Investigation of column capacity of soft storey building by pushover analysis

Abstract

In Bangladesh, many high-rise buildings have open ground storey with masonry infill in the upper storeys. They are mainly constructed to accommodate parking, reception lobbies and other commercial spaces. Conventional practice is to design these buildings as moment resisting frame building considering masonry infill as non-structural element. Thus contribution of infill in the total structural response is neglected. In reality, masonry infill interacts with frame members and makes the structure stiffer than the bare frame except ground storey. The open ground storey building has stiffness irregularity as visualized during an earthquake. In this thesis, an extensive computational study has been conducted to monitor the response of the open ground storey column subjected to ground acceleration. Numerical investigation of present study is meant to evaluate the soft storey effect on the ground storey column of a soft storey building subjected to earthquake loading. Dhaka city is selected as the investigation site of this study. The variable parameters in the investigation are number of bays, number of spans, and percentage of infill. Two 3D models are generated namely as bare frame model and soft storey frame model. The masonry infill is converted to equivalent diagonal strut within the panel. Conventional static linear analysis is carried out for basic loading (i.e., Dead load, Live load and Earthquake load) and load combination is done as per BNBC (Draft), 2015. Pushover analysis is performed to get nonlinear static response of the models. The effects of said parameters on the models are monitored in detail and evaluated in terms of top deflection, shear force and bending moment of ground storey column. Finally, nonlinear responses of soft storey building model are compared with nonlinear responses of bare frame model. It has been found that the base shear, shear force and bending moment of soft ground storey column of soft storey model, as obtained by pushover analysis method, are considerably higher than the corresponding nonlinear values of bare frame model. This indicates that the ground storey columns of soft storey buildings are overstressed under earthquake loading; as such unexpected soft storey failure of such building occurs before reaching to its ultimate performance point of maximum earthquake. Based on findings of the study, few suggestions are made to safeguard buildings with open ground storey from soft storey collapse. At design stage, magnification of design shear force and bending moment of ground storey column of soft storey building is to be done before designing the columns. This shall compensate the absence of infill wall and minimize the stiffness irregularity in vertical direction. It is expected that justified application of the suggestions would be useful for safeguarding buildings with open ground floor.