03/06/2015
REINFORCEMENT DETAILING OF ISOLATED FOOTING
Foundations are of types like isolated footing, combined footing, strip footing, raft footing, pile foundations etc. They are selected based on structural requirement and soil conditions.
Reinforcement detailing of footing is as much important as site investigation for the type of footing and structural design of footing. A good detailing reflects the design requirement of the footing for structural stability. A good detailing of reinforcement covers topics like cover to reinforcement based on environmental considerations for durability, minimum reinforcement and bar diameters, proper dimensioning of footing etc.
DETAILING OF ISOLATED FOOTING:
Reinforcement Cover:
The minimum thickness to main reinforcement in footing should not be less than 50mm f footing is in contact with earth surface directly, and 40mm for external exposed face such as surface levelling PCC. If surface levelling is not used, then it is required to specify a cover of 75mm to cover uneven surface of excavation. For raft foundation, the minimum cover to reinforcement is 75mm whether resting on PCC or directly on earth surface.
Minimum reinforcement and bar diameter:
The minimum diameter for main reinforcement should not be less than 10mm.
Detailing method of Isolated Footing:
It is desirable foundation should be detailed in both plan and elevation in drawings.
The isolated footing are generally square or rectangular in plan based on structural requirement and forces acting on the column. But circular or other shapes can also be used for isolated footings.
Reinforcement Distribution in Footing:
In one-way RCC footing, the reinforcement is distributed uniformly across the full width of footing.
In two-way square footings, the reinforcement extending in both directions is distributed uniformly across the full width of the footing. But in the case of two-way rectangular footings, reinforcement is distributed across the full width of footing in long direction, however for short direction, the reinforcement is distributed in the central band as per calculations below. The rest reinforcement in short direction is distributed equally on both sides of the central band.
