Cellular raft or framed raft with foundations slab, walls, columns and one of the floor slabs acting together to give a rigid structure.Beam and slab raft which can be designed with down standing beam or upstanding beam system.This can be with pedestals or without pedestals. Plain slab rafts which are flat concrete slab having uniform thickness throughout.(b) Based on the Structural System Adopted for the Structure of the Raft
Self-weight of the mat and the total weight of the structure is calculated and is divided by the total area of the foundation, it is covering to calculate the stress on the soil. Stress distribution mechanism of the foundation is very simple. Raft foundation transmits the total load of the building, to the entire building area. The raft may have of reinforced concrete beams with a relatively thin slab underneath. Usually, when hard soil is not available within 3 to 4 m, a Raft foundation is adopted if the soil is weak. For this reason, almost double the settlement of that permitted for individual footings, is acceptable for rafts. A Raft foundation may undergo large settlements without causing harmful differential settlement. Raft foundation is also used to support storage equipment, storage tanks, chimneys and various industrial structures. Mat or Raft foundation is used to reduce settlement on highly compressible soils by making the raft and the weight of structure approximately same to the weight of soil excavated.
They are used for lightly loaded structures on soils with poor bearing capacity or where variations in soil conditions necessitate a considerable spread of the load, for heavy loads in place of isolated footing, where differential settlements are significant. Barry’ (Author of The Construction of Buildings), Raft foundation is continuous in two directions, usually covering an area equal to or greater than the base of a building or structure. The raft tends to bridge over the random deposits and eliminates the possibility of differential settlement.
The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure compared to the traditionally used strip or trench footings. A Raft foundation is a large slab made of concrete which can support a number of columns and walls. It is a combined footing that covers the entire area under a structure and supports all the columns. The Raft foundation is also called mat footing.
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