Road 1 800 000 lb in2 for modulus of elasticity MOE 1 4 MPa. bridge 200 lb in2 for shear strength and 3 9 MPa 560 lb in2 for. compression strength perpendicular to grain All design. values were adjusted by appropriate wet use factors per. Objective and Scope AASHTO requirements, The objective of this project was to evaluate the field per The sawn lumber flanges were constructed with standard. formance of the North Siwell Road bridge for 24 months 38 by 184 mm nominal 2 by 8 in material that was. beginning at bridge installation The scope of the project surfaced on four sides to provide uniform contact between. included data collection and analysis related to the wood laminations Butt joints were used in the flanges because. material was not available in the full 9 1 m 30 ft length. The butt joints were specified at an interval of one butt joint. every four adjacent laminations longitudinally spaced at. 1 2 m 4 ft Fig 4 The design values for the flange were. based on AASHTO specifications for Southern Pine lumber. visually graded No 1 in accordance with Southern Pine. Inspection Bureau rules SPIB 1993 The tabulated design. values were 10 3 MPa 1 500 lb in for bending 11 7 GPa. 1 700 000 lb in for MOE 620 kPa 90 lb in2 for shear. and 3 9 MPa 565 lb in2 compression perpendicular to. Figure 2 Typical configuration for a, stress laminated T system grain All design values were adjusted by the appropriate. wet use factors and laminations were specified to be at or. below 19 moisture content prior to preservative treatment. and bridge installation, The stressing system used ten 15 9 mm 5 8 in diameter.
high strength steel bars that complied with the requirements. of ASTM A722 ASTM 1988 and provided a minimum, ultimate tensile strength of 1 03 GPa 150 000 lb in2 The. bars were inserted through oversized predrilled holes located. at the center of the sawn lumber flange and 92 mm 3 5 8 in. from the top of the glulam webs Fig 3 The bars were. spaced 914 mm 36 in on center starting 457 mm 18 in. from the ends of the bridge Each bar required a tensile force. of 156 kN 35 000 lb to provide 924 kPa 134 lb in2 com. pression stress between the laminations The value for inter. laminar compression was based on the West Virginia. University design method in which the recommended design. interlaminar stress is 2 5 times the minimum interlaminar. stress of 365 kPa 53 lb in2 The effect of this initial pre. stress is examined in detail in the following section on bar. force The bar tension was transferred into the deck using a. discrete plate anchorage system consisting of 184 by 394. by 25 4 mm 7 25 by 15 5 by 1 in bearing plates and a. 50 8 by 127 by 25 4 mm 2 by 5 by 1 in anchor plate. with a hexagonal nut Fig 5, Design of the bridge rail and curb system was based on a. crash tested system for longitudinal spike laminated decks. FHWA 1990 The bridge rail and curb consisted of a 267. by 152 mm 10 6 by 6 in sawn lumber timber rail and a. 140 by 292 mm 5 5 by 11 5 in sawn lumber curb with. 140 by 292 mm 5 5 by 11 5 in sawn lumber scupper, blocks The rail and curb were attached to six 1 370 by. 190 by 305 mm 54 by 7 5 by 12 in sawn lumber posts. For protection from deterioration all steel components. including stressing hardware stressing bars and anchorage. plates were galvanized in accordance with AASHTO M232. AASHTO 1992 All wood components were preservative, Figure 3 Design configuration of the North Siwell treated with creosote in accordance with American Wood. Road bridge Preservers Association standard C14 AWPA 1990 No. asphalt wearing surface was specified for the bridge. Figure 4 Butt joint configuration used with Southern. Pine sawn lumber flange laminations on the North, Siwell Road bridge A butt joint was placed transverse.
to the span in every fourth lamination Longitudinally. butt joints in adjacent laminations were separated by. 1 2 m 4 ft,Figure 5 Discrete plate anchorage configuration. consisting of a bearing plate anchor plate and,hexagonal nut. Figure 6 Assembled L and T sections used,for transportation to bridge site and ease of. Construction installation, Construction of the North Siwell Road bridge was completed. in late 1994 Construction extended over several months and sections were loaded on a flatbed truck and shipped to the. included assembly that was completed by the bridge manu bridge site. facturer and installation that was completed by Hinds County. personnel Installation, After demolition and removal of the existing bridge super.
