Seismic Behaviour of R.C. Buildings with Cut-Outs

Abstract

Reinforced Concrete (RC) buildings with irregularities are prone to damage under seismic event as evident from past experience. Buildings with different types of irregularities are unavoidable due to various reasons like, space constraint, floor space constraint, ventilation, aesthetics and local authority guidelines. Broadly, there are two types of irregularities in buildings: Plan Irregularity and Vertical Irregularity. Plan Irregularities include Torsional Irregularity, Re-entrant Corners, Floor Slabs having Excessive Cut-outs, Out-of-Plane Offsets in Vertical Elements, Non-Parallel Lateral Force System while Vertical Irregularities include Stiffness Irregularity, Mass Irregularity, Vertical Geometric Irregularity, in-Plane Discontinuity in Vertical Elements Resisting Lateral Force, Strength Irregularity and Floating Column. Seismic design codes of most countries provide specific guidelines to balance with irregularities in the buildings out of the two types of irregularities of the building, vertical irregularities in terms of stiffness, mass and asymmetry were all well researched. However, plan irregularities of the buildings are relatively less studied. Out of plan irregularities, irregularities related to excessive amendment of Indian Seismic Design Code recommends excessive cut-outs in diaphragm shall not exceed 50% of the area and should be modelled as flexible diaphragm. Therefore, impact of excessive cut-out on seismic behaviour of the RC Building is important to study in order to understand codal provisions of Indian Seismic Design Code better. Additionally, a parametric study on three-dimensional modelling of diaphragm with rigid and flexible (semi-rigid) diaphragm action are conducted. In the present study, seismic behaviour of single storey and multi-storied RC buildings with different levels of cut-outs in diaphragm are studied. Equivalent Static Method (ESM) and Response Spectrum Method (RSM) as presented by Indian Seismic Design Code are employed to estimate Internal Seismic Force on the buildings comprising of cut-outs in diaphragm in the order of 15%, 30%, 45%, 56%, 72%, 81% are developed using ETABS (v2020) software. Three-dimensional building models are designed for gravity loads; Dead Loads and Live Loads. Seismic response quantities; Peak base shear, peak displacement, peak inter-storey drift ratio, normalised base shear and normalised overturning moment are evaluated. A linear time history analysis is also single storey and multi-storied models with cut-outs in the diaphragm. A parametric study in terms of diaphragm modelling, location of cut-outs in the plan, lateral load distribution and stress concentration stresses are conducted for both single-storey and multi-storied RC building with varied levels of diaphragm cut-outs. It has been observed that diaphragm of the RC Building when modelled using rigid and flexible (semi-rigid) diaphragm do not show any difference in lateral load transformation to lateral load resisting system except peak displacement increases with increasing percentage cut-out area in the diaphragm. The lateral load distribution is found to increase for frames near to cut-out section. Stresses produced in the diaphragm due to flexible diaphragm yield very low shear stress. It can be inferred that excessive diaphragm openings yield higher seismic response due to seismic event.