Structural Implications of Near‐Fault Directivity Representations for Bridges

This study evaluates the seismic performance implications of recently proposed near‐fault directivity factors for the California Department of Transportation (Caltrans) Seismic Design Criteria (SDC). Recent advances in probabilistic seismic hazard analysis have enabled site‐specific incorporation of near‐fault directivity into design spectra, including updates to the Caltrans SDC, yet their structural performance implications remain largely unexplored. To address this gap, nonlinear response history analyses of two typical long‐span ordinary bridges with single‐ and two‐column bents were conducted using detailed three‐dimensional bridge models subjected to bidirectional near‐fault ground motions. Ground motions were scaled to three hazard‐consistent target spectra: (1) a uniform hazard spectrum without directivity, (2) a site‐specific spectrum incorporating directivity models within the hazard integral, and (3) a spectrum based on recently proposed Caltrans directivity amplification factors. Motions were rotated across multiple incidence angles to evaluate direction‐dependent structural response, and analyses were conducted at two seismic hazard levels (1000‐ and 2475‐year return periods). Results indicate that directivity‐amplified spectra produce higher drift demands, higher collapse rates, and greater response variability compared to uniform hazard spectra without directivity, with differences becoming more pronounced at longer return periods. Comparisons between the new SDC factors and model‐based directivity spectra show similar overall trends but case‐dependent differences in deformation demands, reflecting the interaction between directivity representation and structural characteristics. Directional effects significantly influence structural response, with demand variations of up to 35% relative to median responses computed across multiple incidence angles. This study provides one of the first structural evaluations of recently proposed directivity‐based design spectra and underscores the need to explicitly incorporate directivity and ground‐motion incidence angle effects in performance‐based design and assessment of bridges near active faults.