Formulation of design eccentricity to reduce ductility demand in asymmetric buildings

A critical analysis of the large number of papers about the seismic behaviour of asymmetric buildings shows some concordant results: the modal analysis correctly predicts their elastic dynamic response to seismic records, while it overestimates deck rotation in the inelastic range. On this basis, the authors propose to design asymmetric structures by twice repeating the modal analysis: the first one with the actual mass distribution, so as to cover 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 design eccentricity that reduces the maximum ductility demand, the paper statistically analyses the inelastic response of an idealised one storey building, symmetric about one direction, to different sets of accelerograms (both natural and artificial) and compares it to that of the corresponding balanced building; the analysis is repeated many times, so as to evaluate the influence of the different geometrical and mechanical parameters governing the inelastic response. The proposed approach and formulations prove to be effective in evaluating the effects of asymmetry, thus providing a design criterion which limits the ductility demand of asymmetric schemes without relevant increments of structural costs.