Dynamic seismic response of controlled rocking bridge steel-truss piers

The dynamic seismic response of steel braced bridge piers allowed to uplift and rock on their foundation is investigated analytically. Allowing piers to rock provides a retrofit solution with increased seismic performance by limiting demands to existing non-ductile elements while damage can be avoided or forced into replaceable structural elements. Also, an inherent restoring mechanism exists that can provide self-centering following an earthquake. However, during the rocking response, as the pier transfers its axis of rotation from the base of one leg to another, the impact and uplift from the foundation excites vertical modes of vibration, increasing the lateral base shear and the axial force demands on the pier legs. Methods are developed to characterize and quantify the increased dynamic demands in order to capacity protect the existing elements. These simplified methods are then compared with the results of nonlinear time history analysis for a set of frames representative of highway bridge piers with aspect ratios of 4, 3 and 2, and shown to be reasonably accurate in most cases. An example set of calculations and analysis results are also presented.