Static Load and Vibration Performance of Six Timber Bridges Constructed Using Local Species in Northern Michigan

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1 Static Load and Vibration Performance of Six Timber Bridges Constructed Using Local Species in Northern Michigan By: Brian K. Brashaw, Robert Vatalaro and Xiping Wang University of Minnesota Duluth Natural Resources Research Institute 5013 Miller Trunk Highway Duluth, Minnesota and James Wacker USDA, Forest Service, Forest Products Laboratory One Gifford Pinchot Drive Madison, Wisconsin March 2006 Project No NRRI/TR-2006/08 Natural Resources Research Institute University of Minnesota Duluth 5013 Miller Trunk Highway Duluth, MN Michigan Technological University School of Forest Resources and Environmental Science 1400 Townsend Drive Houghton, MI 49931

2 OBJECTIVE The objective for our study was to conduct structural condition assessments of six timber bridges constructed by Michigan Technological University (MTU). These bridges were inspected using visual inspection techniques, flexural vibration and live loading using a field truck. A secondary objective was to provide ongoing monitoring of vibration parameters of one bridge located at the Ford Forestry Center in Alberta, Michigan. BRIDGES AND BRIDGE MATERIALS Six bridges were constructed using eastern hemlock, red maple, big tooth aspen, white pine and red pine in various combinations for abutments, beams, deck boards, rails and running surfaces. Five of the bridges were constructed on MTU cross country ski trails designed to carry a 19,000-pound trail groomer and a packed snow load. The log truck bridge was installed at the MTU Ford Forestry Center in Alberta, Michigan and was designed to carry a HS-20 loading. BRIDGE TESTING Each of the six bridges was inspected using visual inspection techniques to establish a baseline for the bridge. Specifically, we inspected the railing system, the abutment, the curb, post, railing, beams and decking for signs of damage or other deterioration. A forced vibration technique was used to identify the first bending mode frequency of the bridge structures. This method is a purely time domain method and was used because it eliminates the need for modal analysis. An electric motor with a rotating unbalanced wheel was used to excite the structure (Fig. 1), which creates a rotating force vector proportional to the square of the speed of the motor. Placing the motor at midspan ensured that the simple bending mode of structure vibration was excited. Two piezoelectric accelerometers (PCB 626BO2), also at midspan, were used to record the response in the time domain. To locate the first bending mode frequency, the motor speed was slowly increased from rest until the first local maximum response acceleration was located. The period of vibration was then estimated from 10 cycles of this steady-state motion as captured using a Fluke digital scopemeter. Figure 2 shows the setup of the signal acquisition and the motor control system used in this testing. Figure 1.--Rotating motor used to excite the timber bridge.

3 Figure 2.--System used to control motor and acquire vibration response. Because the primary goal of this work was to relate the vibration characteristics of the timber bridge structures to a measure of structural integrity, the bridges were also evaluated with the established method of static load deflection field testing. Stiffness of bridge superstructure (EI product) could then be estimated from the field test results. Static load tests were conducted on each field bridge using a live load testing method. A test vehicle was placed on the bridge deck, and the resulting deflection was measured from calibrated rulers suspended from each timber girder along the midspan crosssection using an optical surveying level. The test vehicle consisted of a Michigan Technological University truck with a gross vehicle weight of 10,000 lb with individual axle weights of 2,000 lbs and 8,000 lbs. The rear axles were spaced 5.5 ft apart and the axles were 11.5 ft apart. A quad axle truck was also used to test the Alberta Ford Forestry Center bridge. It had a rear axle weight of 40,400 lbs with a center-center rear axle span of 54 in. Deflection readings were recorded prior to the testing (unloaded), after placement of the truck for each load case (loaded) and at the conclusion of testing (unloaded). Measurement precision was mm, with minimal vertical movements detected at the bridge supports. The static EI product of each bridge was estimated from the load-deflection data on the basis of conventional beam theory. Figure 3 shows a representative load test of the MTU bridges.

4 8,000 lbs rear axle weight Figure 3.--Typical static load testing of a newly constructed timber bridge at Michigan Technological University. Eastern Hemlock Ski Trail Bridges TESTING RESULTS The first two bridges (20 wide and 20' long) were installed in near proximity on a perennial stream which flows into the Peepsock Creek (a small year round stream within the MTU forest). Figure 4 show the bridges as installed. Figure 4.--Eastern hemlock bridges constructed at Michigan Technological University.

