Seismic retrofit of bridge steel truss piers using a controlled rocking approach

This paper investigates a seismic retrofit technique that allows bridge steel truss piers to uplift and rock on their foundation. Displacement-based passive energy dissipation devices (buckling-restrained braces, or BRBs) are implemented at the uplifting location to control the rocking response while providing additional energy dissipation. The hysteretic behavior of the controlled rocking system is developed for a static cyclic load applied to the top of a bridge pier, representing the dominant mode of vibration. Some existing methods of analysis are considered in predicting the response of the controlled rocking system in terms of maximum displacements. A capacity-based design procedure is established for sizing the BRBs and a design example provided to illustrate the key steps. Methods to predict design response values (displacements, velocity, forces) are discussed, and a parametric study, based on nonlinear time history analysis, is performed to verify the effectiveness of these methods. The parameters in the study include the pier aspect ratio (h/d), the local strength ratio (eta(L)) and an effective period of vibration (T-eff). Results of the study are presented as normalized by the design response values and are shown, in almost all cases, to be conservative.