Research Of Thick Raft Foundation Based On Generalized Conforming Theory

Raft foundation, with high bearing capacity, good overall performance and nice stiffness characteristics, is widely used in high-rise building structures. However, the force mechanism of raft foundation, especially the thick raft foundation, is not clearly known by people. In design time, engineers still use the reverse beams and reverse floors methods, which don't consider the interaction of superstructure, raft foundation and solid. In finite element analysis, thin plate elements are still widely used in raft foundation, which ignores the shear strain. In addition, the results of Mindlin thick plate elements, due to the shear locking effect, is higher than real fact .This paper starts from constructing a finite element ,which is batter fit for raft foundation ,then improves the interaction methods of superstructure, raft foundation and solid. The works are included the following:â‘ Timoshenko thick beam theory is applied to get the thick plate's shearing strain field. The plate's deflection field is assumed to be an imperfection fourth expression individually, then, according to the generalized conforming theory to establish the equations between the two fields, constructs a quadrilateral element, for both thin and thick plates, no shear locking, which is named EAE/Plate. Basing on the definition of rotation freedom of planar membrane element, constructs a quadrilateral membrane element, with high precision, no zero-energy mode and weak trapezoidal locking , which is named EAE/Mem. EAE/Plate and EAE/Mem are composed to be the shell element EAE/Shell. Through the numerical examples of two traditional shell element problems, it shows that: the shell element inherits from the excellence performance of the plate element EAE/Plate and the membrane element EAE/Mem, with high accuracy and good applicability.â‘¡Taking a frame building for example, this paper improves the interaction method of superstructure, raft foundation and solid. Basing on virtual displacement principle, this paper introduces the rotation supporting effect of Winkler's foundation to the plate element's stiffness matrix, which makes up the Winkler's foundation's original deficiencies of only considering the vertical supporting effect and discontinuity. Because of using the shell element, with 6 degrees of freedom, to simulate the raft foundation, this paper not only considers the horizontal displacement, but also guarantees the connectivity between the superstructure and raft foundation at nodes. Through the numerical examples of plate under a center partial load and the comparison with the boundary element method, it shows that: the method this paper improved, with a better accuracy, is a feasible and effective interaction method.â‘¢Based on the finite elements constructed and the interaction method improved above, this paper establishes a special finite element program to analyze a practical framework building for example. Numerical results show that: in thick raft foundation, due to the ignoring of shear strain, the internal force of raft foundation is smaller than fact, which makes the design of raft foundation to be insecure; not considering the interaction will ignores the secondary stress of the upper part of building cased by raft foundation, and the shear and moment are smaller than fact, which makes the design of superstructure to be insecure; horizontal load will increase the internal force of both foundation and the superstructure in its direction, and the traditional method of the superstructure's lateral displacement is smaller than fact, which makes the design of the building to be dangerous.