5 Visual inspection These are new bridges that were installed in Inspection of the bridge showed no damage to any of the members or any loose connections. There was no evidence of any decay or insect damage. Vibration testing The measured natural frequency for the eastern hemlock trail bridges is shown in Table 1. Table 1.--Flexural vibration testing results for eastern hemlock bridges at MTU. Bridge Parameters Motor Parameters Vibration Parameters Bridge C-C and test Length Width Speed Frequency Time/cycle Frequency span number (ft) (ft) (rpm) (Hz) (ms) (Hz) (ft) Hemlock #1 Test Test Test Test Test Test Hemlock #2 Test Test Test Test Test Test Note: Speed = revolutions/minute. Conversion to frequency = RPM/60 for the motor parameters Conversion to frequency = 1/(time/cycle) for the vibration parameters Static load testing Figures 5 and 6 show the static testing deflection of eastern hemlock bridge #1 and #2 under the 8,000 lb load of the rear axles. The truck was placed along the bridge centerline with the rear axle located at midspan and the front axle off the bridge. No permanent deflection was noted on the stringers monitored.

6 2.0 Deflection (mm) Beam No. Figure 5.--Deflection performance of eastern hemlock bridge number 1. Deflection (mm) Beam No. Figure 6.--Deflection performance of eastern hemlock bridge number 2. Based on the static testing results, the structural stiffness (EI product) of the eastern hemlock bridges was 15,195x10 6 lb-in 2 for bridge #1 and 14,110 x10 6 lb-in 2 for bridge #2. The formula used for determining the EI for all bridges was EI=(P*L 3 )/(48*avg defl), where P = load and L = span.

7 Red Pine Ski Trail Bridge The red pine ski trail bridge is 18' long x 20' wide and located on the Peepsock Creek. It has 6"x11"x18' long red pine beams. This bridge, because of terrain, skier speed and trail orientation was built without rails. A simple curb was installed. One 12"x12" by 21' long treated white pine abutment was used due to the shallow terrain and creek bed. Galvanized 4"x4"x3/8" clips were used to fasten the beams to the abutments. The creek flow occurs during spring melt and rain. Figure 7 shows the completed red pine ski trail bridge. Figure 7.--Red pine ski trail bridge constructed at Michigan Technological University. Visual inspection This is a new bridge that was installed in Inspection of the bridge showed no damage to any of the members or any loose connections. There was no evidence of any decay or insect damage. Vibration testing The measured natural frequency for the red pine ski trail bridge is shown in Table 2. Table 2.--Flexural vibration testing results for the red pine skit trail bridge at MTU. Bridge Parameters Motor Parameters Vibration Parameters Bridge C-C and test Length Width Speed Frequency Time/cycle Frequency span number (ft) (ft) (rpm) (Hz) (ms) (Hz) (ft) Test Test Test Average Note: Speed = revolutions/minute. Conversion to frequency = RPM/60 for the motor parameters Conversion to frequency = 1/(time/cycle) for the vibration parameters

8 Static load testing Figure 8 shows the static testing deflection of the red pine ski trail bridge under the 8,000 lb load of the rear axles. The truck was placed along the bridge centerline with the rear axle located at midspan and the front axle off the bridge. No permanent deflection was noted on the stringers monitored. 2.0 Deflection (mm) Beam No. Figure 8.--Deflection performance of the red pine ski trail bridge. Based on the static testing results, the structural stiffness (EI product) of the red pine bridge was 10,107x10 6 lb-in 2. Aspen Ski Trail Bridge The aspen bridge is 20' wide by 19' long and located on the Peepsock Creek. It has 6"x12"x19' long aspen beams. The abutments were constructed using 2-12"x12"x10.5' long treated white pine bolted to 1-6"x12"x20' long treated aspen base. The beams were connected to the abutments using 4 "x4"x3/8" galvanized clips. Figure 9 shows the completed aspen bridge. Figure 9.--Aspen bridge constructed at MTU.

