A piled raft foundation consists of a thick concrete slab reinforced with steel which covers the entire contact area of the structure, in which the raft is supported by a group of piles or a number of individual piles. Bending moment on raft, differential and average settlement, pile and raft geometries are the influencing parameters of the piled raft foundation system. The mutual interaction between the soil, foundation and superstructure is dependent on various parameters including the type and configuration of the structure, type of foundation, type of soil, loading etc. This paper attempts to quantify this interaction effect for plane frame structures resting on two types of foundations viz. piled foundations and raft foundations. The study is made based on the finite element approach with the help of the finite element package ANSYS. A linearly elastic model for structure and a linearly elastic plane strain model for soil is used for the analysis. The variation of stresses for the various components of the frame and footing are analysed for a range of parameters. Non-dimensional tables are arrived at incorporating the parameters under consideration, which may be used to study the effect of inclusion of soil in the analysis for plane frames. The results obtained from this study are compared with the conventional method of analysis. The results obtained from the study suggests that there is some variation due to the inclusion of soil in the analysis but it is very small and thus may not considerably affect the design of such structures except a few locations, where the stresses do not govern the design.
