EFFECT OF SETBACK ON FUNDAMANTAL PERIOD OF RC FRAMED BUILDINGS

The unbalanced load effects can be further exacerbated by non-alignment of center of stiffness of the structural system and center of mass of the floor plates, which result in three-dimensional (3D), coupled mode shapes and experience coupled responses when exposed to wind & Earthquake loads. A structure can be classified as irregular if it contains irregular distributions of mass, stiffness and strength or due to irregular geometrical configurations. Different codes prescribe different limits for these irregularities like as per IS 1893:2002, a storey in a building is said to contain mass irregularity if its mass exceeds 200% than that of the adjacent storey. If stiffness of a storey is less than 60% of the adjacent storey; in such a case the storey is termed as „weak storey", and if stiffness is less than 70 % of the storey above, then the storey is termed as „soft storey". In reality, many existing buildings contain irregularity due to functional and aesthetic requirements. However, past earthquake records show the poor seismic performance of these structures. This is due to ignorance of the irregularity aspect in formulating the seismic design methodologies by the seismic codes (IS 1893:2002, EC8:2004, UBC 1997, NBCC 1989, NBCC 2005 etc.). Seismic Behaviour of asymmetric building may cause interruption of force flow and stress concentration. Due to this, there is produce of torsion in the building which leads to increase in shear force, lateral deflection and ultimately causes failure. Asymmetry can be reason for a buildings poor performance under sever seismic loading. The building with vertical setbacks and L, H, U or T shaped in plans which built as unit are more affected during seismic event. There is horizontal torsional effect on each arm arising from the differential lateral displacement of two ends of each arm. In this paper, inelastic seismic behaviour of multi-storeyed building with vertical setbacks are analysed by IS code approach. The effect of torsion on building are analysed. Designs of asymmetric multi-storeyed building are studied. Study shows that there is increase in shear force due to torsion in column and increase in area of steel reinforcement in column particularly at the edge member of the building. Setback buildings with geometric irregularity (both in elevation and plan) are now increasingly encountered in modern urban construction. Setback buildings are characterised by staggered abrupt reductions in floor area along the height of the building, with consequent drops in mass, strength and stiffness. Height-wise changes in stiffness and mass render the dynamic characteristics of these buildings different from the ‘regular’ building. Many investigations have been performed to understand the behaviour of irregular structures as well as setback structures and to ascertain method of improving their performance. Here an attempt has been made to study the behaviour of different structures of reinforced concrete with different heights with and without shear walls. Coupled shear walls have also been studied to understand the comparative merit or demerit of framed structures with shear wall structures. Studies have been carried out on sample model structures and analysis has been carried out by ETABS software. It has been ensured to consider sample models that represent the current practices in structural design to include different structural configurations. Models having varied structural configurations like framed, shear wall, coupled shear wall, central core shear wall, core in core etc. have been taken into consideration. The inherent asymmetry present in the structures have also been deal