Design of Shallow Foundation (Isolated Footings)
Aim: General design of spread footing and manually calculation with respect to limit state design of soil and structure.
Procedure:
Step 1:
Given data
Size of the column = 400x400
Load = (1.5 D.L + 1.5 L.L) is 2000KN
S.B.C 150KN/M2
Step 2:
To calculate required area
Unfactored load = 2000/1.5 = 1.33 KN
= 1.33x1.1/150 = 9.78 m2
Assume we have square footing
= 3.12 m side footing
Consider 3.2 side isolated footing
Step 2:
To calculate N.U.P
Factored load / actual area of the footing
P/A = 2000/3.2x3.2
= 195.3 KN/M2
Assume the depth of the footing is 500mm and checking for the bearing stress
Calculation for the bearing pressure:
Permissible bearing stress 0.45 x fck
= 11.25 N/mm2
Permissible bearing stress multiplied with area factored = 11.25 x 2 = 22.5 N/mm2
Actual bearing stress
(2000x1000) / (400x400) = 12.5 N/mm2
12.5 N/mm2 is less then 22.5 N/mm2 hance ok;
We proceed with 500mm depth of the footing and 50mm cover of the footing
The effective depth (d) shall be taken as 450mm
Step 3:
Calculation for flexural reinforcement
Convert N.U.P to U.D.L
= 195.3 x 3.2 = 625KN/m
B.M due to U.D.L over cantilever is WL2/2.
M = 625*1.6^2/2
= 800KN-m
Assume Fy = N/mm2 which results = 4350mm2
Proved 16mm dia rebar at 125mm spacing = 3200/125x201
Ast provided = 5146mm2
Step 4:
One way shear stress check
shear force in one way shaded area
= 0.95x3.2x195.3 = 593.7 KN
Shear stress capacity as per IS 456-2000 table 19
= 0.425 N/mm2
Design shear stress at one way
Tv = 593.7x1000/(3200x450) = 0.412 N/mm2
Design shear stress is less then shear capacity [OK]
Two way shear
Shear force in two way shaded area
= (3.2x3.2) – (0.85x0.85) x 195.3 = 1859 KN
Shear capacity of section as per IS 456 – 2000
= 0.25 x 
= 0.25 x 
= 1.25 N/mm2
Design of shear stress at two way cut face
Tv = 1859x1000 / (3400x450) = 1.215 N/mm2
Design shear stress is less then shear capacity [OK].