Optimal intensity measure-based seismic fragility surfaces for curved bridges considering their sensitivity to seismic excitation direction

The effect of seismic directionality is crucial for curved bridges, a subject generally overlooked in seismic vulnerability analysis. This paper focuses on seismic fragility development as a function of seismic incidence directions for a geometrically curved bridge. A series of non-linear time history analyses were carried out for a representative finite element model of the bridge by considering actual ground motions. For reliable seismic demand models, a total of eleven intensity measures (IM) were analyzed based on optimality metrics. To quantify the sensitivity of fragility functions to input incidence directions, fragility surfaces were developed throughout the horizontal plane by considering spectral acceleration at one second (Sa1.0) as the optimal IM. Results show that the optimal IM ranking is insignificantly influenced by seismic directionality. However, seismic orientation influences fragility, which intensifies in higher damage states, particularly for piers. For a bridge system, the differences in median demand corresponding to the least and most vulnerable direction for slight, moderate, extensive, and collapse states are about 9.0%, 7.31%, 10.32%, and 11.60%, respectively. These results imply that while evaluating the vulnerability of curved bridges, the optimality of IM in demand estimation and the impact of seismic directionality should not be disregarded.