Foundations or substructures

UNIT - III

Chapter - 5.(A)

FOUNDATIONS or SUBSTRUCTURES

FOUNDATIONS or SUBSTRUCTURES

Though the foundation once built is invisible to the owner and the public, it is a critical part of the total building system. Therefore, it should be given careful attention in design. Foundation is also known as Substructure. It consists of:

1. Plain Cement Concrete Bed (or Lime Concrete Bed) of about 300 mm thick and

2. Two or Three Masonry Footings.

1. LOADS ON FOUNDATIONS

The type of foundation to be used depends upon the loads carried by it. These loads are:

1. Dead Loads

Dead load is the dead weight or self weight of the foundation and the super structure. It includes the weight of all walls, floors, roofs, etc. To determine the dead load, knowledge of the weight of various building materials is necessary.

2. Live Loads

Live loads are the movable and superimposed loads on the floor. Live loads include all the loads that are not permanent. Live loads are not constant loads, but varying loads. These include the weight of persons on the floor, weight of materials such as furniture, etc., stored temporarily on the floor, etc.

3. Wind Load

The effect of wind should be considered in case of tall buildings. The exposed sides and roofs of tall buildings are subjected to wind pressure. The design wind pressure at a place depends on the wind velocity, height of the building, etc. The effect of wind pressure is to reduce the pressure on the foundation in the windward side and to increase the pressure in the leeward side.

Distribution of Loads

Foundation should distribute the above loads to a large area (in Shallow Foundation) or through end bearing and skin friction (in Deep Foundations).  Distribution of loads is done so that the intensity of stress and the settlement are within limits.

2.  FUNCTIONS or REQUIREMENTS OF FOUNDATIONS

1. Base for the Structure

Foundation forms the base for a building structure. The foundation of a structure should support the weight of the super structure and the load carried by it.

2. Transmitting the Load to the Soil

Foundation transmits the loads of the building (including its own weight) to the soil below.

3. Uniform Distribution of Load on Larger Area

Foundation distributes the total loads of the super structure on larger area of the soil uniformly. This prevents overloading of the soil. Thus, the intensity of the load exerted on the soil is less than the Safe Bearing Capacity of the soil.

4. Minimization of Differential Settlement

By distributing non-uniform load of building uniformly on the sub-soil, the foundation minimizes the differential (unequal) settlement of soil.

5. Stability of the Building

Foundation should ensure stability and safety of the building by preventing the effects due to soil erosion.

6. Firm Level Base

Foundation should provide a firm level base for the construction of the super structure by carrying out concreting and masonry works systematically.

7. Lateral Stability to the Structure

Foundation should impart lateral stability to the super structure by anchoring it to the ground. It gives lateral stability to the super structure against various weathering disturbing forces such as wind, rain, etc.

8. Safety Against Sliding and Overturning

Foundation should provide grip between the ground and the superstructure. Thus, it provides safety against sliding and overturning due to wind or other lateral loads.

9. Safety against Scouring and Undermining

Foundation should protect the building against scouring and undermining by flood water, drainage water, etc.