ANALYSIS OF DIFFERENT TYPES OF RAFT FOR DIFFERENT SOIL CONDITIONS FOR A MULTI STORIED BUILDING

Abstract

Raft foundations are large inverted concrete slabs supporting superstructure through pedestals and walls. The choice of Raft slab can be made under following circumstances: 1) The bearing capacity of soil is low, 2) The columns are spaced so close that the isolated footings overlap 3) The column loads are heavy so that most of the area(>50%) is covered by spread footings. 4) At some locations, there is soft soil. Raft foundations are best suited to use for deep basements, both to spread the column loads uniformly, and to provide basement floor slab. Raft foundations are useful in reducing the differential settlements and sustaining large variations in loads on the individual columns. In conventional analysis of raft foundation, the reactive soil pressures due to the loads from the structure is important as the raft is subjected to bending due to loads from the structure and also from the reactive pressure offered by the soil. These effects considerably alter the forces and the moments in the structural members. In the present work, analysis is carried out for six different types of raft(i.e., flat plate raft, slab beam raft, flat plate raft thickened under column, flat plate raft with pedestal, box type raft and pile raft) on six different soils. These raft foundations are analyzed by using SAFE-v12software for a fifteen storied building on six different soil conditions. This fifteen storied building of 3 bay x 3 bay is analysed to get base reactions under columns using ETABS software. The deflections, punching shear, shear force and bending moments obtained for these rafts on various soils are compared. The result indicates the significant effects of shape of the flat plate thickened under column and flat plate with pedestal raft in resisting punching shear is more efficient among these types. Box type raft and Beam slab raft is more efficient in resisting shear force and bending moments.