Behaviour of randomly infilled RC frames with soft ground floor subjected to seismic loading

Abstract

Multistoried masonry infilled reinforced concrete (RC) frame with open ground floor is a common building construction practice in Bangladesh. Masonry infills in upper floors make the corresponding floors stiffer; resulting stiffness irregularity in RC frames. Consequently stress concentration occurs at open ground floor level in the event of any seismic load. But, this interaction of masonry infill panels with frame elements is often neglected in the conventional design analysis of such structures. Therefore, an extensive analysis has been performed in the present study to determine the seismic performance of masonry infilled RC soft story buildings and to propose appropriate mitigating measures against their earthquake vulnerability. In the numerical analysis, several soft story 2D frames with variation in floor and span numbers, infill percentages, slenderness of frames as well as randomness of infill positions have been considered to investigate their corresponding seismic performances. The infills are modeled as equivalent diagonal struts. Beams and columns are modeled using two-dimensional elastic frame element. Considered loads during the analyses are dead load, live load, earthquake load and their combinations. Earthquake loads have been applied following both the equivalent static force method (ESFM) and the dynamic response spectrum method (RSM). The base shear, sway pattern and drift demand etc. are evaluated and compared following ESFM as well as RSM. In addition, slenderness of frame and base shear ratio is compared with percentage of infill. Numerical analysis has revealed sudden increases of sway at soft ground floor level whereas it decreases gradually in the upper floors due to presence of infills. The presences of infills stiffen the upper floors resulting major deflection at ground floor level. Also the base shear is significantly increased in presence of structurally active infill as compared to static analysis. This soft story behavior has been clearly identified in dynamic analysis while conventional static analysis cannot predict such behavior. It has been observed from analysis that randomness in the distribution of infill shows no effect on base shear value. Also the observations clearly indicate that the ground floor columns in soft story buildings are, in general, significantly under-designed for seismic loads found from ESFM and vulnerable during earthquake. Finally based on the findings of the present study, a magnification factor has been proposed as a function of number of floors to magnify the base shear, moment and shear force found from ESFM. It is expected that design of ground floor columns based on magnified moments and forces will safeguard the soft story buildings from catastrophic failure at the event of earthquakes.