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 shortcoming, 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 the 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-story systems subjected to bidirectional ground motions. The corrective eccentricities depend on four parameters: the rigidity eccentricity er, the strength eccentricity es, the ratio Ωθ of the torsional to lateral frequencies of the corresponding torsionally balanced system, and the ratio Rμ of the elastic strength demand to the actual strength of the system. In this paper, the effectiveness of the proposed method is evaluated on a set of r.c. framed multi-storey structures characterized by different values of er, es, Ωθ and Rμ. The seismic response of these structures is evaluated by nonlinear time-history analyses 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.