Towards reliable seismic fragility assessment of highway bridges with oblong columns considering the drift-based capacity directionality effect

Highway bridges in seismic zones are commonly designed with oblong columns, featuring different force-displacement relationships and seismic capacities along different response directions. However, the state-of-art seismic fragility assessment treats seismic demand and capacity models as two independent models, which cannot capture the in-between directional dependence and correlation for oblong columns. This study develops direction-dependent seismic capacity limit state models and proposes a multi-step analysis framework to assess the seismic fragility of highway bridges with flexure-controlled oblong columns. Cyclic pushover analyses are conducted along different loading directions to identify direction-sensitive column drift ratios against four damage states. These drift ratios are utilized to regress a closed-form model, namely the directional capacity ratio model, to quantify the changes in drift capacities as a function of the column response angle. This model is integrated into an analysis framework to directly simulate the demand-capacity ratio (DCR) time history, where the peak value is utilized to assess the seismic fragility of the oblong column. The case study indicates that the most severe seismic damage to an oblong column is not necessarily along its principal axis. The proposed analysis framework outperforms existing approaches in having more reliable fragility estimates for the oblong column and bridge system.