Rational Analysis of Raft Foundation

An analysis of a raft foundation is developed which takes into account the interaction of the three elements of the system, i.e., the structure, the raft, and the supporting soil. The stiffness of the structure is incorporated into the stiffness matrix of the raft which is treated as a thin elastic plate supported on a soil mass modeled either by the Wrinkler (spring) or by the linear elastic model. A three-bay by three-bay multistory space frame supported on a square raft is analyzed and the effects of structural stiffness, relative raft-soil stiffness, and soil stiffness are considered in detail for both the Winkler and linear elastic soil models. It is shown that use of the linear elastic model leads to the conclusion that the settlement profile of a flexible raft is concave, thus there is a transfer of load to the outer columns compared with a rigid raft. In contrast, the Winkler analysis predicts a convex settlement profile. Special methods of analysis are developed for rigid structures and for highly flexible rafts.