Modified response spectrum model for the design of structures subjected to spatially varying seismic excitations

Abstract

An important aspect of earthquake loads exerted on extended structures, or structures founded on several foundations, is the spatial variability of the seismic motion. Hence, a rigorous earthquake resistant design of lifeline structures should account for the spatial character of the seismic input, at least in an approximate way. A procedure for the modification of the design response spectrum is proposed. It enables addressing the problem of multiply-supported structures subjected to imperfectly correlated seismic excitations by means of an extension to the response spectrum method. A modified response spectrum model is developed for the design of extended facilities subjected to single and multicomponent ground motion, and a modal combination rule is proposed for each case. The modification procedure is based on adjusting each spectral value of the given design response spectrum by means of a correction factor which depends on the structural properties and on the characteristics of the wave propagation phenomenon. Finally, the theoretical model is validated through digital simulation of seismic ground motion, whereby model predictions are found to be in satisfactory agreement with exact results. In chapter 2, a review of past work on the subject of strong motion arrays and spatial variability of ground motion is made, and the relevance of the present thesis work is put into evidence. In chapter 3, the general derivation of the modified response spectrum model is presented. It addresses the cases of discrete systems subjected to both single and multicomponent ground motion, and the case of bridges subjected to a single ground motion component. In chapter 4, modal combination rules are developed for the cases of single and multicomponent ground motion, accounting for the spatial variability of the seismic excitation. In chapter 5, the theoretical response spectrum model is validated through digital simulation of seismic ground motion. Finally, chapter 6 summarizes the results and provides suggestions for further research.