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 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.).