What is Footing?

Footing is that part of the substructure that ultimately transfers the load to the supporting soil in such a way that the safe load bearing capacity of soil is not exceeded. Footing is a structure constructed in brick work, masonry or concrete under the base of a wall or column for distributing the load over a large area. The most important part of footings is the plan dimension, i.e, the bearing area which is ultimately resting on the soil and dispersing off the building load to the underneath soil. The plan dimension of footing is determined by the nature of soil, as from the chart of safe bearing capacity of soils given above, it is evident that footing resting on rocks will have less plan dimensions compared to footing resting on soft clay or fine sandy soil.

Generally a 3-storeyed building resting on clayey/sandy soil should have minimum 5.5ftx5.5ft to 6ftx6ft footing size, 2-storeyed building should have 5ftx5ft and single storied building 4ftx4ft footing sizes. These sizes should be provided if no soil data is available for proper design by a licenced structural engineer.

RCC Footing should have a minimum depth of 150mm for footing on soil, and minimum 300mm above the top of piles for footing on piles. Minimum clear cover for footings is generally 50mm.

Importance of footing

MAXIMUM SAFE BEARING CAPACITY OF SOILS
TYPE	DESCRIPTION	T/M2
ROCKS
	Hard rock without lamination and defects e.g granite,trap , diorite.	330
	Laminated rocks in sound condition e.g sand stone	165
	Residual deposits of shattered and broken bed rock and hard shale,cemented material.	90
	soft rock	45
NON -COHESIVE SOILS
	Gravel, sand and gravel (compact and offering high resistance to penetration when excavated by tools)	45
	Coarse sand ,compact and fry.	45
	Medium sand ,compact and dry.	25
	Fine sand ,silt (dry lumps easily pulverised by the fingers)	15
	Loose gravel or sand gravel mixtures; loose coarse to medium sand,dry.	25
	Fine sand, loose and dry	10
	Kankar	32
	Sand with clay	20
COHENSIVE SOILS
	Soft shale, hard or stiff clay in deep bed,dry	45
	Medium clay, readily indented with a thumb nail	25
	Moist clay and sand clay mixture which can be indented with strong thumb pressure.	15
	Soft clay indented with moderate thumb pressure	10
	Very soft clay which can be penetrated several inches with the thumb.	5
	Black cotton soil or other shrinkable soil or expansive clay in dry conditions (50 % Saturation)	10
	Red earth	30
	Alluvial soil	3 to 9
	Alluvial loam	9 to 17
	Peat	5 to nill
	Fills or made up ground(Consolidated)	5