International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064 Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438 Volume 4 Issue 6, June 2015 www.ijsr.net Licensed Under Creative Commons Attribution CC BY Seismic Soil Structure Interaction of Buildings with Rigid and Flexible Foundation Amar R Chougule 1 , S S Dyavanal 2 1 Research Scholar and Asst. Professor, Department of Civil Engineering, KLECET, Chikodi, 2 Professor, Department of Civil Engineering, B.V.B.C.E.T., Hubballi Abstract: The effect of soil-structure interaction is generally ignored in the design process of low-rise buildings resting on shallow foundations though it has been shown that ignoring such effect may lead to unsafe seismic design. When a structure is subjected to an earthquake excitation, it interacts the foundation and soil, and thus changes the motion of the ground. It means that the movement of the whole ground structure system is influenced by type of soil as well as by the type of structure. An attempt has been made in this paper to study the effect of Soil-structure interaction on multi storeyed buildings with various foundation systems. Also to study the response of multi storeyed buildings subjected to seismic forces with Rigid and Flexible foundations subjected to seismic forces were analysed under different soil conditions like hard, medium and soft. A conventional G+6 storied building when rests on different soils is chosen for the study. The influence of soil-structure interaction is compared with the results obtained when the structure is assumed to be fixed at the base. Keywords: Soil structure interaction, Natural period, Base shear, Fixed base, Flexible base, Soil stiffness, Storey drift. 1. Introduction In the last three decades, the effect of SSI on earthquake response of structures has attracted an intensive interest among researchers and engineers. Most of these researches focus on theoretical analysis, while less has been done on the experimental study. The interaction among the structure, foundation and soil medium below the foundation alter the actual behavior of the structure considerably as obtained by the consideration of the structure alone. Flexibility of soil medium below foundation decreases the overall stiffness of the building frames resulting in an increase in the natural period of the system. Soil-Structure Interaction (SSI) is a collection of phenomena in the response of structures caused by the flexibility of the foundation soils, as well as in the response of soils caused by the presence of structures. Analytic and numerical models for dynamic analysis typically ignore SSI effects of the coupled in nature structure- foundation-soil system. It has been recognized that SSI effects may have a significant impact especially in cases involving heavier structures and soft soil conditions. A parametric study is carried out for determining the lengthened lateral natural period of building frame due to incorporation of the effect of soil structure interaction. The study includes the building with isolated footing on soft, medium and hard soil and comparison between the natures of change in lateral natural period has been presented. Such a study may help to provide guidelines to assess more accurately the seismic vulnerability of building frames and may be useful for seismic design. The primary issues involved in the phenomenon of soil-structure interaction is the seismic waves propagate through soil during an earthquake, a discontinuity in the medium of wave’s propagation is encountered at the interface of soil and structural foundations. The change in the material properties leads to scattering, diffraction, reflection and refraction of the seismic waves at the soil foundation interface their by changing the nature of ground motion at that point from what would have otherwise been observed in the absence of structure and foundation. The overall lateral stiffness of any building decreases due to the compressibility of soil. This leads to a subsequent increase in the natural periods of the structural system. Hence the effect of soil-structure interaction on the structural system resting on isolated foundation needs a detailed investigation. The soil-structure interaction may not be considered in the seismic analysis for structure supported on rock or rock like material. 2. Pushover Analysis The recent advent of performance based design has brought the nonlinear static pushover analysis procedure to the forefront. Pushover analysis is a static, nonlinear procedure in which the magnitude of the structural loading is incrementally increased in accordance with a certain predefined pattern. With the increase in the magnitude of the loading, weak links and failure modes of the structure are found. The loading is monotonic with the effects of the cyclic behavior and load reversals being estimated by using a modified monotonic force-deformation criteria and with damping approximations. Static pushover analysis is an attempt by the structural engineering profession to evaluate the real strength of the structure and it promises to be a useful and effective tool for performance based design. The ATC-40 and FEMA-273 documents have developed modeling procedures, acceptance criteria and analysis procedures for pushover analysis. These documents define force-deformation criteria for hinges used in pushover analysis. As shown in Figure 1, five points labeled A, B, C, D, and E are used to define the force deflection behavior of the hinge and three points labeled IO, LS and CP are used to define the acceptance criteria for the hinge. (IO, LS and CP stand for Immediate Occupancy, Life Safety and Collapse Prevention or Structural stability level respectively.) The values assigned to each of these points vary depending on the Paper ID: SUB155258 513