Mode shape modification of irregular design of buildings

Abstract

Earthquakes are the most unpredictable and devastating of all natural disasters. In view of the extensive damage it can cause, it is very much necessary to understand the seismic response of buildings under various conditions. One such situation is buildings with plan irregularity, and poses a challenge to structural engineers because of the damage it can cause. These torsional irregular buildings are subjected to large rotations about the vertical axis due to the eccentricity between centre of mass and with centre of rigidity. Table 5 of IS 1893(Part-1):2016 mentions re-entrant corners, large cut outs in the slabs, non-parallel lateral force system are some of the causes of torsional irregularity. These buildings will have torsional mode as the fundamental mode and will be subjected to extensive damage during seismic activity. In situations where the irregular shape is inevitable because of architectural and functional demands, the structural engineer has to find means to transform the fundamental mode shape from torsional mode to translational mode. Adding structural walls, bracings at the appropriate locations are a few of the viable solutions. In the present study, an attempt is made to investigate the appropriate location and geometry of structural walls for few torsional irregular buildings. Three-dimensional dynamic analysis (Response Spectrum Method) of 15 storied L and C- shape buildings is carried out as per IS 1893 (part 1) using ETABS. Response quantities like Time Periods, Base Shear, Storey Moments, Modal Participation factors, Joint displacements, Storey Drifts are evaluated. A comparative study is made to arrive at the optimum location of shear walls with minimum torsion displacement and where translational mode becomes the fundamental mode.