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CHAPTER 5 SHAKE TABLE TEST INTRODUCTION The full scale GRS bridge abutment test was performed at the U.S. Army Engineering Research and Development Center â Construction Engineering Research Laboratory (ERDC-CERL) using the Triaxial Earthquake and Shock Simulator (TESS). Figure 5.1 shows the bare TESS platform before the model has been constructed. The GRS bridge abutment model was tested using a staged sinusoidal horizontal motion with increasing amplitude (up to 1 g). Figure 5.1: TESS Platform Prior to Construction of GRS Abutment TEST CONFIGURATION Figure 5.2 shows the test configuration. The GRS abutment model was built on the TESS platform while the steel safety and bearing frame was built off the TESS platform. Twelve
100 MC10x28.5 channels were bolted together to create the two bridge girders. The concrete slabs and steel plates provided the dead load; the total dead load was 445 kN acting on a 6.7 m simply supported bridge. Figure 5.2: Bridge Abutment Model Figure 5.3 shows the completed bridge abutment model at the left side of the picture. The far left side of Figure 5.3 shows the backwall which makes up the fourth face of the abutment model. Six 3.65 m columns were bolted to the TESS platform at 60 cm on center and braced with the diagonal channels. A heavy steel plate was bolted to the top of the columns to provide lateral
101 support for all of the columns to resist the lateral soil pressure. The wall was made adequately stiff to limit wall displacements to an acceptable level. Two 2 cm thick sheets of plywood were bolted to the columns and a 5 cm thick Styrofoam layer was fastened to the plywood. This Styrofoam layer is in direct contact with the GRS abutment and is used to alleviate compressive waves reflected by the rigid backwall. Figure 5.3: Completed Bridge Abutment Model The TESS platform was protected from the soil by bolting 2 cm tongue and groove plywood to the surface. Yellow pine 2 x 12 lumber was used to frame a perimeter where the foundation soil was compacted below the GRS abutment. Additional protective wood coverings were installed to ensure the safety of the TESS hydraulics in the event of a collapse.
102 CONSTRUCTION OF THE BRIDGE ABUTMENT MODEL Figure 5.4 shows the installation and compaction of the 20 cm thick foundation soil. The soil was placed and compacted in 10 cm lifts. Figure 5.5 shows the placement of the first course of CMU split face block after the second lift (20 cm total depth) of foundation soil was placed and compacted. A layer of geotextile fabric (GEOTEX 4x4) was laid below the first course of blocks, the fabric was placed only beneath the block and did not cover the interior soil area. After each layer of block was placed, soil was placed and compacted in two 10 cm lifts using a plate compactor. Engineering and Research International, Inc. measured the moisture content and relative density every 10 cm lift using a nuclear density gauge. The ILDOT CA-6 material used in the GRS abutment had an optimum moisture content of 6.8% and a maximum density of 21.52 kN/m3 as determined from a modified Proctor compaction test. Lifts were kept above 97% relative compaction throughout the model while the moisture content ranged from 6.4% to 6.9%. After every two lifts of soil placed and compacted, a geotextile layer was placed over the entire soil area and full width of the CMU blocks. The top three courses of CMU blocks were grouted together for added stability during seismic loading (Figure 5.6). As was described in Chapter 4, two elastomeric pads are used to support the bridge at the GRS-abutment end as shown in Figures 5.7 and 5.8. The other bridge end is supported using two rollers (slide bearings) as shown in Figure 5.9.
103 Figure 5.4: Placement and Compaction of the First 10 cm Layer of Soil
104 Figure 5.5: First Course of Block Placement
105 Figure 5.6: Grouting the Top Three Courses of Blocks
106 Figure 5.7: Completed GRS Abutment with the Bridge
107 Figure 5.8: Elastomeric Pad Close-up Figure 5.9: Back View Showing the Two Rollers (Slide Bearings)
108 INSTRUMENTATION The response of the bridge abutment model was measured using several sensor types: accelerometers, extensiometers, linear variable differential transducers (LVDTs), pressure transducers, and strain gauges. Figure 5.10 shows the locations of accelerometers. Accelerometers A1 through A13 are attached at the center of the front face of the GRS abutment. Each accelerometer measures the motion in the longitudinal direction. Accelerometer A14, located directly below A1, is attached directly to the plywood surface that is bolted to the TESS platform at the bottom of the foundation soil. Accelerometers A15 and A16 are embedded in the soil at the same elevation as A1 and at the center of the model in the east-west direction (perpendicular to the page of Figure 5.10). Accelerometers A17 and A18 are similarly embedded in the soil at the same elevation as A6, while A19 and A20 are at the same elevation as A13. A21 and A22 measure the longitudinal acceleration of the sill and the girder respectively as shown in Figure 5.10. Accelerometer A23 measures the longitudinal acceleration at the exterior surface of the top CMU block while A24 and A25 measure the acceleration at locations in the soil at the same elevation as A23. Figure 5.11 shows the location of the pressure transducers, strain gauges and LVDTs. Pressure transducer P1 measures the vertical pressure directly beneath the first course of blocks at the center of the front face of the abutment. P2 measures the lateral earth pressure against the first course of blocks at the center of the front wall of the abutment. Figure 5.12 shows a close-up photograph of P2 before it was covered with soil. Pressure transducers P2 through P8 measure the lateral earth pressure against the front wall of the abutment. Pressure transducers P9 and P10 are positioned under the sill and measure the vertical bearing pressure at the north and south edges of the sill.
