Shake table testing and performance evaluation of a post-seismic repaired and retrofitted Oak Street Bridge bent with fibreglass wrapping system
Creators: name: Chen, Ge
Publisher: name: University of British Columbia
Year: 2009

description: A 27% scaled as-built model of the Oak St. Bridge bent S28 (denoted OSB-O) had been severely damaged when tested on the shake table at the University of British Columbia Earthquake Lab (E. Davey, 1996). During testing the structure's overall behavior was brittle, and little plastic deformation was observed before sudden shear damage occurred in the cap beam at a very low seismic load level. An earlier slow cyclic (or Quasi-static) test program on 45% scaled models of the same bent had generated similar failure mode and mechanism (Anderson et al. 1995). It was decided to repair and re-strengthen the 27% scaled bent model, and to test this retrofitted model (denoted OSB-R) by subjecting it to the same seismic loading conditions. The main objectives of this research program were: to develop a repair and retrofit scheme for the damaged Oak Street Bridge bent as-built model, to re-test this re-strengthened model on the shake table and experimentally determine the dynamic response of the model under seismic loading condition, and to examine the effectiveness of the retrofit system by data interpretation and performance comparison between the as-built and re-strengthened models. To repair the damage to the OSB-O, concrete patching and epoxy injecting techniques were employed. A newly developed FRP (fiber reinforced plastic) jacketing system, QuakeWrap™ was chosen to retrofit the repaired bent model. The performance of the re-strengthened bent model was overall satisfactory. It behaved in a ductile manner instead of a brittle one, as plastic hinges were developed at the top of bent columns as anticipated. The retrofitted model suffered far less damage than the as-built one, and no major shear damage was observed anywhere on the structure, especially on the cap beam. A significantly higher strength and ductility level were achieved by this re-strengthened model. The structural integrity and stability of the specimen were well maintained at the end of testing, when further testing was prevented by the shake table limitation. The results also matched the testing results from the earlier slow cyclic tests on a 45% scaled model retrofitted using a similar technique. The testing results indicated that the 27% scaled repaired and retrofitted model had an ultimate strength of 160kN, and a yield strength of 120kN at a displacement of 6.6mm; while the values scaled from the slow cyclic test results showed that, an untested as-built Oak Street Bridge bent model retrofitted with the FRP jackets had an ultimate strength of 166kN, and a yield strength of 129kN at a displacement of 6.5mm. The research results have indicated that the FRP wrapping system could provide the structural members with sufficient and reliable shear strength so that brittle shear damage could be prevented or minimized during a real seismic event. Better confinement in the section comers also can reduce the severity of cover concrete spalling and minimize the steel buckling. Installed at designated locations of the structure, the system would help the structure to deform in a plastic ductile manner and to dissipate seismic energy in the process, and eventually to reach large displacements and ductility levels without significant reduction of the structure's load resisting ability and without losing its structural stability.