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INTRODUCTION
2.1 Foundation:
A foundation is a part of a building or structure which is in direct contact with the ground and which transfers the loads imposed upon it to subsoil beneath.
Foundation may be broadly classified into shallow foundation and deep foundation based on ratio of depth and width.
A foundation consists of:
The choice of foundation will depend on site condition, ground water conditions and bearing capacity of the soil. These are normally taken to be:
Compact gravel, solid chalk | >600kN/m2 |
Compact sand | >300kN/m2 |
Clays, stiff | 150-300kN/m2 |
Clays, firm | 75-150kN/m2 |
Loose gravel | <200kN/m2 |
Loose sand | <100kN/m2 |
Soft silt, clays | 75KN/m2 |
Very soft silt and clay | <75KN/m2 |
2.2 FOOTING:
■ A footing is a structural element that transfer force from a structure to soil.
■ The soil has much lower strengths than do the structural materials, so the force needs to be distributed over a large area in order to get the normal stresses to be lower than what the soil is capable of supporting.
Combined footing:
Whenever two or more columns in a straight line are carried on a single spread footing, it is called a combined footing.
4.1 Factors influencing Bearing Capacity:
Bearing capacity of soil depends on many factors. The following are some important ones:
8 .Depth of water table
10 .Inclination of load on footing
11 .Inclination of ground level
The bearing capacity is fixed by:
The load bearing capacity of the soil is 450KN/m2.As bearing capacity of soil is high, combined footings at shallow foundation is suitable to carry the superstructure loads.
2.3 FACTOR OF SAFETY:
Sub soil is a mixture of solid particles, air and water. with any other material it has to be a certain strength to resist to compressive force, but it will fall if it is overloaded.
In order to provide a factor of safety, it is usual to design a foundation so that the bearing pressure on the ground, imposed by the structure through the foundation, is about one third of the bearing pressure of the subsoil. As per IS code the partial safety factor is taken as 1.5.
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