Accuracy of Simplified Procedures for the Fragility Assessment of Bridges

This paper explores the accuracy of simplified methods based on direct displacement based assessment (DDBA) for the derivation of fragility curves for typical highway bridges. Two variants of the DDBA procedure are implemented: Effective Modal Analysis (EMA), setting a given target deformation level and computing the corresponding seismic level, and Direct Effective Modal Analysis (dirEMA), computing directly the structural response from an input ground motion. These are applied to 16 bridge models, differing for the level of continuity of their deck and the type of pier-to-deck connections. The results obtained with this two simplified analysis approaches are used to derive component- and system-level fragility curves, which are then compared to fragility functions obtained from nonlinear dynamic analyses. While significant discrepancies are observed for some of the component fragility curves, it is found that good agreement can still be found at the system level, possibly due to the existence of minor failure modes that have a negligible impact on the global damage probability. Therefore these simplified methods could be helpful for the efficient derivation of fragility curves for a larger number of bridge typologies.