ISSN: 2319-8753 International Journal of Innovative Research in Science, Engineering and Technology (An ISO 3297: 2007 Certified Organization) Vol. 3, Issue 9, September 2014 DOI: 10.15680/IJIRSET.2014.0309012 Copyright to IJIRSET www.ijirset.com 15871 Seismic Behaviour of Setback Buildings Milind V. Mohod 1 , Nikita A. Karwa 2 Assistant Professor, Department of Civil Engineering, PRMIT & R, College, Badnera, Maharashtra, India 1 Assistant Professor, Department of Civil Engineering, PRMIT & R, College, Badnera, Maharashtra, India 2 ABSTRACT: A common type of vertical geometrical irregularity in building structures arises from abrupt reduction of the lateral dimension of the building at specific levels of the elevation. This building category is known as the setback building. Various researchers have studied the behavior of setback buildings by taking into account different approaches, which revolve mainly around geometric, mass, stiffness and different methods of seismic analysis. But the value of critical setback ratio for which the structure is less prone to earthquake forces has not been reported. Hence, a need has risen to study and specify some improvements in codal provisions for understanding the behavior of setback buildings. Reference structure results were adopted for validation of results obtained from all these models, which helped us to reach the desired output of the task. Nodal displacement and storey drift criteria was considered for ascertaining the optimum value of critical setback ratios. The optimum value of setback ratio came out to be RA=75% and RH=6/5, where the nodal displacement and storey drift values are affecting structure in negligible amount as in comparison to other setback ratio values. Hence, as we go through seismic code, the revision of seismic codes provisions for geometric vertical irregularities seems to be essential to stipulate more restrictive limits or apply more accurate analytical procedures to predict the seismic performance of setback structures under the seismic excitations, especially for structures with critical setback ratios. Keywords: Setback buildings, Setback ratios, IS 1893, Vertical irregularity, Storey drift. I. INTRODUCTION A common type of vertical geometrical irregularity in building structures, needed from various functional and aesthetic architecture requirements, is the presence of setbacks i.e. the presence of abrupt reduction of the lateral dimension of the building at specific levels of the elevation. This building category belongs under setback building. In particular, such a setback form provides adequate daylight and ventilation for the lower storey in an urban locality with closely spaced tall buildings. This type of building form also provides for compliance with building bye-law restrictions related to ‘floor area ratio’ (practice in India). Setback buildings are characterized 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. The increasing number of damage after seismic ground motion has provided strong evidence that setback buildings exhibit inadequate behavior though they were designed according to the current seismic codes. This inferior seismic performance has been attributed to the combined action of structural irregularities i.e. to the combined non-uniform distribution of mass, stiffness, and strength along the height of setback frames, and to concentration of inelastic action at setback level. [1]. So effective procedures to estimate seismic deformation demands i.e. damage in setback frames is certainly needed. Many investigations have been performed to understand the behavior of irregular structures as well as setback structures and to ascertain method of improving their performance. But it is seen that most of the previous studies were based on elastic response of simple structural system and were not satisfied the current code requirements. So there is a need to study