Seismic Analysis of RC Building with Plan Irregularity : Excessive Cut-Outs

Abstract

The prevalence of irregular building construction has significantly surged due to the scarcity of available land, different local authority regulation and aesthetic appeal. Prior research has demonstrated that constructions with irregular configurations are susceptible to damage when subjected to seismic ground motion. Structural irregularities are major factors that decrease the seismic safety of buildings owing to unequal distribution of story drift and excessive torsion. In general, irregularities in structures can be classified into two main types: Plan Irregularity and Vertical Irregularity. Plan irregularities cover torsional irregularity, re-entrant corners, floor slabs with excessive cut-outs, out-of-plane offsets in vertical elements, and a non-parallel lateral force system. Vertical irregularities include stiffness irregularity, mass irregularity, vertical geometric irregularity, in-plane discontinuity in vertical elements resisting lateral force, strength irregularity, and floating columns. One common type of irregularity encountered in the field is the presence of excessive cut-outs in slabs. Reinforced concrete buildings with openings in the middle and other areas of the floor plan are commonly used in university buildings, museums, airport terminals, sports arenas, hospitals, business centers, industrial facilities, and other industrial buildings to meet varied functional needs.

In the present study, the seismic analysis and design of Basement+Ground+5 story RC building with cut-outs at various floor levels has performed. The hotel building has varying percentages of cut-outs at different floor levels serving functional needs. Total five numbers of building models representing different percentage of cut-outs at different floor levels were developed along with one regular building model for comparison seismic response parameter. Those buildings models were analyzed using ETABS (V.21.0) software. The structural components of the RC hotel building were designed following IS 456:2000. The earthquake load was estimated using the equivalent static method and response spectrum methods following IS 1893:2016. Various seismic response parameters; peak displacement, inter-storey drift ratio, normalized displacement ratio, storey shear, normalized base shear and normalized overturning moment for each RC building model were evaluated and were compared with regular RC building model. Time history analysis was also performed on RC building models under EL Centro seismic ground excitation. It was observed that codal specification about cut-outs given in Indian seismic code were found to be sufficient from the present study. However, it was seen that peak normalized displacement ratio of RC building model with cut-outs at various floor levels yields higher values vis-a-vis regular RC building model. Peak inter-storey drift ratio of all RC building models was found to be well within the limit specified by the Indian seismic code, through building model with cut-outs at various floors shows amplified response as compared to regular building model. The said limit found to be breached by each RC building models under EL Centro seismic excitation. The peak normalized base shear and peak normalized overturning moment parameters shows identical response to that of regular building model. It was noticed that shear force demands varies for columns around extreme cut-outs.