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DESIGN OF SHALLOW FOUNDATION ISOLATED FOOTING Aim: To design a square footing for a column of size 400x400 using some of the informations such as The compression axial load for the load combination of (1.5 DL + 1.5 LL) = 2000 KN. The safe soil bearing capacity = 150 KN/m2 at a depth of 2 metres below E.G.L.…
Praveen Ps
updated on 29 Jul 2022
DESIGN OF SHALLOW FOUNDATION ISOLATED FOOTING
Aim:
To design a square footing for a column of size 400x400 using some of the informations such as
Then to perform the following checks:
Introduction:
Procedure:
- Given datas include:
- Here a sloped footing is designed and the steps involved in the design are as follows:
1) Size of footing:
Unfactored or service load = 20001.5=1333.33kN
Safe bearing capacity of soil , qc = 150kN/m2
Hence, the required base area = 1333.33⋅1.1150=9.78m2
Hence, provide 3200mmx3200mm which gives 10.24m2 base area for footing. (This area later proves to be insufficient).
2) Thickness of footing slab based on one way shear.
Factored bearing pressure(Net soil pressure) = 20003.2⋅3.2=195.31kN/m2
Now, assuming 0.15% reinforcement in footing slab.
From table 19 of IS456:2000, design shear strength of concrete, τc= 0.29N/mm2 for M25 grade concrete.
From the condition, One way shear resistance≥One way shear force -->
0.29d x 3200 x d≥0.195x(1400-d)x3200
d≥562.88mm ≈600mm
So, provide d = 600mm
Assuming 20mm dia bars and 70mm cover , total depth of the footing = 600+70+20+10 =700mm
3) Check for two way shear
Here, the critical section is ABCD, so shear resistance at critical depth = area of critical section x permissible shear stress
i.e, shear resistance = (400 + 600) x 4 x 600 x Ksτc =
Ksτc = 0.25√fck= 1.25N/mm2
Hence, shear resistance = 1000 x 4 x 600 x 1.25 = 3000kN
Now, the shear force = [3.2 x 3.2 - (0.4+0.6)2] x 0.195 = 2944.5kN < 3000kN
Hence, safe.
4) Gross bearing capacity
Assuming unit weight of concrete and soil as 25kN/m3 and 18kN/m3, we have,
Load on footing = 1333.3kN
Weight of footing = 3.2x3.2x0.7x25=179.2kN
Weight of soil = 3.2x3.2x(2-0.7)x18=239.61kN
Total load = 1751.81kN
Hence, gross bearing capacity = 1751.813.2⋅3.2=171.07kN/m2 > 150kN/m2. Hence, we need to increase the size of footing to 3600mmx3600mm.
So, now the weight of footing = 226.8kN
Weight of soil = 303.26kN
Total load = 1863.06kN
Now, gross bearing capacity = 1863.063.6⋅3.6=143.75kN/m2<150kN/m2. Hence safe.
So, the corrected net soil pressure = 20003.6⋅3.6=154.32kN/m2
5) Bending moment
We need to determine the area of steel in one direction as it is a square footing.
Consider effective depth = 600mm.
Bending moment at critical section is obtained as , Mu= 3600x16000.154x1600/2= 709.63kNm
Mubd2=709.63⋅1063600⋅600⋅600=0.5475N/mm2
Now, from table 3 of SP16 for fck = 25N/mm2 and fy = 500N/mm2, % of reinforcement = 0.1294%
Hence, area of steel = 0.1294100⋅3600⋅600=2795.04mm2.
So, no. of bars required is given by, 2795.04314=8.9≈10bars
Hence, provide 10 nos of 20mm dia bars both ways.
6) Development Length
For 20mm dia bars, Ld = 0.87fyϕ4τbd=970.98mm≈1000mm.
7) Providing slope in footing slab
Since the three critical sections are within a distance of 600mm from the face of the column , the full depth of footing slab is to be atleast 700mm, which is the provided depth of footing.
Hence, the one way shear force = 0.154(1600-600)3600=554400N=554.4kN
One way shear resistance is given by 0.29 x 3600 x 600= 626.4kN which is greater than the shear force. Hence safe.
So, we need to provide a depth of 700mm near the column and the depth can be reduced to about 400mm at the edges of the footing.
Result:
Sloped isolated footing for a square column has been designed with the given specification.
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