Seismic Response of a RC Framed Building with SSI

Author Name(s): Udaysingh Patil , Vivek Purushottam Joshi, Ankush Asati, Nakul Kabra
Author Email: patil.udaysingh4@gmail.com

Abstract

Earthquake is the seismic activity that can cause catastrophe and take millions of lives in one go. From the beginning of time and beginning of recorded history it is noticed that Earthquake is one of the many wraths of god which causes so much damage to the livelihood of the people. Conventional buildings are designed with no consideration for what seismic zones they are divided in, this causes a huge amount of risk for the residents of that particular structure. The zones are divided according to the risk of earthquakes in that particular area. This ignorance of zones can cause irreplaceable damage to the future of the building Along with this there is mysterious interaction of the soil beneath on the capacity of the foundation which is yet to be fully studied and concluded, as the variety of soil is vastly dependent on the different geographical locations and cannot be easily compared. In this paper 3, buildings with different storey heights were analyzed (i.e G+4, G+6, and G+ 8) for different zones of India in SAP with different soil condition using equivalent spring constants.

Introduction

Loads from structural systems are transferred to the ground through foundation. When these structural systems are subjected to seismic forces, relative motion of superstructure, substructure and the ground takes place. This process of the response of the soil influencing the motion of the structure and the motion of the structure influencing the response of the soil is termed as soil-structure interaction (SSI).[3] Effect of SSI is generally neglected while doing structural design. SSI can be neglected for light structures viz. low rise buildings & retaining walls in comparatively stiff soil. But it must be considered for heavy structures (High rise buildings, nuclear power plants, elevated highways etc.) resting on relatively soft soils. Many researchers have highlighted the response of foundation and ground during past earthquakes. Therefore design and analysis must include combined response of superstructure, substructure and ground during lateral loadings causing base shear and moment at the base, ignoring SSI [5] in the analysis may lead to unsafe design for both the building and construction. Many numerical analyses have shown that if there is increase in the natural period of a structure due to SSI it is not always helpful as thought earlier in simplified design codes. Softer soils increases the period of seismic waves and increase its natural period, possibly leading to resonance with the longer vibrations. It is noteworthy that, due to SSI ductility demand can very much increase with the increase in the natural period of the construction. The primary concept lies in the concept of Kinetic energy and Inertia.[1] Since the soil’s inertia gives kinetic energy to the building and its foundation and the inertia again gives momentum to the structure.

Conclusion

1. Base shear value for 4 storey building in fixed base condition remains constant irrespective of soil condition. 2. Base shear value for 6 storey and 8 storey building increases as soil condition becomes loose. 3. Base shear value has unpredictable nature for soil structure interaction with varying soil condition. 4. Base shear value for both fixed base condition as well as for soil structure interaction increases with increase in storey height. 5. Base shear value, keeping storey height constant increases with increase in zone number for both soil structure interaction and fixed base condition. 6. The percentage of storey drift value decreases with respect to lower storey as storey height increases. 7. The maximum displacement value of building increases with zone no. for both soil structure interaction as well as fixed base condition. 8. The results shown are with respect to Winkler’s Model , if we use any other improved model the result would be different since Winkler’s model only takes into account only the effect of soil directly below it ignoring the effect of surrounding soil. 9. The base shear values for a fixed foundation are always greater than the values of base shear for a spring foundation.

660 total views, no views today

Download PDF File

About the author: admin