Seismic behaviour of fixed and flexible 2d RC frame: A case Study

Abstract

Response of a structure subjected to gravity and lateral loads depends on the boundary conditions assigned at the base of structure in numerical modeling. Most of the structures are analyzed considering fixed base, but in reality, foundation is not fixed. The fixity depends on the interaction between the soil and foundation. In most of the cases base of a structure undergoes small amount of rotation because of flexibility induced by soil especially at the time of earthquake. Difference in boundary conditions used in analysis and in actual conditions will lead to improper estimation of the design forces, to reduce such effects in numerical analysis fixed base of structure can be replaced by springs or structure base resting on soil to obtain results closer to that of actual base conditions. Stiffness of the spring depends on geotechnical parameters as well as on the dimensions of the foundation. In the present paper an attempt is made to understand the difference between the Linear and Non Linear response of a frame having fixed base, spring base and with soil. Stiffness of springs for spring base structure is calculated as per ATC-40 [1] and JICA [7]. Analysis of structure with soil modeling is complex, specifically for Non Linear behavior for this Applied Element Method is used. Linear and Non Linear response is compared in terms of stress resultants and capacity of the frame respectively, for non linear response static pushover analysis is done. From the study, it is observed that the actual capacity of structure is overestimated assuming fixed base. The fact that the initial stiffness of the structure is governed by the soil stiffness can be clearly stated from the obtained pushover curves. It is also recommended that analysis of structure should be done considering spring base in place of fixed support.