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Jawaharlal nehru technological university anantapur college of engineering
**Specialization:
**Civil Engineering**Views:
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Design of Square Footing:
Footing or foundation is defined as the part of substructure, which transmits the loads from the super-structure to
surrounding soil stratum safely. Foundation are classified as two types,
1. Shallow foundation
2. Deep foundation
The depth of the foundation is less than or equal to the width of the foundation then the foundation is said
to be shallow foundation. If the depth of the foundation is greater than width of the foundation then the foundation
is said to be Deep foundation.
Design of footing mainly depends on the safe bearing capacity of the soil on which the footing rests and the load
coming from the superstructure. Footings may be isolated, combined.
Isolated or independent footings are the footings that support the individual columns. They distribute and
spread the load over a sufficiently large of the soil stratum to minimize the bearing pressure. Isolated footings may
be square, rectangular or circular.
In general, it is assumed that the soil behaves elastically that is the strain in the soil is proportional to applied
stress and strain distribution in the soil immediately under the base of the footing is linear. Stress distribution is
different soils.
For analysis purpose, a footing can be compared with a rigid body in equilibrium subjected to loads. Like
other structural members, a footing is designed to resist shear forces and bending moments. In design, for any soil
the pressure distribution is assumed to uniform.
In design, the critical section for one way shear (beam shear) is at a distance equal to the effective depth, d
from the face of column footing. The critical section for two way shear or slab type shear shall be at a distance d/2
from the periphery of column, perpendicular to the plane of the slab. The critical section for bending moment is at
the face of the column. Generally the footing is sensitive to punching shear.
IS-CODE PROVISIONS FOR DESIGN OF FOOTINGS:
1.
Footings shall be designed to sustain the applied loads, moment and forces. And safe bearing capacity is not
exceeded.
2.
In R.C.C. footing, the thickness at the edge shall not be less than 15cm for footing on soil.
3.
The greatest bending moment to be used in the design of an isolated concrete at the face of the column.

The critical section for diagonal cracking is taken at a distance equal to the effective depth from the face of the
column in hard soils and shall not exceed nominal shear stress.
No-26
1) Type of footing
= Square footing
2) Size of the column
= 230 X 450 mm
3) Load on footings:
Axial Load on footing (ETABS) (P) = 850 KN
= 250kN/m2
(Assume) S.B.C of soil
STEP β 1:
Self-weight of footing
= 10 % of axial load.
= 850 X 0.10 = 85 KN
Total load transmitted to the soil
= axial load + self-weight
= 850 + 85 = 935 KN
= 250kN/m2
S.B.C of soil
STEP β 2:
Area of footing (A)
= Total load/SBC of soil
= 935 / 250 =3.74 β
4.0 m2
Size of the square footing
= βπ΅ = β4 = adopt = 2m x 2m
ULTIMATE BEARING CAPACITY;
qu =
ππ’
ππππ ππ ππππ‘πππ
=
850
2π₯2
= 220 N-M β
0.22 πΎπ β π
BENDING MOMENT:
M U = qu x B x
( Bβb )2
= 0.22 x 2000 x
8
( 2000β230)2
8
= 172.3 x 106 N-mm

Calculating depth (d)
β΄ MU = 0.138 fck bd2
172.3 x 106 = 0.138 x 25 x 2000 x d2
d = 158 mm
β΄ So provide twice the depth (Take 350mm)
Assume Cover as 50mm
Overall depth = 350 + 50 = 400 mm
d = depth β half dia of bar β effective cover
d = 400 β 12/2 β 50 = 344mm (Provide 12mm bar)
Area of Steel:-
Ast
= 0.5 x
fck
x
fy
4.6MU
[1 β β1 β fckbd2 ] x bd
4.6 x 172.3 x 106
25
= 0.5 x 415 x [1 β β1 β 25 x 2000 x3442 ] x 2000x344
= 1437.84 mm2
SPACING:
S = ast x
B
ππ π‘
= 113.09 x
2000
1437.84
= 157 mm β
160 c/c

Provide 12mm @ 160 c/c in both direction
Check for one way shear:
Critical section for one way shear isβdβ from face of the column.
ο© ( B ο b)
οΉ
ο dοΊ
ο« 2
ο»
Shear Force Vu ο½ Pu xBοͺ
ο© (2000 ο 230)
οΉ
ο 350οΊ
2
ο«
ο»
Shear Force Vu ο½ 0.22 x 2000οͺ
Shear Force Vu = 235400 N.
Nominal Shear stress
ο΄ v=
235400
= 0.33
2000π₯350
N/mm2
Percentage of steel:Pt =
π
π₯122 π₯100
100 Ast
=4
= 2.01
spacingxdepth 160π350
From IS 456-2000 Table No-19
Shear stress in concrete = Οc= 0.82 N/mm2
Οv<Οc
Hence it is safe
1) Check for two-way shear:
2000
b+d
230
2000
2000
450
450
b+d
b+d
230
2000
b+d

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