Reinforcement detailing of footing is as much important as site investigation for the 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. It is desirable foundation should be detailed in both plan and elevation in drawings.
That is why different aspects of reinforcement detailing of isolated footing is discussed in the following sections.
Reinforcement detailing of isolated footing include:
Concrete cover of reinforcements
Minimum reinforcement and bar diameter
Reinforcement distribution in isolated footing
1. Concrete cover of Reinforcements
According to IS 456-200, the minimum thickness to main reinforcement in footing should not be less than 50mm if 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.
2. Minimum reinforcement and bar diameter
Minimum reinforcement shall not be less than 0.12 percent of the total cross sectional
area.The minimum diameter for main reinforcement should not be less than 10 mm.
3. 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.
Where y is the long side and x is the short side of the footing.
Fig.1: Reinforcement distribution in square isolated footing
Fig. 2: Reinforcement distribution is rectangular isolated footing
4. Dowel reinforcement
Dowel reinforcement is used to tie the isolated footing to the above column. with regard to dowel reinforcement development length, the development length of dowel bars into the column and the isolated footing shall be provided and clearly shown in the design drawings.
Fig. 3: Dowel bars
5. Lap splice
Splice length of dowel and column reinforcement shall be clearly shown. Anchorage of both flexural and dowel reinforcement lengths shall be checked to prevent bond failure of the dowels in the footing and to prevent failure of the lap splices between the dowels and the column bars.
Fig. 4: Anchorage reinforcement
Fig. 5: Section view of reinforcement details of isolated footing (Typical reinforcement detailing)
Fig. 6: Plan view of reinforcement details of isolated footing (Typical Reinforcement details)