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.