Analytical and Experimental Investigation of Controlled Rocking Bridge Steel Truss Piers

An innovative seismic retrofit strategy that allows uplift and rocking of bridge steel truss piers (Fig. 1, Lions ’ Gate Bridge North Approach) on their foundation is investigated both analytically and experimentally. Allowing rocking response effectively increases the structures period of vibration. To control response of the system, displacement-based steel yielding devices are implemented at the uplifting location. A retrofitted bridge pier using this concept is shown in Fig 2. The devices’ strength and stiffness characteristics are calibrated to capacity protect the existing, vulnerable members and foundation of the structure. The system provides a significant restoring force that can allow re-centering of the structure following excitation. 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 dynamic shake table testing of a rocking pier specimen 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, maximum developed forces, etc. Also, behavior of the devices during the rocking response is observed. BACKGROUND 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 will likely suffer significant 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. The built-up members have high potential for buckling in compression with significant strength and stiffness degradation. The anchorage connections have inadequate strength to resist even moderate seismic demands, especially for piers with significant overturning (slender piers). Thus these piers have very little effective ductility. Also, these types of bridges may be a vital geographical link to a region and must remain operational following a major earthquake. Therefore, 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 rocking bridge pier concept has been implemented into a few bridges including the Lions Gate Bridge (north approach) in Vancouver and the South Rangitikei Rail Bridge in New Zealand. Both use a steel yielding device at the foundation interface to dissipate energy. Some major bridges in California have also allowed at least partial uplift of pier legs as a means of seismic resistance; including the Carquinez, San Mateo-Hayward, and Golden Gate Bridges.