SEISMIC RESPONSE OF BUILDINGS WITH FLOATING COLUMNS

Abstract

Gujarat earthquake disaster of January 26,2001 has been an eye opener for the architects, designers, administrators and town planners. It has been observed that their existed many lacunae in the existing construction practices and bye-laws. There are certain rules and regulations" laid down by the municipalities or town planning authorities or urban improvement boards, in their jurisdiction. These have to be considered by an architect while planning and designing the layout of the buildings. The building bye-laws in Ahmedabad and Gandhidham (and rest of the country) require spaces to be left open on all sides of the building which are measured from the edge of building footprint in the ground storey. Designers often take advantage of this aspect of measuring the offset from the edge of building footprint in the ground storey and have covered balconies on cantilever overhangs in the upper storeys. This configuration invariably results in floating column located at the end of the overhangs, giving the buildings a mushroom like appearance on top of the ground floor columns. The exterior columns in the upper storeys are terminated at the first floor level, which results in a sudden change in the load-path of the seismic forces. This type of construction does not create problem under gravity loading condition but during earthquake a clear load path is not available for transferring the lateral forces which endangers the safety of the building. There are several examples where such buildings suffered damage during earthquakes. In the present study an attempt has been made to understand the seismic behavior of such buildings. In the present study 3D linear dynamic analysis of a G+4 storey residential reinforced concrete ordinary moment resisting building with iii floating columns, located in seismic zone III, designed as per IS: 456-1978, has been carried out using response spectrum method by STAAD.Pro 2001 software. Results of the dynamic analysis show that this type of buildings are very flexible and vibrates dominatingly in torsional mode, which is undesirable because buildings are not normally designed for such actions. In order to avoid the torsional mode of vibration four retrofitting schemes have been suggested. These schemes involve use of shear walls at different strategic locations. Results of dynamic analyses on the building retrofitted with different schemes show that shear walls are capable of averting the torsional mode of vibration of the study building. Care should be taken in choosing the location of shear walls and their connections with slab/beam/columns. Iv