Assembly structure a new substructure was installed which consisted. The assembly process began with manufacturing the glulam of timber piling wing walls and caps The new substructure. beams and drying and surfacing of the sawn lumber flange was installed behind the existing substructure to provide. material Following manufacturing and drying all sawn additional erosion protection for the new substructure. lumber and glulam members were prefabricated cut and Approximately 0 5 to 0 7 m 2 to 3 ft from the top the. drilled and pressure treated with creosote preservative After existing substructure was removed to provide clearance for. preservative treatment the treated wood was fabricated into the new bridge. sections to facilitate shipping and installation These sec. tions were made by nailing flange pieces to the glulam webs On December 20 1994 installation of the new superstructure. to form either an L or T module Fig 6 The L module was commenced by lifting one assembled L section into place on. used on the exterior of the bridge and the T configuration the abutments with an overhead crane After several T sec. was used for the interior After assembly the L and T tions were in place next to the L section the high strength. steel bars were inserted through predrilled holes in the flange. and webs All sections were placed in a similar manner until. the final L section was placed on the opposite end After all. sections were placed and the stressing bars were fully in. serted the steel bearing and anchorage plates were installed. A single hydraulic pump and jack was used to partially. tension the steel stressing bars to bring the T and L sections. into contact After all sections were in contact full design bar. force was then introduced into the bridge The Hinds County. bridge crew began at one end and tensioned each bar along. the length of the bridge Fig 7 After all bars were fully. tensioned the tensioning process was repeated to ensure that. the interlaminar compression level was uniform and at the. required design level Seven days after this initial tensioning Figure 7 Bar tensioning using a single hydraulic. a second design tensioning was introduced into the steel pump and jack. bars The third tensioning was completed January 24 1995. approximately 5 weeks after the initial stressing, After the second bar tensioning the bridge was attached to. the substructure by connecting each glulam web to the tim. ber cap with steel angles bolted to the webs and abutment. cap Fig 8 At this time the timber curb and rail system. was also installed Fig 9 When the rail system was com. plete a tack coat of asphalt and coarse rock was applied as a. wearing surface and the bridge was opened for traffic The. bridge was posted with a reduced speed limit because the. approach roadway was very rough The completed bridge is. shown in Figure 10,Figure 8 Bridge attachment to the sawn lumber. The total cost of the North Siwell Road bridge superstructure substructure with steel angle saddles. was 46 190 which included design fabrication materials. and construction To compare the cost of this bridge with. others the total cost of the bridge is divided by the total. deck area to ascertain a cost per unit area This equates to. approximately 582 m2 54 ft2 for the North Siwell Road. Evaluation Methodology, To evaluate the structural and serviceability performance of. the North Siwell Road bridge Hinds County personnel. contacted the FPL for assistance Through a cooperative. agreement with Hinds County FPL and Federal Highway. Administration FHWA a 2 year bridge monitoring plan. was developed and implemented The plan included perform. ance monitoring of the deck moisture content stressing bar Figure 9 Attachment of bridge curb and rail system. force static load test behavior and general bridge condition following the second bar tensioning. The evaluation methodology employed procedures and. equipment previously developed by FPL and used on other. similar structures Ritter and others 1991, Figure 10 Completed North Siwell Road bridge top side view bottom end view. Moisture Content, To characterize changes in moisture content an electrical.
