Impact of seismic excitation direction on the fragility analysis of horizontally curved concrete bridges

The focus of this study is the impact of the seismic excitation direction on the fragility of horizontally curved bridges. Nonlinear time history analyses are performed on a typical, curved concrete bridge in China using a set of real ground motions with different incident angles. To build reliable probabilistic seismic demand models, ten commonly used intensity measures (IMs) are assessed in terms of various metrics to determine the optimal IMs, which account for the influence of the seismic excitation directions. Subsequently, fragility surfaces with respect to both the optimal IM and incident angles are generated to qualify the fragility sensitivity for various components and the bridge system to the seismic excitation directions. Moreover, the rationality and applicability of the methods recommended by the Caltrans, Eurocode 8 and Chinese codes for determining the seismic excitation direction of curved bridges are evaluated. The results indicate that the excitation direction imposes a minor impact on the optimal IM rankings. Compared to structure-independent IMs, structure-dependent IMs are more appropriate for predicting the demands of horizontally curved concrete bridges. However, the seismic excitation direction significantly affects the component fragilities, and the level of the effect intensifies with increasing limit states. If the incident angle occurrence probability is not provided, the Chinese code method for the seismic excitation direction is more suitable for the horizontally curved concrete bridge fragility assessment, which has the advantages of computational efficiency when compared to the Caltrans code and relatively conservative results when compared to Eurocode 8.