Torsional Response of Asymmtric Buildings under Earthquake Loads

Abstract

Irregular buildings constitute a major fraction of the urban infrastructure due to various occupational and architectural demands.Most buildings are irregular to varying degrees due to asymmetry in plan, elevation, distribution of vertical members or mass distribution on the floors. Perfectly regular buildings are more of an idealized concept and in practice this condition is rarely satisfied. Under seismic loading, the presence of structural irregularity in buildings leads to large displacement amplifications and stress concentrations in the members which lead to their severe damage and ultimately, early collapse. The presence of irregularities in mass, stiffness, strength or geometry along the elevation of the building is categorized as vertical irregularity. Torsional irregularity or in-plan irregularity can be considered to exist if the building possesses non-concurrency in the lines of action of centers of mass and stiffness on a common vertical axis at each floor level. During earthquakes or any other lateral loads, the inertia force acts through the center of mass and resistive force through the center of stiffness or resistance. If an in-plan eccentricity is present, a time varying twisting moment is generated causing torsional vibration. The proposed study investigates the effect of in-plan irregularity and vertical irregularity on the seismic response of buildings. Irregularities in mass and stiffness along the height of the buildings in combination with torsional irregularities along the plan of the buildings are evaluated. Transient analysis is carried out to analyse the seismic response of the shear wall buildings, mass irregular buildings and stiffness irregular buildings with in-plan eccentricity using LS-DYNA software. The responses of the irregular buildings and the effect of in-plan eccentricity in terms of variation in natural period, base shear, storey drifts, roof deflection, torsional resultant and roof rotations obtained from the analysis due to asymmetry have been studied in detail.Based on the seismic responses of the irregular buildings, equations and irregularity coefficients are proposed to quantify and compare buildings with of vertical and torsional irregularity in combination. It is also attempted to suggest modification for the approximate natural period expression given in the IS 1893:2016 and ASCE 7-16 to incorporate the in-plan eccentricity and evaluate the natural period of irregular buildings. Also, the effects of frequency content of ground motions on the seismic responses of the irregular buildings are also evaluated. It is observed that the presence of in-plan eccentricity if present singly or in combination with any other irregularities, determines the overall seismic behavior of a building and tends to modify it's response.