resistance moisture meter was used to obtain wood moisture. content readings on a quarterly basis Moisture meter meas. urements were taken by Hinds County personnel from the. bottom of glulam beams and the underside of the sawn lum. ber flanges and were assumed to be representative of the. overall moisture content of the bridge Measurements were. obtained in accordance with ASTM D 4444 84 ASTM, 1992 by driving the insulated moisture pins into the. underside of the bridge at depths of 50 to 75 mm 2 to 3 in. recording the moisture content value from the unit then. adjusting the moisture content value for temperature and. wood species, To monitor stressing bar force two calibrated load cells were. placed on the stressing bars just prior to the third stressing. These cells were placed between the bearing and anchorage. plates to monitor the bar forces based on the strain variations. in the load cell On a monthly basis load cell measurements. were obtained by Hinds County personnel with a portable. strain indicator The measurements were then converted to. force based on laboratory load cell calibrations to determine. the tensile force in the bar At the conclusion of the monitor. ing period the load cells were removed adjusted for zero. balance shift and re calibrated in the laboratory In addition. hydraulic stressing equipment was used at the site visits to. verify bar force levels obtained from the load cells. Load Test Behavior, To determine the live load behavior of the bridge under. vehicle loading two static load tests were conducted during. the monitoring period The first load test was completed. immediately following the third stressing on January 24. 1995 The second load test was completed January 28 1997. 2 years after the first load test, The static load test consisted of positioning one or two fully. loaded trucks on the bridge then measuring the resulting. deflections along the transverse centerline of the bridge. Deflection measurements from an unloaded to loaded condi. tion were obtained by hanging calibrated rules on the under Figure 11 Transverse load positions looking north. used for all static load tests on the North Siwell Road. side of the deck and reading values with a surveyor s level bridge. The accuracy of this method for repetitive readings is. estimated to be 1 mm 0 04 in, For both load tests the trucks were positioned for six trans on the bridge in the same positions as load positions 1 and.
verse load positions three centric and three eccentric 2 The fourth and fifth load positions placed one vehicle on. Fig 11 The first and second load positions placed one the bridge near each rail The sixth load position placed both. vehicle on the bridge near the longitudinal centerline in vehicles on the bridge in the same positions as load posi. opposite lanes The third load position placed both vehicles tions 4 and 5 Fig 12. Figure 12 Transverse load test positions used for both load tests a load position 1 0 6 m upstream of longitudinal. center line b load position 2 0 6 m downstream of longitudinal center line c load position 3 both trucks in same. locations as load positions 1 and 2 d load position 4 1 5 m upstream of longitudinal center line. e load position 5 1 5 m downstream of longitudinal centerline f load position 6 both trucks in same locations. as load positions 4 and 5, Load Test 1 centerline of the bridge bisecting the tandem rear axles and. The test vehicles were fully loaded three axle dump trucks the front axles off the bridge Both trucks faced north for all. with gross vehicle weights of 268 8 kN 60 440 lb for truck load positions Data points were positioned along the trans. 1A and 273 9 kN 61 570 lb for truck 1B Fig 13 The verse centerline at each glulam web. vehicles were positioned longitudinally with the transverse. Predicted Deflection Analysis, At the conclusion of load testing predicted deflections were. calculated for AASHTO HS20 44 loading The procedure. was based on the measured load test deflection and a ratio of. deflection coefficients DCs as determined through computer. analysis Murphy 1994 The following relationship was. established to find the predicted deflection under HS20. HS20 Load test,DC Load test,HS20 is HS20 predicted deflection mm. Load test maximum measured load test deflection mm. DCHS20 HS20 deflection coefficient kN m4 and, Figure 13 Load test truck configurations and DC Load test load test vehicle deflection coefficient kN m4. axle loads for load test 1 All vehicle track,widths measured center center of the rear.
tires were 1 8 m 6 ft Condition Assessment, The general condition of the bridge was assessed at the time. of the two load tests The assessments involved visual in. spections measurements and photograph documentation of. the bridge Items of specific interest included geometry. wood condition wearing surface and stressing system. Results and Discussion, Performance monitoring of the North Siwell Road bridge. extended for 24 months beginning in January 1995 Results. and discussion of the performance data follow,Moisture Content. The average trend in electrical resistance moisture content. readings is shown in Figure 15 As shown there are slight. variations between the readings for the glulam webs and the. sawn lumber flanges At the beginning of the monitoring. Figure 14 Load test truck configurations and,axle loads for load test 2 All vehicle track. period the glulam web moisture content was approximately. widths measured center center of the rear 12 5 During the monitoring period the moisture content. tires were 1 8 m 6 ft exhibited seasonal changes of 5 At the end of monitor. ing the moisture content was approximately 15 The sawn. lumber flange moisture content was slightly higher at the. Load Test 2 start of monitoring at approximately 17 During the first. The test vehicles were fully loaded three axle dump trucks year of the monitoring the sawn lumber flange moisture. with gross vehicle weights of 233 2 kN 51 950 lb for truck content varied similarly at 5 Readings were not taken. 2A and 233 5 kN 52 490 lb for truck 2B Fig 14 The during the second half of the monitoring period except during. vehicles were positioned longitudinally with the transverse the final quarter of 1996 Near the end of the monitoring. centerline of the bridge bisecting the tandem rear axles and period the sawn lumber flange moisture content was ap. the front axles off the bridge Both trucks faced north for all proximately 12 5 Both the glulam webs and the sawn. load positions Data points were positioned along the trans lumber flange were installed at a moisture content less than. verse centerline at each glulam web the recommended value of 19. 1996 showed that the bar force level decreased at a higher. rate than did the North Siwell Road bridge where the wood. moisture content was less than 19 at installation, The amount of initial interlaminar compression stress can.
