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Design specifications for footings following IS 456: 2000
As per standard IS 456: 2000, the following guidelines are given for the design of isolated footings :-
The objective of Footings is to retain the applied loads, moments and forces and the driven reactions and make sure that any prospective settlement is as almost undeviating as possible, and the safe bearing capacity of the soil is not surpassed.
In sloped or stepped footings the operative cross-section in compression is restricted with the area over the neutral plane as well as the angle of slope or depth and location of steps is arranged in such a way that the necessities for design are fulfilled at each section. Sloped and stepped footings which are designed as a unit should be built up to guarantee action as a unit.
Thickness at the Edge of Footing
1. In reinforced and plain concrete footings, the thickness at the edge should be over 150 mm for footings on soils, nor below 300 mm over the tops of piles for footings on piles.
2. For plain concrete pedestals, the angle among the plane moving toward the bottom edge of the pedestal and the equivalent junction edge of the column with pedestal and the horizontal plane should be managed by the expression: 2) where = computed optimal bearing pressure at the base of the pedestal in N/mm2 fck = characteristic strength of concrete at 28 days in N/mm2.
Moments and Forces
1. Toward the footings on piles, calculation for moments and shears are dependent on the proposition that the reaction from any pile is set at the centre of the pile.
2. In order to calculate stresses in footings which support a round or octagonal concrete column or pedestal, the face of the column or pedestal is assumed as the side of a square imprinted within the perimeter of the round or octagonal column or pedestal.