Seismic planning For Irregular Structures to Avoid Torsion in First Two Primary Modes - A Literature Review

During strong earthquakes, it is likely that buildings with torsional irregularity in the plan have can be seriously damaged, partially collapsed or fully collapsed. This is because Torsionally Irregular Buildings may have significant aerodynamic torsion loads that increase the eccentricity between the center of mass and the center of rigidity, especially in dominant torsion modes. For this reason, torsion leads to excessive increase in lateral motions when dynamic loads excite the buildings. Torsional irregularity is one of the main failure causes during strong dynamic excitations due to earthquakes. Ignoring torsional irregularity in seismic design analysis can cause unexpected damages and losses. To enhance the safety and performance of buildings, most of the current seismic provisions address this irregularity in two main ways. The first is computing torsion-al moment at each floor by using equations provided in various current seismic code provisions. After they are applied on each floor, the seismic analysis will be performed. The second is shifting the centre of mass or stiffness to eliminate the eccentricity by putting additional masses or structural components such as braced frame systems on buildings. Recently, reports showing the damage caused by earthquakes indicate that, torsion-al effects often cause considerable damage to structures leading to their collapse. The response of asymmetric buildings towards torsion is one of the most crucial factors for their damage. Torsion in such buildings is due to irregularity in plan, mass and stiffness which may cause severe damage in structural systems. Due to various reasons structures acquire asymmetry. Asymmetric structures have irregular distribution of plan, stiffness and mass, its centre of mass and centre of rigidity do not coincide and hence cause the torsion-al effect on the structures which is one of the most important factor influencing the seismic damage of the structure. Structures with asymmetric distribution of mass and stiffness undergoes torsion-al motions during earthquake. The performance of the structures is assessed as per the procedure prescribe in IS 1893:2016. To assess the torsion-al effect on the structures in the present study, different models with irregular shapes with the different orientation of columns and other lateral load resisting elements were prepared and analyzed using E-tabs 2018 software. For the purpose of study, L shaped building of G + 30 storeys with same columns sizes subjected to gravity loads and seismic loads are analysed using linear dynamic analysis.