Nonlinear time history analysis of reinforced concrete frames with brick masonry infill

Abstract

In Bangladesh, most of the multistoriedbuildingsare found to have open storeyat ground floor with masonryintillin the upper stories.This is primarilybeing adoptedto accommodate parking or reception lobbies in the ground floor. Conventionalpractice is to design these buildings as RC frames considering the masonry infill as non structural component, thus neglecting the contribution of the infill in the total structural response. But in reality masonry infills interact with the frame members and make the structure stiffer; resulting stiffuess irregularity in buildings with open ground floor. In this thesis, an extensive computational study has been conducted to monitor the response of RC building frames with open ground floor subjected to ground accelerations, and to determine the seismic vulnerability of such building frames in selected seismic zone of Bangladesh. The numerical investigations performed, in this present study, are meant to evaluate the soft storey effects on a typical interior bay of a RC building frame with open ground floor subjected to earthquake loadings. Infill percentage in the upper stories and number of stories (i.e., total height) of the building frames are considered as the main parameters. Plane frame is generated with 2D frame elements and the infills are considered as equivalent diagonal strut within the panels. Conventional static analysis is carried out for the basic loads (i.e., Dead load, Live load, Wind and Earthquake Load) and for all load combinations following Bangladesh National Building Code (BNBC, 1993); particularly earthquake load is analyzed using equivalent static force method. In order to get the dynamic response of the frames, time history analyses of the frames have been performed through application of real earthquake time history accelerograms following UBC 1997. The effects of the said parameters on the frames natural time period, sway pattern in terms of interstorey drift, forces and bending moment in the ground floor columns etc. are monitored in detail and evaluated. Important basic responses are compared with that from 'equivalent static force method. Time history analysis demonstrates influences of infill percentage on the vibrational characteristics of RC frames with open ground floor. Presence of infill in upper stories increases their relative stiffuess comparing with ground floor. As a result the upper floors behave as a single unit and motion of an inverted pendulum takes place. Thus time history analysis clearly identifies the soft storey effects in presence of infill, while conventional static analysis fails to predict such behaviour. It has been found ,from time history analysis that the design base shear is significantly increased due to presence of structurally active infill as compared to static analysis. This clearly indicates that the ground floor columns in buildings with soft storey are significantly under-designed for earthquake loading through equivalent static force method; as such catastrophic soft storey failure of such buildings may occur during earthquakes. Based Onthe findings of the study suggestions are made to safeguard buildings with open ground floor from soft storey collapse. At design stage, amplification of design shear force and bending moment, as obtained from conventional equivalent static force method, shall demand for greater geometry and increased reinforcement for the open ground floor columns. This shall compensate the absence of infill walls between the columns and minimize the stiffness irregularity in vertical direction. It is expected that justified application of the suggestion would be useful for safeguarding buildings with open ground floor and human life.