When we design a wall footing, the ratio of all the dimensions should be as shown in the below-given drawing.
Given:
Suppose, we take the width of the superstructure wall as 300mm. (i.e. T ), then all other dimensions should be as follows.
Calculation:
1. Concrete bed:
The width of the concrete bed
= 3T = [ 3 × 300mm.]
= 900mm.
The thickness of the concrete bed should not be less than 200mm.
2. Step-1 footing:
The width of the 1st footing
= 2.5T = [ 2.5 × 300mm.]
= 750mm.
The depth of the 1st footing
= [(3T - 200mm.) ÷ 2]
= [( 3 × 300mm. - 200mm.) ÷ 2]
= [ 700mm. ÷ 2]
= 350mm.
Note: The depth of the footing should be equal for the 1st & 2nd steps of footings.
3. Step-2 footing:
The width of the 2nd footing
= 2T = [ 2 × 300mm.]
= 600mm.
The depth of the 2nd footing = The depth of the 1st footing = 350mm.
4. Plinth wall:
The width of the plinth wall
= 1.5T = [ 1.5 × 300mm.]
= 450mm.
Normally, we keep the depth of the plinth wall between 450mm. to 650mm., depending upon the site conditions.
Note: The offset of all the steps should be equally distributed on both sides.
For eg:
The extra length of the concrete bed
= [900mm. - 750mm.]
= 150mm.
Offset or projection on both sides
= [150mm. ÷ 2] = 75mm.
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Thank you for going through these calculation steps❤. Have a good day 😄.
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