Investigation of the Dynamic Properties Effect of Site on Seismic Ground Response in Analysis and Design of Buildings

The study of variations of damage intensity for different earthquakes indicates the importance of the site effects on earthquake induced damage and ground motion parameters such as peak ground acceleration and induced amplification. In the occurrence of an earthquake, local site conditions such as soil characteristics, dimension of topographic irregularities, seismic bedrock depth, etc. as well as characteristics of incident wave have important e ects on seismic ground responseLocal site effects play an important role in earthquake-resistant designs and should be assessed carefully. By assessment of induced damages to structures and major infrastructures, seismic geotechnical researchers have concluded that the site conditions significantly influence on the failure distribution in urban and rural areas. Consequently, to determine the characteristics of seismic motions of the ground, it is essential to study the effective geotechnical factors. Consideration of the effects of the site response in the design of civil structures systems is of important to mitigate the damages to a certain extent on structures and the environment. Hence, it is relatively crucial to reliably attain the dynamic soil parameters of an earthquake-prone city/state. The nature of local site effects can be explained by using different methods such as simple theoretical analysis of ground response, measuring of surface and subsurface actual motion at the same site, and measuring the movements of the ground in sites with different conditions in comparison with the proposed site. In this paper, the effective factors on ground motion and site response in the soil classes II and III for Tehran city under the influence of Tabas and Bam earthquake were reviewed with numerical method and by using the finite difference software (FLAC 2D). Some of the effective parameters of the site dynamic characteristics on the seismic response of the ground surface have been studied and the response of peak ground acceleration and the resonance function has been compared. The results of this study show that at both sites, the horizontal maximum acceleration of the ground surface increases with respect to the bedrock shear velocity, reflecting the influence of the site on ground movement. At both sites, with increasing soil internal friction angle, the maximum acceleration at ground level increases and the acceleration response decreases. Also, as the shear wave velocity increases, the horizontal maximum acceleration at ground level for both structures decreases. Increasing the shear wave velocity in the surface layer, assuming the thickness of the layer is constant, results in a decrease in the natural frequency, which is far from the normal frequency range of conventional structures in Tehran. This indicates a decrease in the earthquake's dynamic force on surface structures. The natural frequency of five to seven story buildings in Tehran is about 0.5 to 0.7 Hz. As the surface layer thickness and bedrock depth increase, the horizontal maximum acceleration of the ground surface also decreases, meaning that the structural response can also be reduced.