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, isв easy to make when structure the is symmetrical, but for unsymmetrical structures the choice of seismic excitation direction cannot be made arbitrarily. The critical seismic excitation direction provides a way to identify the most unfavourable direction of seismic excitation for various modes 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. Mass perturbation procedure together with critical direction alleviates the need for dual dynamic and static analysis to model the torsional response as all effects are accounted for within the realm of dynamic analysis only.