COMPUTER AIDED DESIGN OF RAFT FOUNDATION

Abstract

Foundation is very important part of any structure. Loads from the superstructure are transferred to the soil through the foundation. Foundations can be of many types depending upon the type of loading and the soil condition, but if the bearing capacity of the soil available at the location is low and it contains compressible lenses then there will be more differential settlements if individual footings are provided. To overcome this problem raft or mat foundations are provided Here finite difference technique is adopted to analyze the raft considering that raft is on elastic foundation. So here raft slab is divided into small square grids having nodal points at the junction of elements. The plate bending differential equation is written at each nodal point in finite difference form. For the boundary nodes certain boundary conditions are applied to get the difference equation at those points. A set of simultaneous equations can be obtained and solution of those equations give deflection at each nodal point which is then used to obtain bending moments and shear forces at those points. A computer program has been developed in 'C' language which is capable of calculating nodal displacements, support reactions and bending moments at various nodal points. The program also calculates the effective depth required for the slab as per bending moment and punching shear criteria. Few raft problems have been solved using the program and same problems are modeled on STAAD PRO software also using quadrilateral plate elements. The nodal displacements, support reactions and bending moments are then obtained and compared with the values obtained from the program. Comparisons of results show good agreement. The results also show that there are 10%45% variations in the design bending moment between program and STAAD results.