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 axis, schematised as one-storey models with resisting elements arranged along two orthogonal directions. Their strength is designed by means of the standard application of the modal analysis and by a procedure already proposed by the authors with reference to asymmetric models subjected to unidirectional ground motions. The stochastic nature of the seismic excitation is considered by analysing the structural inelastic response to 30 pairs of artificially generated accelerograms matching the elastic response spectrum proposed by Eurocode 8 for hard layer soil. The secondary horizontal seismic component is scaled to different values so as to examine the influence of its intensity on the ductility demand. The analyses show that the inelastic response is affected only in a minor way by the contemporary presence of the principal and secondary components of the seismic action, although the results are more scattered and significant increases of ductility demand in the elements along the asymmetric direction may sometimes arise. The proposed design 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 components allows the limitation of the orthogonal elements ductility demand.