9 Visual inspection This is a new bridge that was installed in Inspection of the bridge showed no damage to any of the members or any loose connections. There was no evidence of any decay or insect damage. Vibration testing The measured natural frequency for the aspen ski trail bridge is shown in Table 3. Table 3.--Flexural vibration testing results for the aspen ski trail bridge at MTU. Bridge Parameters Motor Parameters Vibration Parameters Bridge C-C and test Length Width Speed Frequency Time/cycle Frequency span number (ft) (ft) (rpm) (Hz) (ms) (Hz) (ft) Test Test Test Average Note: Speed = revolutions/minute. Conversion to frequency = RPM/60 for the motor parameters Conversion to frequency = 1/(time/cycle) for the vibration parameters Static load testing Figure 10 shows the static testing deflection of the aspen ski trail bridge under the 8,000 lb load of the rear axles. The truck was placed along the bridge centerline with the rear axle located at midspan and the front axle off the bridge. No permanent deflection was noted on the stringers monitored. Deflection (mm) Beam No. Figure 9.--Deflection performance of the aspen ski trail bridge at MTU.

10 Based on the static testing results, the structural stiffness (EI product) of the aspen bridge was 18,662x10 6 lb-in 2. Red Maple Ski Trail Bridge The red maple bridge is 20' long by 20' wide and located on an intermittent stream. It has 6"x12"x20' long red maple beams. The abutments are constructed the same as for the aspen bridge as 2-12"x12"x10.5' long treated white pine bolted to two a 6"x12"x20' long treated white pine base. The beams were connected to the abutments using 4"x4"x3/8" galvanized clips. Figure 11 shows the constructed red maple bridge. Figure 10.--Red maple ski bridge constructed at MTU. Visual inspection This is a new bridge that was installed in Inspection of the bridge showed no damage to any of the members or any loose connections. There was no evidence of any decay or insect damage. Vibration testing The measured natural frequency for the red maple ski trail bridge is shown in Table 4.

11 Table 4.--Flexural vibration testing results for the red maple ski trail bridge at MTU. Bridge Parameters Motor Parameters Vibration Parameters Bridge C-C and test Length Width Speed Frequency Time/cycle Frequency span number (ft) (ft) (rpm) (Hz) (ms) (Hz) (ft) Test Test Test Test Test Test Note: Speed = revolutions/minute. Conversion to frequency = RPM/60 for the motor parameters Conversion to frequency = 1/(time/cycle) for the vibration parameters Static load testing Figure 12 shows the static testing deflection of the red maple ski trail bridge under the 8,000 lb load of the rear axles. The truck was placed along the bridge centerline with the rear axle located at midspan and the front axle off the bridge. No permanent deflection was noted on the stringers monitored. Deflection (mm) Beam No. Figure 11.--Deflection performance of the red maple timber bridge at MTU. Based on the static testing results, the structural stiffness (EI product) of the red maple bridge was 21,949x10 6 lb-in 2.

12 Ford Forestry Center Log Truck Bridge The log truck bridge is 18' long by 15' wide and located on the Plumbago Creek. It has eleven 6"x15"x18' long eastern hemlock beams. The decking is constructed with 4" thick red maple planks and has 2.5" thick red maple running surface. The abutments consisted of 2-12"x12"x16' long treated white pine on each end of the bridge. The location of this bridge was originally planned for the MTU ski trails. The location was changed to the MTU Ford Forestry Center where it will be subjected to more frequent and heavier loaded traffic. This bridge is instrumented to periodically check performance. This bridge has curbs similar to the red pine ski trail bridge but was designed to ASSHTO HS-20 Standard. Figure 13 shows the completed Ford Forestry Center Vehicle bridge. Figure 12.--Completed bridge at Ford Forestry Center, Alberta, Michigan. Visual inspection This is a new bridge that was installed in Inspection of the bridge showed no damage to any of the members or any loose connections. There was no evidence of any decay or insect damage.

