With the reduction of available space, the structures are developing vertically rather than horizontally and increase in height of structures tends to attract more seismic forces. IS:1893 (2002) stated that seismic analysis should be performed on structural model which is excited with forces in two orthogonal directions separately. But an actual earthquake will have its effect in both the directions simultaneously. Limited research has been carried out on the effect of such biaxial excitation on multistorey building having biaxial eccentricity. The paper deals with the comparison of the nonlinear behavior of G+5, G+10 and G+15 storey buildings with biaxial eccentricity during biaxial excitation by time-history analysis. The angle of incidence of earthquake forces is varying between 0 and 360o. The building having regular plan with eccentricity along each of x and y directions, has been studied. Time-history analysis has been carried out using SAP2000 after validating a preliminary model with the experimental results available in the reference literature.

Structures are becoming more vulnerable towards seismic forces with the increase in the height of the building. However, there are two basic recommendations required for sound design of earthquake-resistant structures: symmetry and geometric regularity of plan. It is not always possible to attain these two recommendations in structure due to some constraints like aesthetic constraints, architectural constraints, purpose of the building, etc. Even after following all the code provisions, it has been observed that those buildings which possess irregularities have had an unsatisfactory behavior during strong earthquakes. In these types of structures, the reason for partial or complete collapse has been attributed to the actual three-dimensional nature of earthquake effects. Till date, the three-directional analysis has not been explored in depth. Although some light has been thrown on biaxial excitation of structures, most of the literature describes only the qualitative effects, and this literature includes only single-storey structures. Particularly, much effort has been made in studying the effect of seismic behavior of the design eccentricities already adopted by the Indian Standard code IS: 1893 (2002); for example, the study of influence of vertical irregularity (Cassis and Cornejo, 1996) and estimation of accidental torsion effects for seismic design of buildings (Juan and Anil, 1995), which was included in IS: 1893 code as 5% of lateral dimension.

Most of the above-mentioned literature basically focussed on issues related with design problems, and numerous studies on analytical aspects have also been carried out. However, in all these studies, the authors have taken single-storey structure, whereas currently most of the structures are multistoried. However, these investigations have helped in developing analysis methods to achieve a certain level of efficacy, for example, the nonlinear seismic response on asymmetric plan buildings by Lucchini et al. (2000), influence of bidirectional seismic motion on the response of asymmetric building (Julio and Oscar, 2000), nonlinear response of two-way asymmetric single-storey building under biaxial excitation (Lucchini et al., 2011). This literature shed light on several gray regions of analytical understanding of structures. However, final conclusions of earthquake forces in biaxial excitation on multistorey structures with biaxial eccentricity and other significant parameters are still lacking.

Structural Engineering Journal, Seismic analysis, Biaxial excitation, Multistorey building.