Structural Demand on Bridges Subjected to Bidirectional Ground Motions

AbstractBridge design spectra in the current design specifications are geometric mean based. In other words, the underlying research on which ground motion data are based predicts the geometric mean spectral acceleration of two horizontal components. This is, perhaps, transparent to many engineers performing bridge design in regions subject to extreme seismic loading. For response spectrum analyses, this does prove problematic when estimating structural response to bidirectional ground motions. Prevailing combination rules rely on the assumption that, when one component produces the maximum structural response, the second component produces 30% of the corresponding maximum. The purpose of this paper is to assess the validity of this prevalent rule for sites characterized by large magnitude earthquakes (MW = ±7.7) and Site Class D/E subsurface conditions. This study further provides estimates of maximum-to-geometric mean displacement ratios for various levels of ductility and provides estimates of dispersi...