SEISMIC RESPONSE OF AN INSTRUMENTED BUILDING INCLUDING SOIL-STRUCTURE INTERACTION

Abstract

In this work seismic response of the instrumented multi-storied reinforced concrete building (G +9) has been studied using strong motion records of Bhuj earthquake, 2001. The analysis includes determination of modal parameters, amplification factor of accelerations at various floors; velocity time history, displacement time histories; drift index ; transfer function / amplification spectra of acceleration time histories between top floor and base of the building; Short Time Fourier Transform (STFT) / window analysis to obtain variation of building rocking motion in two horizontal directions over the whole duration of strong motion record and floor spectra of instrumented floors. The Ambient Vibration Testing (AVT) has also been conducted to measure the modal parameters of the building under ambient environmental forces. Using velocity time histories obtained by AVT, modal parameters of first five modes are obtained using FDD technique. In order to study the effect of structural and non-structural members on the seismic response of the building, a series of five three-dimensional finite element (FE) models of the building have been created by considering the number of structural and non-structural components consisting of the specific geometry, material properties and section properties. The fixed base FE model consisting of columns, beams, floors slabs, stair case and infill with modal parameters close to ambient vibration test is considered to model foundation and layered soil below the building. Soil properties are determined at different depths of soil by a combination of in-situ and laboratory tests of founded soil. The shear wave velocity of founding soil at different depths upto 30 m below the ground level is measured directly by cross borehole tests using three boreholes at a distance of 50 m from the building. In the FE model viscous boundary condition is applied using COMB1N14 element in the general purpose finite element package ANSYS. The FE model developed by using soil properties obtained from soil tests is used to compute seismic response of the building in Bhuj earthquake by applying the input excitation at the base of the soil block. The excitation at the base of soil block is computed from the strong motion record at the ground floor of the building using the transfer function approach. Linear dynamic time history analysis is performed by applying computed base rock motion at the base of soil block in FE model. Thus, seismic response of the building is computed using soil structure interaction FE model. The seismic response of the building is also carried out by varying soil properties of the soil layers to consider variation in properties. The shear wave velocity for varying soil properties are 0.6, 0.8, 1.4 and 2.0 times of the actual shear wave velocities of the soil as obtained from the field test. The reduction and increment in the shear wave velocity is done for each soil layer of the founding soil to compute the seismic response. The seismic response of the building is also computed by applying El Centro earthquake motion which is scaled down to the same peak acceleration as computed for the Bhuj earthquake at the base rock motion. The observed natural frequencies during strong motion are smaller than the ambient vibration testing. The difference in the frequencies may be caused by several factors including possible soil structure interaction and interaction of structural and non structural elements. Also, the modal pattern of first five modes obtained from strong motion records, ambient vibration records and finite element model including floor slabs, staircase and stiffness due to infill wall are identical. Seismic response of the Modal frequencies of the first two modes of the FE model including SSI effect are close to as obtained from the strong motion record.