Determining seismic response of mid-rise building frames considering dynamic soil-structure interaction
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Structures are often mounted on layers of soil unless bedrock is very close to the ground surface. Based on the fact that seismic waves pass through kilometres of bedrock and usually less than 100 meters of soil, soil layers play a significant role in assigning the characteristics of the ground surface movement. 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 structure will be different from the fixed base condition owing to the interaction between the soil and the structure. This difference in behaviour is because of the phenomenon, commonly referred to as soil-structure interaction (SSI), which if not taken into account in analysis and design properly; the accuracy in assessing the structural safety, facing earthquakes, could not be reliable. Performance-based engineering (PBE) is a technique for seismic evaluation and design using performance level prediction for safety and risk assessment. Soil-structure interaction particularly for unbraced structures resting on relatively soft soils may significantly amplify the lateral displacements and inter-storey drifts. This amplification of lateral deformations may change the performance level of the building frames. Thus, a comprehensive dynamic analysis to evaluate the realistic Performance level of a structure should consider effects of SSI in the model. In this study, an enhanced numerical soil-structure model has been developed which treats the behaviour of soil and structure with equal rigor. Structural elements of the soil-structure model are capable of capturing both elastic and inelastic structural behaviour as well as structural geometric nonlinearity (large displacements) in dynamic analysis. Adopting direct method of analysis, the numerical model can perform fully nonlinear time history dynamic analysis to simulate realistic dynamic behaviour of soil and structure under seismic excitations accurately. Fully nonlinear method precisely follows any prescribed nonlinear constitutive relation and adopts hysteretic damping algorithm enabling strain-dependent modulus (G/Gmax - ɤ) and damping functions (ζ - ɤ) to be incorporated directly to capture the hysteresis curves and energy-absorbing characteristics of the real soil. In order to avoid reflection of outward propagating waves back into the model, viscous boundaries comprising independent dashpots in the normal and shear directions are placed at the lateral boundaries of the soil medium. In addition, the lateral boundaries of the main grid are coupled to the free-field grids at the sides of the model to simulate the free-field motion which would exist in the absence of the structure. The proposed numerical soil-structure model has been verified and validated by performing experimental shaking table tests at the UTS civil laboratories. For this purpose, a prototype soil-structure system including a building frame resting on a clayey soil has been selected and scaled with geometric scaling factor of 1:30. The soil-structure physical model consists of 15 storey steel structural model, synthetic clay mixture consists of kaolinite, bentonite, class F fly ash, lime, and water, and laminar soil container, designed and constructed to realistically simulate the free field conditions in shaking table tests. A series of shaking table tests were performed on the soil-structure physical model under the influence of four scaled earthquake acceleration records and the results, in terms of maximum structural lateral and vertical displacements, were measured and compared with the numerical predictions. Comparing the predicted and observed values, it is noted that the numerical predictions and laboratory measurements are in a good agreement. Therefore, the numerical soil-structure model can replicate the behaviour of the real soil-structure system with acceptable accuracy. In order to determine the elastic and inelastic structural response of regular mid-rise building frames under the influence of soil-structure interaction, three types of mid-rise moment resisting building frames, including 5, 10, and 15 storey buildings are selected in conjunction with three soil types with the shear wave velocities less than 600m/s, representing soil classes Cₑ (Vs=600m/s), Dₑ (Vs=320m/s), and Eₑ (Vs=150m/s) according to Australian Standards, having three bedrock depths of 10, 20, and 30 metres. The structural sections are designed after conducting nonlinear time history analysis, based on both elastic method, and inelastic procedure considering elastic-perfectly plastic behaviour of structural elements. The designed frame sections are modelled and analysed, employing Finite Difference Method adopting FLAC2D software under two different boundary conditions: (i) fixed base (no soil-structure interaction), and (ii) flexible base considering soil-structure interaction. Fully nonlinear dynamic analyses under the influence of four different earthquake records are conducted and the results in terms of lateral displacements, inter-storey drifts, and base shears for both mentioned boundary conditions are obtained, compared, and discussed. According to the numerical and experimental investigations, conducted in this study, soil-structure interaction has significant effects on the elastic and inelastic seismic response and performance level of mid-rise moment resisting building frames resting on soil classes Dₑ and Eₑ. Thus, the conventional elastic and inelastic design procedures excluding SSI may not be adequate to guarantee the structural safety of regular mid-rise moment resisting building frames resting on soft soil deposits. Based on the numerical results, a simplified design procedure is proposed in which inter-storey drifts under the influence of soil-structure interaction for each two adjacent stories can be determined and checked against the criterion of life safe performance level. This can be used to ensure the performance levels of the mid-rise moment resisting building frames under the influence of SSI remain in life safe level, and the seismic design is safe and reliable. Structural engineers and engineering companies could employ the proposed simplified design procedure for similar structures as a reliable and accurate method of considering SSI effects in the seismic design procedure instead of going through the whole numerical procedure which could be complicated and time consuming.
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