Behavior of multistoried RC framed buildings with open ground floor under seismic loading

Abstract

In urban areas of Bangladesh it is a very common practice to keep the ground floor of multi storied buildings open for parking facility. These buildings-are generally designed as RC framed buildings without considering the masonry infill contribution. Conventional practice is to consider the masonry infill as non structural element. But in reality masonry infills make the structure stiffcr; resulting stiffness irregularity, in buildings with open ground floor. In this thesis, an extensive computational study has been conducted to find out the nature of buildings with open ground floor and to determine the seismic vulnerability of such a building. A numerical investigation has been performed to evaluate the naturc of RC framed building with open ground floor, i.e. soft story building. Several soft story buildings with variation of number of floors, number of span, number of bays, floor panel size, percentage of infills etc. are considered during the investigation. The infills are modeled as equivalent diagonal strut. Beams and columns are modeled using threedimensional frame element and floor slab using shell element. Loads considered during analysis are dead load, live load, wind and earthquake load. Bangladesh National Building Code (BNBC, 1993) is followed in the analysis. Earthquake load is studied with both equivalent static force method and response spectrum method. The time period, force, bending moment, sway pattern as well as stecl percentage of the same buildings is also evaluated and compared using equivalent static force method and response spectrum method. It has been found from analysis that natural period of the building decreases with increasing amount of in fill as found from dynamic analysis while conventional Equivalent Static Force Method (ESFM) does not show any variation in period with varying amount of infill. Presence of infill in upper stories makes those floors much stiffer compared to ground floor. As a result the upper floors behave togethcr in a rigid manner causing excessive sway at ground floor level. This soft story behavior is clearly identified in dynamic analysis in prescnce of infill while conventional static analysis cannot predict such behavior. It has been found from 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 story are, in general significantly under-designed for earthquake loading. As such the possibility of collapse of such buildings by soft story mechanism is very high during earthquakes. Based on the findings of the study suggestions are made to safcguard buildings with open ground floor form soft story collapse. Two alternative suggestions arc made, namely, a) base shear amplification for design of ground floor columns at design stage and b) Isolation of infill form the frame elements at construction phase to make them structurally inactive. It is expected that these suggestions would be useful for safeguarding buildings with open ground floor and human life.