Brief overview of foundation construction
Definition of foundation: The foundation can be defined as follow :-
Any structure is segregated into 2 parts:
1. Superstructure: It is the portion of structure located over Ground level (GL)
2. Substructure/ Foundation: It is the portion of structure under GL
Constituents of foundation: Generally there are 3 constituents of foundation: (i) Footing, (ii) Column, (iii) Tie beam
Footing belongs to that portion of substructure that fundamentally delivers the load (dead/live/wind/seismic) to the soil that provides support to the structure, in a manner so as to maintain the safe load bearing capacity of soil under control. Footing refers to a structure that is built up in brick work masonry or RCC under the base of a wall or RCC column for allotting the load over a extended area.
The footings are very useful for the plan dimension, i.e., the bearing area that is ultimately supported with the soil and distributing the building load to the soil at below. The plan dimension of footing is defined with the nature of soil. The footing that is supported with the rocks will contain less plan dimension with regards to that situated on soft clay or fine sandy soil.
RCC Footing should contain a least depth of 150mm for footing on soil, and lowest 300mm over the top of piles for footing on piles. Lowest clear cover (concrete cover from outline of concrete to outline of steel) for footings is usually 50mm.
Structural characteristics of footing:
1. Total settlement of the structure is restricted to values as indicated in following table 2 [extracted from IS1904].
2. Differential settlement (variation in total settlement) among various footings has to be circumvented as much as practicable, and should be restricted to values as stated in following table 2 [extracted from IS1904].
3. Footing has to be designed and placed at the level where safe bearing capacity of soil is recognized from soil testing report.
4. Centre of gravity (CG) of the footing has to be matching with the CG of loading.
5. Stress distribution under the footing should be in a way that maximum permissible stress should not surpass anywhere under the footing.
6. Nowhere tension should be acceptable to develop under the footing. Eccentricity check for footing should be accomplished.