CRITICAL EARTHQUAKE DIRECTION AND MASS PERTURBATION IN THE• SEISMIC ANALYSIS OF FRAMED BUILDINGS

Abstract

The conventional approach of seismic analysis of multistorey buildings involves dynamic analysis under a pure translational excitation, usually applied in two orthogonal directions combined with an additional quasi-static analysis for torsional response. The choice, of two orthogonal directions for applying seismic excitation, is easy to make when structure is symmetrical. But for unsymmetrical structure the choice of seismic excitation direction cannot be made arbitrarily. In this thesis the concept of critical direction of seismic excitation is studied. The critical seismic excitation direction provides a way to identify the most unfavourable direction of seismic excitation for a particular mode but fails to do so in case of a symmetric. structure having pure torsional modes.. To this end mass perturbation procedure has been studied, primarily to account for accidental unsymmetry. By mass perturbation torsional modes can be excited in symmetric structures. In this thesis mass perturbation procedure together with critical direction is used. Thus the need for dual dynamic and sLaL lc analyse:; to modal the torsional response is avoided, as all effects are accounted for within the realm of dynamic analysis only.