Keywords: spatial variability, asynchronous excitation, bridges, seismic design..

Although bridge structures might seem at a first sight to be rather linear and simple structural systems, their actual performance under earthquake loading is more complicated than that of ordinary buildings, since bridges typically have an order of magnitude larger cross-section dimensions, they cross non-uniform soil profiles, whereas the contribution of higher modes of vibration is more important. Bearing in mind that to date significant research effort has already shed some light on many bridge engineering problems, one of the most difficult aspects, which is related to the highest relative uncertainty compared to that related to the superstructure, is the definition of a 'realistic' earthquake motion and its variation among the bridge supports, a problem known as spatial variability of ground motion. It is the objective of this chapter to investigate and quantify the degree of detrimental influence, if any, of the above phenomenon, by attempting to apply a comprehensive methodology that simultaneously considers spatial variability, site effects and soil-structure interaction to the analysis of long bridges and other extended structures such as the Byzantine city walls. Moreover a review is made of current seismic code provisions and recent developments regarding the available methods for generating spatially variable earthquake ground motion scenarios. The chapter concludes with practical recommendations to be used in the design process.

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