Analytical and experimental investigation of a controlled rocking approach for seismic protection of bridge steel truss piers
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A large number of steel truss bridges were constructed in the U.S. when seismic resistance was not considered. Recent structural analyses of these bridges have revealed that they will likely suffer significant seismic damage and have a risk of collapse during their remaining service life. Contributing significantly to their poor seismic behavior is the built-up, lattice type members used to resist the lateral seismic forces and the pier anchorage connections resulting in very little effective system ductility. Also, these types of bridges may be a vital geographical link to a region and must remain operational following a major earthquake. Therefore, seismic retrofit strategies that enhance the global structural ductility, limit maximum forces transmitted to existing members and the foundation (capacity protect), and prevent residual deformations are needed. A seismic design (or retrofit) strategy allowing uplift and rocking of steel truss piers on their foundation is investigated both analytically and experimentally. To control system response, the use of displacement-based steel yielding devices and velocity-dependant viscous dampers, implemented at the uplifting location, are considered. The devices can be calibrated to capacity protect the existing vulnerable members and the foundation of the structure. The system provides a significant restoring force that can allow re-centering of the structure with proper selection of device properties. The behavior of 2-legged and 4-legged bridge steel truss piers is considered and methods of predicting response under multiple components of seismic excitation are evaluated using nonlinear, inelastic time history analyses. The analytical investigation of seismic response includes ground motions typical of far-field rock sites and near-field ground motions with pulse-type characteristics. Also, the response of 4-legged piers that resist transverse and longitudinal demands in bridges is investigated with three components of ground motion. Experimental investigations include shake table testing of a rocking pier with the added passive energy dissipation devices to verify analytical methods and further investigate the dynamic response. Response quantities of interest include pier displacements, impact velocity, and maximum developed forces. Overall system behavior and the methods of response prediction are shown to be reasonably accurate using the analytical and experimental techniques.