Predicting displacement demand of multi-storey asymmetric buildings by nonlinear static analysis and corrective eccentricities

Nonlinear static methods are not always effective in the assessment of multi-storey asymmetric structures because of the errors committed in the evaluation of the deck rotation. To overcome this problem, some of the authors have recently proposed the use of two nonlinear static analyses, characterised by lateral forces applied to different points of the deck. For each of these two analyses, the distance between the point of application of the lateral force and the centre of mass of the deck (corrective eccentricity) is given by relations resulting from a parametric study on a large set of single-storey systems subjected to bidirectional ground motions. The corrective eccentricity depends on four parameters: the rigidity eccentricity, the strength eccentricity, the ratio of the uncoupled torsional to lateral frequencies and the ratio of the elastic strength demand to the actual strength of the system. In this paper, the effectiveness of the proposed method is evaluated for different values of the abovementioned parameters, considering two seismic excitation levels and multi-storey buildings with reinforced concrete framed structure or steel structure equipped with buckling restrained braces. The seismic response of the structures is evaluated by nonlinear time-history analysis and compared with that resulting from the application of the proposed nonlinear static method. To highlight the accuracy of the proposed method, a comparison is also made with the response obtained from the standard application of the nonlinear static method, i.e. without corrective eccentricities, and with the response predicted by the extended N2 method.