109 Figure 5.10: Location of Accelerometers
110 Figure 5.11: Location of Pressure Transducers, Strain Gauges and LVDTs Figure 5.12: P2 Sensor Measuring Lateral Pressure at 1st CMU Course
111 Strain gauges were attached to geosynthetic layers 3, 6, 11 and 15 at the center of the model in the east-west direction (perpendicular to the plane of Figure 5.11). Figure 5.13 shows a close-up photograph of geosynthetic layer 3 with strain gauges before it was covered with soil. Figure 5.13: Strain-Gauge Instrumented Geosynthetic Layer Placed Above 2nd CMU Course Figure 5.11 shows LVDTs L1-L13 that measure longitudinal (x-direction) deformation between the center of the 1st, 3rd, 5th, 7th and 8th through 16th course of the front wall of the abutment and the reference frame. The LVDTs are located at the center of the model in the east-west direction. Two additional LVDTs measure girder motion, L14 measures the relative motion between the North end of the girder and the supporting steel frame while L15 measures the relative motion between the South end of the girder and the top course of the front wall (18th course). Figure 5.14 shows the cable extensiometers used to measure the longitudinal (x-direction) and vertical (z-direction) between the reference frame and points 1-6 as indicated in the figure. In the figure, points 1-6 signify respectively: the top north corner of the 16th CMU course (C1x and C1z); the top north corner of the sill (C2x and C2z); the top south corner of the sill (C3x and C3z); the top south end of the girder (C4x and C4z); the top of the 18th CMU course (C5z and Strain gauge
112 C5z); and the top surface of the soil near the south rigid wall (C6x and C6z). Figure 5.15 shows a close-up photo of selected LVDTs, accelerometers, and extensiometers Figure 5.14: Location of Cable Extensiometers Figure 5.15: Instrumentation Close-up: LVDTs, Accelerometers, and Extensiometers Accelerometer LVDT Extensiometer
113 TEST MOTIONS On April 6, 2010, the bridge abutment model was tested using system identification tests as well as longitudinal sinusoidal wave tests. System Identification (SI) Tests Longitudinal and low level vertical SI tests were conducted in order to measure the natural frequencies of the model using sine-sweep motions. These motions began with amplitudes of 0.05 g, and swept from 1.25 to 80 Hz, at a sweep rate 2 octaves per minute, for a total of 6 octaves and duration of 3 minutes. A single low level vertical SI test was conducted, because there were no vertical accelerometers, the vertical modes were measured through transfer functions between the highest amplitude cable extensiometer records and TESS vertical accelerations. The longitudinal modes were defined by transfer functions between the accelerometers near the top of the model and the accelerometers inside the TESS. One critical mode measured was defined by longitudinal motion of the girder relative to the sill due to deformation of the bearing pad. The frequency of this mode was measured through a transfer function between A22 and A21 (see Figure 5.10). Other transfer functions were also used to measure this frequency and other modes. The estimated frequency for the first mode of the bearing pad was approximately 2.24 Hz (see Chapter 4) which was a critical mode; above this frequency the girder would be isolated from the longitudinal motion of the model, significantly reducing the longitudinal loading on the model. The elastomeric bearing pads being just over 52 mm thick could withstand significant deformation without reaching displacements that would either damage or stiffen them. However, any motions at this frequency would create an amplified response at this natural frequency. As the sine-sweep motions pass through this frequency the response of the girder and slab system above the model would be amplified, significantly loading the model. The degree of amplification depended on the damping of the elastomeric bearing pad and the sweep rate. The frequencies of the fundamental vertical modes were also calculated based on transfer functions between select vertical cable extensiometers and vertical table accelerations. Lateral modes were not defined due to the lack of instrumentation in the lateral direction and the potential for damaging sensors attached to the instrumentation frame.
114 Sine-Sweep Tests at Increasing Amplitude Uniaxial sinusoidal tests were conducted in the longitudinal direction which coincides with the axis of the girders. The testing amplitude gradually increased while maintaining a set frequency. The frequency chosen to test at was decided after system identification tests were completed and the natural frequency of the abutment model and the bearing pads were known. From the SI tests, the horizontal natural frequency of the bearing pads was found to be 2.3 Hz while the longitudinal natural frequency of the abutment model was found to be 8.5 Hz. Based on these results, a testing frequency of 1.5 Hz was decided upon, well below the natural frequencies of the models components. The first test was conducted at an amplitude of 0.15 g with a frequency of 1.5 Hz for 20 seconds. All further testing was performed at 3 Hz, a frequency significantly higher than the natural frequency of the bearing pads causing the horizontal motion of the superstructure to be isolated from the substructure. Testing at 3 Hz was performed at amplitudes of 0.3 g, 0.45 g, 0.67 g and 1.0 g.
