Orthogonal Effects in Nonlinear Analysis of Bridges Subjected to Multicomponent Earthquake Excitation

Abstract

Choosing the direction of applied earthquake forces has always been an important issue in earthquake engineering, especially as it refers to nonlinear analysis. Determination of the response to all possible incident angles is too time consuming and not practical. Fortunately, in recent years, some studies have been conducted on the response spectrum method of analysis of structures subjected to multicomponent earthquake excitation that enables the designer to find the most critical angle of excitation. However, the results from these studies are only valid for the response spectrum method for a linear elastic analysis. As such, there is no method to find the critical angle of excitation for a nonlinear time history analysis. This paper investigates the orthogonal effects in nonlinear analysis of candidate single-span bridges subjected to multicomponent earthquake excitations, and evaluates the effects of nonlinearities in candidate bridges affected by varying angles of excitation to determine the most critical angle. Also a comparison will be made between the results of the multicomponent earthquake excitations and AASHTO combination rule for combining the effects of separate unidirectional excitations. Although seismic analysis is not necessary for single-span bridges based on AASHTO, this paper applies AASHTO procedure on single-span bridges to compare the procedure to the results of nonlinear time history analyses (NTHA) at various excitation angles. Results show that the critical excitation angle is not the same in linear and nonlinear models and nonlinear models are more sensitive to the angle of excitation. It is also concluded that AASHTO procedure may underestimate the maximum probable response.