Influence of bi-directional ground motions on the inelastic response of one-storey in-plan irregular systems

This paper examines the influence of bi-directional seismic excitations on the inelastic behaviour of in-plan irregular systems having one symmetry ax is, schematised as one-storey models with resisting elements arranged along two orthogonal directions. Their strength is designed by means of the stan dard application of the modal analysis and by a procedure already proposed by the authors with reference to asymmetric models subjected to unidirectio nal ground motions. The stochastic nature of the seismic excitation is cons idered by analysing the structural inelastic response to 30 pairs of artifi cially generated accelerograms matching the elastic response spectrum propo sed by Eurocode 8 for hard layer soil. The secondary horizontal seismic com ponent is scaled to different values so as to examine the influence of its intensity on the ductility demand. The analyses show that the inelastic res ponse is affected only in a minor way by the contemporary presence of the p rincipal and secondary components of the seismic action, although the resul ts are more scattered and significant increases of ductility demand in the elements along the asymmetric direction may sometimes arise. The proposed d esign procedure is almost always able to reduce the ductility demand of the resisting elements along the asymmetric direction to values comparable to those required by torsionally balanced systems. In most cases the adoption of Eurocode 8 provisions to combine the effects of the two seismic componen ts allows the limitation of the orthogonal elements ductility demand. (C) 2 001 Elsevier Science Ltd. All rights reserved.