Structural Design for Multicomponent Seismic Motion Oscar A. LÓPEZ 1 and Julio J. HERNÁNDEZ 2 SUMMARY This paper aims to discuss structural design procedures for multicomponent seismic motion and to present application examples in structures such as bridges and platforms. The seismic motion is described by the three principal ground acceleration components that can form any angle with the structural axes. The main properties of the principal seismic components, needed to determine the structural response, were identified for an ensemble of 97 earthquake records. One principal component have a mean inclination angle of 11.4° with respect to the vertical axis and a standard deviation of about 10°. Spectral ratios of the minor and the major quasi-horizontal spectra are found to be between 0.63 and 0.87, depending on vibration period T. Spectral ratios of the quasi-vertical and the major quasi-horizontal spectra are between 0.3 and 1.33 depending on T and the distance to the fault. The CQC3-rule was applied to determine the critical structural response to two seismic components on a vertical plane: a quasi-horizontal and a quasi- vertical component that has an inclination with respect to the vertical axis. The inclination of the quasi- vertical component may significantly increase the response of structures with close periods of vibration, up to 1.37 times the standard SRSS response. The critical response to three seismic components that may have arbitrary directions with respect to the structural axes, including a restriction to the maximum inclination of the quasi-vertical component, was determined using the GCQC3-rule, a generalization of the known CQC3-rule that considers one principal component to be vertical. An upper bound of the critical response is determined by combining the eigenvalues of the response matrix R and the spectral ratios of the three components. When the inclination is considered the critical response can be up to 1.26 times the standard SRSS response for the structures considered. INTRODUCTION Earthquake-resistant design requires consideration of multicomponent seismic motion as specified in current building codes [1-3]. Under the framework of the response spectrum method of analysis the seismic components are described in terms of the response spectra associated to the principal directions of ground motion [4,5]. Although there is a great deal of information regarding the properties of the response spectra of the recorded seismic components [6,7], very little information is available concerning the principal components. In a previous study the authors evaluated the properties of the principal spectra, limited to the horizontal components of motion, using a small sample of 17 seismic events finding that the major and minor components have different spectral shapes [8,9]. In preliminary studies using three 1 Professor, IMME, Engineering School, Central University of Venezuela. oslopez@reacciun.ve . 2 Associate Researcher, IMME-CDCH-FONACIT; julher@cantv.net 13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 2171