Shake table testing of an Oak Street Bridge bent model

Further to the slow cyclic testing of a 0.45 scale model of a two column bent from the Oak Street Bridge from 1993-1995, a testing program was devised to test a smaller 0.27 scale model of the same bent in its as-built condition on the shake table at UBC's Earthquake Engineering Research Laboratory. The prototype bent was scaled to 0.27 size, and the dead load was incorporated as a mass block attached to the cap beam. The specimen was instrumented internally with strain gauges on some longitudinal and transverse reinforcing steel, and externally with accelerometers and displacement transducers. The bent was subjected to scaled seismic motions in the in-plane horizontal direction. A representative record of the 1992 California Landers Earthquake was chosen as the input motion. A high speed camera recorded the development of cracking and damage. Several test runs at different levels of shaking were executed. After each run, the dynamic characteristics of the bent were determined from impact hammer tests. Complementary analytical studies were performed to predict the behaviour of the specimen. The aims of this program were: to identify damage, failure modes and dynamic characteristics of the bent, to compare the experimental results with analytically determined values of damage index and dynamic characteristics, to relate the experimental results to the results from the earlier larger scale slow cyclic tests, and to gain further insight into the operating characteristics of the recently upgraded digitally controlled shake table. It was found that the modes of failure from the shake table and slow cyclic tests were very similar, both being due to shear failure of the cap beam, however the shake table tested bent was capable of withstanding a more severe load. The peak relative displacements experienced by the shake table bent were also larger than those by the slow cyclic test. The hammer testing showed that the natural frequency of the bent decreased throughout the test, until the final value was less than half that of the initial one. The analytical predictions of the dynamic characteristics of the model correlated well with the test results, but underpredicted the failure load by about 20%. Some initial problems with the operation of the shake table were experienced, but these were corrected and the table worked well for the rest of the test. The experimental evidence supported the earlier testing that was done, and highlighted some of the differences between the two testing methods.Applied Science, Faculty ofCivil Engineering, Department ofGraduat