Formulation of design eccentricity to reduce ductility demand in asymmetric buildings

A critical analysis of the large number of papers about the seismic behavio ur of asymmetric buildings shows some concordant results: the modal analysi s correctly predicts their elastic dynamic response to seismic records, whi le it overestimates deck rotation in the inelastic range. On this basis, th e authors propose to design asymmetric structures by twice repeating the mo dal analysis: the first one with the actual mass distribution, so as to cov er the elastic behaviour; the second one by considering the centre of mass displaced towards the centre of rigidity by a design eccentricity, so as to fit the inelastic response. In order to assess a formulation for the desig n eccentricity that reduces the maximum ductility demand, the paper statist ically analyses the inelastic response of an idealised one storey building, symmetric about one direction, to different sets of accelerograms (both na tural and artificial) and compares it to that of the corresponding balanced building; the analysis is repeated many times, so as to evaluate the influ ence of the different geometrical and mechanical parameters governing the i nelastic response. The proposed approach and formulations prove to be effec tive in evaluating the effects of asymmetry, thus providing a design criter ion which limits the ductility demand of asymmetric schemes without relevan t increments of structural costs. (C) 2000 Elsevier Science Ltd. All rights reserved.