Dynamic Behaviour of Raft Foundation for Tall building with Variable Subsoil

Structures are often constructed on layers of soil unless bedrock is very close to the ground surface. When the ground is stiff enough, the dynamic response of the structure will not be influenced significantly by the soil properties during the earthquake, and the structure can be analysed under the fixed base condition. When the structure is resting on a flexible medium, the dynamic response of the complete structure will be different from the fixed base condition, where the interaction between the soil and the structure has to be incorporate. This behavioural difference because of the phenomenon commonly referred to as Soil-Structure Interaction (SSI), which if not considered in analysis and design properly; the accuracy in assessing the structural safety, response for earthquake excitation could not be reliable solution. Hence evaluation of the site, specific effect of soil stiffness on structure becomes important to understand behaviour of structure. Flexibility of soil increases natural period of structure, which basically turn changes the seismic response of structure. The interaction among structure, their foundation and soil media below foundation alter the actual behaviour of structure. Here G+25 storey building is modelled and analysed, employing Finite Element Method adopting Commercial code SAP2000 V19 under fixed base (no soil-structure interaction) and flexible base considering soil-structure interaction. An attempt has been made to evaluate the effect of soil structure interaction of super structure by considering the systematic parameters like time period, lateral displacement, storey drift, bending moment in dual global structural axis i.e., X-X and Y-Y direction.