The seismic design of RC buildings requires determining the expected base shear, lateral drift at each story level and internal forces of the structural elements. In the analysis, it is common for the structural engineers to consider a fixed base structure which means that the foundations and the underlying soil are assumed to be infinitely rigid. This assumption is not proper since the underlying soil in the near field often consists of soft soil layers that possess different properties and may behave nonlinearly leading to drastic variation of the seismic motion before hitting the structure foundation. In addition, the mutual interaction between the structure, its foundation and the underlying soil during the vibrations can substantially alter the structure response. This response variation depends on the structure characteristics, the soil properties and the nature of the seismic excitation. Consequently, an accurate assessment of inertial forces and displacements in structures requires a rational treatment of soil structure interaction (SSI) effects. In this paper, comprehensive numerical study is carried out to investigate the seismic response of mid-rise RC buildings subjected to different seismic excitations assuming full nonlinear SSI employing PLAXIS V8.2 software. Three types of two dimensional mid-rise moment resisting frames consisting of five story (S5), ten story (S10) and fifteen story (S15) are analyzed. Each building is considered to be founded on three types of soil representing firm soil (class A), medium soil (class C), and loose soil (class D) conditions with shear wave velocity (Vs) of 1000, 270, and 90 m/s, respectively. For comparison, each building intermediate frame has been analyzed with different base boundary conditions assuming: (i) fixed base; (ii) equivalent soil springs; (iii) flexible base considering full SSI. The results showed that it is essential to consider SSI effects in the procedures of the seismic design of concrete mid-rise moment-resisting frames. Generally, decreasing the dynamic stiffness of the subsoil (by decreasing Vs and shear modulus G) the base shear ratios decrease while inter-story drifts of the frames increase relatively. Moreover, assuming fixed base can lead to high overestimation of the structure design forces and seismic response. |