also affect the bar force performance In this case 47 of. initial interlaminar compression stress was lost during the. monitoring period which is similar to other bridges The. initial interlaminar compression stress level was higher on. the North Siwell Road bridge than other typical stress. laminated bridges Thus the final interlaminar compression. stress level remained within acceptable levels,Figure 15 Average trend in electrical. resistance moisture content readings Stress laminated T beam bridges can experience slip between. the web and flange when the interlaminar compression drops. below 345 kPa 50 lb in The final interlaminar compres. sion stress of 490 kPa 71 lb in2 for the North Siwell Road. bridge is well above this level The North Siwell Road. bridge has performed well during the 2 years of monitoring. therefore it is assumed that with standard periodic mainte. nance i e bar force checks routine inspections the bar. force level should remain acceptable,Load Test Behavior. In this section results of the static load tests and analytical. assessment of the North Siwell Road bridge are presented. Figure 16 Average bar force from load For each load position transverse deflection measurements. cells installed after the third bar tensioning, are given at the transverse mid span as viewed from the east. end looking west No permanent residual deformation was. Bar Force measured at the conclusion of load testing and there was no. The average bar force based on load cell readings is shown in detectable movement at bridge supports At the time of load. Figure 16 At the third stressing all bars were tensioned to test 1 the interlaminar compression stress was approxi. 142 kN 32 000 lb which corresponds to a 848 kPa mately 841 kPa 122 lb in2 For load test 2 the interlaminar. 123 lb in2 interlaminar stress Two months later the bar compression stress was approximately 490 kPa 71 lb in2. force decreased to 98 2 kN 22 100 lb or 579 kPa Load Test 1. 84 lb in2 The rate of loss declined and the bar force then. remained relatively stable for the remainder of the monitoring Transverse deflections for load test 1 with the locations and. period At the conclusion of monitoring the average bar force magnitudes of the maximum measured deflections are shown. was 87 6 kN 19 700 lb which corresponds to a level of in Figure 17 The maximum measured deflection was 9 mm. 490 kPa 71 lb in2 interlaminar compression stress 0 35 in for load position 1 8 5 mm 0 34 in for load. position 2 and 11 5 mm 0 45 in for load position 3 For. The observed bar force loss is most likely the result of the eccentric load positions the maximum measured deflec. stress relaxation in the sawn lumber flange laminations tion was 8 5 mm 0 34 in for load positions 4 and 6 and. Stress relaxation is a time dependent phenomenon caused 8 3 mm 0 33 in for load position 5 As shown in Fig. by the long term compressive force of the steel bars acting ure 17 the maximum measured deflections occurred near the. on the wood microstructure In the design of stress laminated wheel lines of the test vehicles These maximum measured. bridges it is assumed that the bridges will lose approxi deflections are symmetrical between similar load positions. mately 60 of the interlaminar compression over the life of such as load positions 1 and 2 and load positions 4 and 5. the structure as a result of stress relaxation The stress relaxa For the load positions with two vehicles the deflection from. tion behavior of this bridge was similar to previous moni load position 3 was greater than all other measured deflec. tored bridges Wacker and Ritter 1995 Ritter and others tions and the smaller deflection from load position 6 was. 1995 1996 However the amount of stress relaxation in the similar to load positions 1 2 4 and 5 This was due to the. North Siwell Road bridge was affected by the wood moisture transverse stiffness of the deck that was created by the inter. content Recent monitoring completed on a bridge installed laminar compression stress. with a moisture content greater than 19 Kainz and others. Figure 17 Transverse deflections measured at mid span looking north for load test 1 Bridge cross sections and. vehicle positions are presented to aid interpretation and are not to scale. Assuming uniform material properties symmetric loading. and accurate deflection measurements the summation of. bridge deflections from two single truck load positions. should equal the deflections from the load position with both. trucks Figure 18 displays the summation of load positions. 