A simplified method to predict torsional effects on asymmetric seismic isolated buildings under bi-directional earthquake components

The assessment of maximum displacement demand is a crucial point in the design of seismic isolating systems, in particular when the non linear behaviour of devices is modeled through visco-elastic equivalent schemes, as common in the design practice. Several phenomena influence the maximum demand assessment, among which the torsional and earthquake directionality effects can be of great impact. International codes use some formulations which allow to consider torsional effects, while the impact of the other phenomena is commonly assessed through time-history analyses. In this paper an innovative design method is developed based on an exact linear elastic formulation with response spectrum, which keeps in count both torsional and directivity effects considering natural and accidental eccentricity and by using the CQC3 (Menun and Der Kiureghian in Earthq Spectra 153–163, 1998. https ://doi.org/10.1193/1.15860 25) as directional combination rule. The method models the seismic action through the response spectra of a set of natural recorded ground motions, properly oriented along their principal axes to assess the correct ratio between the horizontal components of spectral accelerations; thus accounting for the sitespecific earthquake source, without the need to perform time-history analyses. A specific formalization of the dynamic problem is presented to emphasize the parameters which more affects the response (e.g. torsional factor, eccentricity, geometrical aspect ratio) and simplify its interpretation. Results obtained on two case studies are compared with timehistory analyses to show the effectiveness of the procedure.