13 Vibration testing The measured natural frequency for the Ford Forestry Center vehicle bridge is shown in Table 5. Table 5.--Flexural vibration testing results for the bridge at the Ford Forestry Center. Bridge Parameters Motor Parameters Vibration Parameters Bridge C-C and test Length Width Speed Frequency Time/cycle Frequency span number (ft) (ft) (rpm) (Hz) (ms) (Hz) (ft) Test Test Test Average Note: Speed = revolutions/minute. Conversion to frequency = RPM/60 for the motor parameters Conversion to frequency = 1/(time/cycle) for the vibration parameters Static load testing Figure 13 shows the static testing deflection of the Ford Forestry Center vehicle bridge under the 8,000 lb load of the rear axles. The truck was placed along the bridge centerline with the rear axle located at midspan and the front axle off the bridge. No permanent deflection was noted on the stringers monitored. Deflection (mm) Beam No. Figure 13.--Deflection performance of the Ford Forestry Center vehicle bridge. A larger truck was also used to conduct static load testing of the Ford Forestry Center vehicle bridge. Figure 14 shows the static testing deflection of the Ford Forestry Center vehicle bridge under the 40,400 lb load of the rear axles. The truck was placed along the bridge centerline with the rear axle located at midspan and the front axle off the bridge. No permanent deflection was noted on the stringers monitored.

14 Deflection (mm) Beam No. Figure 14.--Deflection performance of the Ford Forestry Center vehicle bridge. Based on the static testing results, the structural stiffness (EI product) of the Ford Forestry Center bridge was 19,578x10 6 lb-in 2. Testing Summary The testing data is summarized in Table 6. In this table, the measured EI stiffness is reported for each bridge along with the measured frequency of vibration. For the eastern hemlock bridges and the red maple bridge, we chose to report both of the frequencies identified during testing. Further analyses need to be conducted to determine the actual flexural mode of vibration. The results are reported graphically on Figure 15. When more than one frequency was reported for a bridge, the lower frequency was selected for inclusion on the graph. As shown on Figure 15, the data for these six bridges is overlaid with several previous data sets from the Ottawa National Forest and Minnesota. The data generally fits the previous results, although there are substantial opportunities for error in determining the first bending mode of vibration, the true mass of the bridge, settling at the bridge supports, and moisture content.

15 Table 6.--Summary of vibration testing and measured bridge stiffness. Bridge Measured bridge stiffness (EI product) Measured Forced Vibration Bending Mode Frequency (Hz) (x10 6 lb-in 2 ) Frequency 1 Frequency 2 Hemlock 1 15, Hemlock 2 14, Red pine 10, n/a Aspen 18, n/a Red maple 21, Ford Forestry 19, none Note: In some cases, a second mode of vibration was noted. It was not clear which of these modes was the natural bending mode frequency, so both frequencies were reported. The numbers in bold were used to generate Figure Frequency (Hz) EI/WL 3 (1/in.) free-free fixed-fixed MI Tech U MN Bridges Ottawa NF Bridges Figure 15.--Comparison of measured bridge stiffness and measured forced vibration bending mode frequency. Note: This chart shows the Michigan Technological University bridges evaluated during this study and previous data sets from Minnesota, and the Ottawa National Forest.

16 FUTURE MONITORING The performance of these bridges should be monitored on a yearly basis for the next five years. Budgeting for these activities will have to be resolved. A request will be submitted to the USDA Forest Service at a future date to cover MTU and UMD costs. The monitoring will include retesting of each bridge. The base line data mentioned has been established using forced vibration and natural frequency. This method will be used in the future. Static load will not used since the primary purpose of these tests was to confirm design load performance and relationship between NDE methods (vibration) and static load tests. CONCLUSIONS The following conclusions can be made from this study: Visual inspection of the bridge showed no damage to any of the members or any loose connections of any of the six bridges built during this project. There was no evidence of any decay or insect damage. Each bridge performed according to design under static live loading. There was some settling at the support locations identified during the loading tests. The use of forced vibration testing shows promise as an indicator for bridge superstructure stiffness (EI product). The results of this testing fits previous vibration and load testing data completed by the project team in the Ottawa National Forest and in Minnesota. Further evaluation and testing of the bridges using new testing equipment will be performed in the future to better understand and define the bending mode frequency of each bridge. ACKNOWLEDGMENTS This project was funded through a grant awarded by the Northeastern Area State and Private Forestry, USDA Forest Service. Roland Huhtala Construction, L Anse, Michigan, donated the use of a loaded quad-axle dump truck and operator for dead load testing of the vehicle bridge.