1 and 2 overlaid on load position 3 As shown there are. slight deflection variations near the longitudinal centerline. but the deflections are essentially the same,Load Test 2. Load test 2 transverse deflections are shown in Figure 19. The maximum measured deflection was 7 4 mm 0 29 in. for load position 1 7 3 mm 0 29 in for load position 2. and 9 3 mm 0 37 in for load position 3 The maximum. measured deflections for load positions 4 through 6 were. 8 3 mm 0 33 in for load position 4 7 2 mm 0 28 in for. Figure 18 Comparison of measured deflections for, load position 5 and 7 6 mm 0 30 in for load position 6 load test 1 showing the actual deflection of load.
Observations from load test 1 were also made for load test 2 position 3 and the sum of load positions 1 and 2. The maximum measured deflections occurred near the wheel looking north. lines of the test vehicles These maximum measured deflec. tions were symmetrical between similar load positions such. as load positions 1 and 2 and load positions 4 and 5 For the Predicted Deflection Analysis. load positions with two vehicles the deflection from load. To compare the North Siwell Road bridge with other. position 3 was greater than all other measured deflections. bridges a theoretical deflection based on a standard HS20 44. and the smaller deflection from load position 6 was similar. truck was determined Using the method previously de. to load positions 1 2 4 and 5 Again this was due to the. scribed maximum deflections of 11 mm 0 43 in for load. transverse stiffness of the deck that was created by the inter. test 1 and 10 4 mm 0 41 in for load test 2 were determined. laminar compression stress, for load position 3 These deflections correspond to L 809 for. As for load test 1 the summation of bridge deflections for load test 1 and L 849 for load test 2 which are significantly. two single truck load positions should equal the deflections less than the minimum design deflection criteria of L 500. from the load position with both trucks Figure 20 displays and the actual design deflection of L 579. the summation of load positions 1 and 2 overlaid on load. position 3 for load test 2 Aside from slight deflection varia Condition Assessment. tions the deflections are essentially the same, Condition assessments of the North Siwell Road bridge. indicated that structural performance and serviceability were. Load Test Comparison,good Inspection results for specific items follow. An examination of the results from load tests 1 and 2 re. vealed similar maximum deflection locations for each load Bridge Geometry. position Fig 21 As shown the second load test exhibited. Measurements obtained during site inspections reveal that. smaller deflections than the first load test The decreased. the bridge was slightly narrower at mid span than at the. deflections were due to 15 lighter loading on the second. abutments This behavior commonly called hour glassing. load test If the deflections for load test 2 were factored to. is a result of reduction of lamination size caused by compres. account for the 15 load reduction they would appear very. sive deformation creep over time at mid span and resistance. similar to the deflections for load test 1, to movement provided by the attachment of the webs at the. On most stress laminated bridges with butt joints there is a abutments Hour glassing does not affect the structural per. correlation between the transverse stiffness of the bridge and formance of the bridge but can have an adverse aesthetic. the level of interlaminar compression stress It has been effect if severe The hour glassing on the North Siwell Road. observed on several stress laminated bridges that the trans bridge is minor and generally not noticeable. verse stiffness decreases when the level of interlaminar com. pression in the deck decreases Wacker 1996 The change in. interlaminar compression seemed to have little effect on the. transverse stiffness of the North Siwell Road bridge. Figure 19 Transverse deflection measured at mid span looking north for load test 2 Bridge cross sections and. vehicle positions are presented to aid interpretation and are not to scale.

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