Asymmetric-Plan Buildings: Irregularity Levels and Nonlinear Seismic Response

Results from both linear and nonlinear dynamic analyses exploring the seismic torsional behaviour of a multi-storey RC frame structure asymmetric in plan and regular in elevation are reported. Based on the obtained findings, the conceptual gaps in the current prescriptions given by the OPCM3431 and the EC8 on the evaluation method for the irregularity level produced by the mass and stiffness asymmetry and on the definition of the range of applicability of the linear analyses methods are identified. Comments on the code prescriptions relative to the number of accelerograms needed in the nonlinear time-history analyses and on the proposed method for modifying the seismic response obtained with a pushover analysis in order to account for the torsional behaviour of asymmetric-plan buildings are also presented. Possible proposals for a code provision are finally suggested.