Assessment of EC8 procedures for the asynchronous excitation of bridges based on numerical analyses and recorded data

The significance of the spatial variability of earthquake ground motion SVEGM is recognized by all modern seismic codes (EC8, AASHTO, ATC, JRA) but it is only EC8 that provides a clear, though simplified, methodology to account for asynchronous motion for design purposes. Currently, two amendments of the EC8 methodology have been proposed by Sextos & Kappos (2009) and Nuti & Vanzi (2009) both aiming to improve the applicability and robustness of the EC8 simplified procedure. In the present study, the above EC8 framework is comparatively assessed in the light of actual, low-amplitude measurements obtained along the Evripos cable stayed bridge. A permanent accelerometer special array of 43 sensors was installed on the bridge in 1994 by the Institute of Engineering Seismology and Earthquake Engineering. Since then, the bridge's behavior to seismic excitations has been continuously monitored. A reliable numerical model was first developed and was updated to match the monitored response given the available free-field recordings. Subsequently, various (synchronous and asynchronous) seismic scenarios were developed to identify the impact of ground motion spatial variation on the action effects of the bridge using the 7/9/1999, Athens earthquake (Ms=5.9) records. The results indicate that spatial variability of earthquake ground motion has generally a favorable effect on the Evripos bridge response, primarily due to the significant flexibility of its deck. It is also concluded that the Eurocode 8 simplified method, being essentially a pseudo-static loading is inable to capture the salient features of dynamic response under multiple-support excitation.