Assessment of the Seismic Demands Posed to Suspension Bridges in the Near Field with Site-Specific Arrays of Simulated Ground Motions

Abstract

A key component of infrastructure seismic risk assessment is the execution of nonlinear time-history analyses with a suite of selected ground motions scaled to site-specific spectra. When interested in the evaluation of distributed systems residing near major active faults, such an approach is challenged by the scarcity of records adequately incorporating region and site-specific features known to impact the nonlinear response of distributed infrastructure substantially (pulses, spectral shape, spatial variability). This paper assesses the differences in the seismic demand posed to a suspension bridge as obtained from real ground motions selected based on state-of-the-art methods and arrays of site-specific simulated ground motions generated from physics-based wave-propagation models. The objective is to characterize the biases in structural response estimates introduced by the use of real ground motions in code-compliant approaches. A total of 15 realizations of an M7 Hayward fault earthquake and a detailed nonlinear simulation model representative of the West San Francisco–Oakland Bay Bridge are utilized as a case study. Damage limit states associated with structural drifts are defined for each bridge component as a function of the spread of plasticity. The results indicate that the median and variability of the demands posed by the simulated motions and real ground motions scaled to the same site-specific spectra are markedly different. Evidence from this study highlights the sensitivity of complex structures to multiple characteristics of the ground motions that might be biased by current scaling methods. Finally, fundamental characteristics of the scaled records leading to differences in the bridge response are discussed providing the basis to inform current ground motion selection and scaling procedures.