International Journal of Engineering and Technical Research (IJETR) ISSN: 2321-0869, Volume-3, Issue-6, June 2015 187 www.erpublication.org Abstract— A structure can be considered as irregular, if it has irregular distributions due to irregular geometrical configurations i.e. set-back, strength, mass and stiffness. Different codes recommend different limits for these irregularities like as per IS 1893:2002, when the horizontal dimension of the lateral force resisting system in any storey is more than 150% of that in an neighboring storey is termed as irregular geometric configuration i.e. set-back, storey in a building is said to contain mass irregularity if its mass exceeds 200% than that of the neighboring storey. If stiffness of a storey is less than 60% of the neighboring storey, in such a case the storey is termed as weak storey, if stiffness is less than 70% of the storey above or less than 80% of the combined stiffness of the three stories above, then the storey is termed as soft storey. In reality, many existing buildings contain irregularity due to visual and functional requirements. In particular, such a set-back form provides for suitable daylight and ventilation for the lower storey in an urban locality with narrowly spaced tall buildings. This set-back affects the mass, strength, stiffness, center of mass and center of stiffness of building as compare to regular building. Dynamic characteristics of such buildings differ from the regular building due to changes in geometrical and structural property. Many researchers have measured the behavior of set-back buildings by taking into account different approaches like geometric, mass, stiffness irregularity and different methods of seismic analysis. But value of critical set-back ratio for which the structure is less prone to earthquake forces has not been reported. Hence, it is require study and specifying some upgrading in codal provisions for appreciative the behavior of set-back buildings. In this present paper lateral storey displacement of different three types of models (nine cases) with constant in bay length i.e. 5x5m and with change in storey height is examined. Nodal displacement criteria were considered for the best value of critical set-back ratio. The most favorable value of set-back ratio comes out to be A/L=0.75 and H=8/25 where nodal displacement value are affect structure in small amount with comparison to other set-back ratio values. Index Terms— IS 1893, Nodal displacement, Set-Back structure, Vertical irregularity I. INTRODUCTION The presence of set-backs i.e. the presence of immediate reduction of the lateral dimension of the building at specific levels of the elevation is a very common kind of vertical Aashish Kumar, did Bachelor of Technology from B.R.C.M College of Engineering and Technology, Bahal, Haryana in 2013 and pursuing Master of Technology in Structural Engineering from P.D.M College of Technology and Management, Bahadurgarh, Haryana. Aman Malik, received his Bachelor of Technology from B.R.C.M College of Engineering and Technology, Bahal, Haryana in 2013 and pursuing Master of Technology in Structural Engineering from Royal Institute of Technology and Management,, Sonepat, Haryana. Neeraj Mehta, earned Bachelor of Technology from B.R.C.M College of Engineering and Technology, Bahal, Haryana in 2013 and pursuing Master of Technology in Structural Engineering from B.R.C.M College of Engineering and Technology, Bahal, Haryana geometrical irregularity in building structures which needed from various functional and aesthetic architecture requirements. This building type belongs under set-back building. This type of building form also provides for compliance with building bye-law restrictions related to „floor area ratio‟. In particular, such a set-back delivers sufficient daylight and ventilation for the lower storey in an urban locality with closely spaced high rise buildings. As per IS:1893:2002, a structure is said to be a set-back structure if ratio between A/L>0.25 as shown below: Fig: 1 A/L ratio as per IS:1893-2002 The set-back structure is characterized by staggered sudden reductions in floor area along the height of the building, with subsequent drop in mass, stiffness and strength. Height change in stiffness and mass render the dynamic characteristics of these buildings dissimilar from the regular building. The rising number of damage after seismic ground motion has provided powerful sign that set-back buildings show poor behavior though they were designed according to the current seismic codes. This poorer seismic performance has been attributed to the combine action of non-uniform distribution of mass, stiffness, and strength along the height of set-back frames and to concentration of inelastic action at set-back level. So effective procedures to estimate seismic deformation demands i.e. damage in set-back frames is certainly needed. Many investigations have been performed to understand the behavior of irregular structures as well as set-back structures and to ascertain method of improving their performance. It is possible to evaluate the seismic performance of set-back structure accurately using STAAD. Pro. V8i software. II. SET-BACK STRUCTURE ANALYSIS METHOD & MODELS A. Analysis Method: The analysis is done on the structure whose related member properties are known. The ways for the seismic assessment of the building requires an appreciative of equivalent lateral force procedure also Seismic Response of Set-Back Structure Aashish Kumar, Aman Malik, Neeraj Mehta