Seismic Design Considering Soil - Pile - Structure Interaction

Abstract eng:
The state of the art on subject of soil-pile-structure interaction is summarized, and focused on the practical applications in seismic design. The nonlinearity of soil is accounted for approximately using a boundary zone model with non-reflective interface. The ratio of Gi / Go indicates the nonlinear properties of soil, where Gi and Go is the shear modulus of soil in the boundary zone and the out zone respectively. The curves of stiffness and damping of soil-pile system vs the ratio of Gi / Go provided in this study. The radiation damping (geometric damping) of foundation is also an important factor in dynamic or seismic design. The soil is assumed normally to be a homogeneous isotropic medium based on the elastic or visco-elastic theory. As a matter of fact, the soil is not a perfect linear elastic medium. It is well recognized in the soil dynamics field that the radiation damping is overestimated with the elastic theory.The effects of radiation damping on seismic response evaluated and the values of damping ratio modified based on a series of dynamic tests of single pile and group pile in the field. A simplified mathematical model of the coupled horizontal and rocking vibration of an embedded foundation was proposed. Only four impedance parameters are used in the displacement expression not six parameters as in traditional method, since the coupled ítems can not be balanced with the superstructure as using the substructure method. The effect of soil-pile-structure interaction was evaluated based on the different conditions. In the first case, the interaction was accounted for fully, that is, all of the soil, pile and structure are flexible. In the second case, the soil-pile system is flexible, but the structure was assumed to be rigid (no deformation in the superstructure). In the third case, the structure is flexible but fixed (or pinned) to the rigid base, no deformation in base soil (without SSI). As for practical applications, a vacuum tower structure was examined in severe seismic zone as a typical industrial structure supported on pile foundation. The vacuum tower with diameter of 8.5 m and weight of 5,600 KN were installed in a steel frame. There are 25 steel piles in the foundation. Three base conditions were assumed to illustrate the soil-pilestructure interaction: rigid base (i.e. no deformation in the foundation), linear soil-pile system, and nonlinear soil-pile system. The stiffness and damping of pile foundation were calculated with linear and nonlinear soil respectively. A case of liquefaction was discussed for the pile foundation, considering the potential liquefaction in a shallow saturated sand layer. The seismic response were calculated using the response spectrum analysis and time history analysis, and compared with the method of equivalent static loads.

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