Research On Effects Of Foundation Stifness On High-rise Building Seismic Design

So far most of the structure seismic theories and calculation methods are basedon the rigid foundation assumption and the structure seismic design methods aremainly based on the embedded fixed end calculation model. The rigid foundationassumption has been widely adopted by seismic specifications overseas and inland,and the corresponding design methods have been proposed. The foundation stiffnessand radiation damping effect are approximately considered by introducing areduction factor, which tends to think that the calculation result based on theembedded fixed end calculation model is sure to form an envelope. In other words,the foundation stiffness must be a favorable factor for anti-seismic design ofhigh-rise buildings. It is found in this paper that the dynamic characteristic andseismic response of the upper structures could be changed significantly by theobjective existence of the foundation stiffness, and the partial response can be evenamplified. That means that the prevailing view in structural engineering of thealways-beneficial role of limited foundation stiffness is an oversimplification whichmay lead to unsafe design.This thesis mainly focuses on three contents: the influence of limitedfoundation stiffness on structural lateral stiffness, internal force distribution and thestructural stability. The coupling effect between horizontal foundation stiffness androtation foundation stiffness has also been considered. On the basis of theoreticanalysis, the modified D-value method has been proposed to calculate the structurallateral stiffness of shearing type structure with limited foundation stiffness. Thecalculation method for structural lateral stiffness of bending type structure withlimited foundation stiffness is also proposed. The proposed modified D-valuemethod covers conventional D-value method, and expands its application range.The formula is simple in form and easy to use. The influence of limited foundationstiffness on structural gravity second-order effect and overall stability is alsodiscussed in the paper. A formula which regards the ratio of rigidity-to-gravity ascontrol target is proposed for calculating the growth of structural lateraldisplacement, and the results have been compared with that of the embedded fixedend calculation model. The study found that the ratio of rigidity-to-gravity shouldbe more strictly controlled when taking the limited foundation stiffness into account.The formulas proposed above have good applicability and can be applied to othercases, which overcome the shortcomings of the traditional SSI method.We can get the limited-foundation-stiffness calculation model by means ofchanging the structural boundary conditions, and the finite element analysis of some typical high-rise buildings that constructed on pile foundations is carried out. Bothdesign response spectrum analysis and dynamic elastic time history analysis havebeen performed. Simulation results show that the theoretical analysis is correct andthe effects of limited foundation stiffness on seismic response of the upperstructures are obtained simultaneously. It is found that embedded fixed endcalculation model exaggerates the lateral displacement stiffness of the bottom framecolumns, which means the inter-floor deflections of the bottom story may be biggerthan expected under earthquake actions. It is seen that the lower the stiffness of thefoundation, the higher requirement for structural ductility is needed. Although thetotal earthquake action tends to decrease when limited foundation stiffness isconsidered, the internal force redistribution will emerge. Results show that thebending moments at the upper section of bottom frame columns increasesignificantly, which may be dangerous for the seismic design. The relationshipbetween lateral displacement growth and the foundation horizontal stiffness isdiscussed, and the minimum foundation horizontal stiffness is proposed. The greatWenchuan earthquake disasters, shows that bending destroy at the upper section ofbottom columns is one of the most typical seismic damages of frame structures.Theoretical analysis and numerical simulation results in this paper can explain thereasons well from the point of limited foundation stiffness.