Analysis And Optimization Of Raft Foundation Considering The Effect Of Superstructure

Raft foundation, which is extensively used, is an important foundation pattern of high-rise buildings. The researches of raft foundation have been being paid close attention to by academic and engineering circles. Though a lot of achievements have been obtained, many problems remain unsolved for their complex nature. On the basis of investigation on the related literature of raft foundation at home and abroad, this paper puts emphasis on studying the deformation behavior, internal force and optimization of raft foundation by coupling of boundary element, finite element and infinite element. The main works and conclusions in this paper are as follows:(1) According to the Kirchhoff's theory, the raft foundations are treated as the thin plate on two-parameter elastic foundation by boundary element method. The ribs are analyzed by finite element method. Then, the coupled equation is obtained with the help of the equilibrium of force as well as the compatibility of displacement. Moreover, the corresponding program for computer is designed. The computational results of the examples show that: a) The application of coupling method presented for analysis of ribbed raft foundation on two-parameter elastic foundation is reasonable and feasible.b) The differential settlement of ribbed raft foundation is mainly adjusted by thickness of plate.c) The rib of raft foundation will take advantage of reducing the differential settlement of ribbed raft foundation.d) The cantilever part of raft foundation has the advantage for raft foundation to suffer load. (2) According to the Reissner's theory, the thick raft foundations are treated as the thick plate on two-parameter elastic foundation by boundary element method. The coupled equation of ribbed thick raft foundation on two-parameter elastic foundation is obtained by coupling. The analytical program for the method is worked out. The computational results of the examples show that:a) The effect of rib in raft foundation is opposite to the thickness of plate. b) It is apparent that the differential settlement at the rib very between Reissner's theory and Kirchhoff's theory. (3) For considering the effect of superstructure, the finite element method is used to analyze the superstructure. The coupled equation is established by using the equilibrium of force and the compatibility of displacement. Based on the coupled equation, computer program is implemented for analysis of superstructure-raft foundation-soil interaction. The numerical results of the examples show that it has the advantage of reducing deformation and moment in raft foundation.(4) Using 3D 20-node iso-parameter element, 3D 12-node infinite element, and 2D 8-node iso-parameter bending element, a coupled non-linear numerical approach for the raft-soil system loaded laterally is set up. The feasibility and rationality of the approach are proved by the analysis of examples. The approach can model the non-linear characteristics of raft foundation and soil, and the method in this paper is expected to being used in engineering project. (5) By using above coupled non-linear numerical approach, the influence of concrete strength and thickness of raft foundation is analyzed and discussed. Through a large number of calculations for optimization of raft foundation, the results show as follow: a) The thickness of raft foundation exerts larger influence than concrete strength on differential settlement and bending moment. b) It is rational that the technical code for tall building box foundations and raft foundations takes more than C30 for raft foundation. c) It is shown that when the differential settlement is at 0.001L, the thickness of raft foundation is reasonable and economical.