Design of Shallow Foundation (Isolated Footings)
1.
AIM- in this challenge is to be design an isolated footing.
STEP 1
GIVEN DATA-
column size = 400*400
compression axial load p= 2000KN
fck= M25
fy= Fe500
soil bearing capacity= 150KN/m2
STEP 2
DESIGN
GROSS SOIL PRESSURE AND FOOTING AREA
unfactored load with surcharge + self weight of the footing/ allowable soil pressure
10% of the load is takenfor self weight= (2000/1.5*1.1)/150 = 9.77m2
we assume a square footing
therefore,
sqaure root of 9.77m2 = 3.12 each side .
net soil pressure and depth of the footing.
factored load/ actual area of footing.
2000/(3.2*3.2)
=pn = 195.3KN/m2
assume the depth of the footing 500mm.
cheak it for bearing stress 0.45*Fck= 0.45*25= 11.25*2=22.5N/mm2
permissible bearing stress <permissible bearing stress.
12.5N/mm2<22.5N/mm2
hence, 500mm depth is okay and provide 50mm for cover
effective depth= 450mm
STEP 3
BENDING MOMENT AND FLEXURAL REINFORCEMENT
bending moment due to UDL=WL2/2
Where
W= net pressure into UDL
W= 195.3*3.2= 652KN/M
= WL2/2
= 625*1.6*1.6/2
Mu= 800KNM
Area of steel for 1m
Ast= 0.5*25/500*(1-``11-(4.6*800*106)/25*1000*5002)*1000*500
Ast= 1450 mm2
total Ast reqiured= 4350mm2
no of bars
= mu /22.7/4*162
22 nos
spacing,
s= L/nos pf bars
s= 3200/22
s= 145.45mm
therefore,
provide 22 bars of 16mm dia at 140mm spacing.
STEP 4
CHEAK FOR ONE WAY SHAER STRESS
shear stress at one way cut face = tv= 593.7*1000/(3200*450)
= 0.412N/mm2
shear capacity cheak as per IS 456-2000 TABLE 19
=.425N/mm2
design shear stress is less then capacity shaer stress.
hence okay
STEP 5
CHEAK FOR TWO WAY SHEAR PUNCING SHEAR.
shear force in two way = p0*(B*B-b0*b0)
b0= 400+450= 850mm
= ((3.2*3.2)- (.85*.85))*195.3
= 1.25n/mm2
cheak for shear capacity as per is 456-2000 ci 31.6.3.1
=0.25*rootFck
0.25*root 25
= 1.25N/mm2
design shear stress is less than shear capacity hence
ok.
STEP 6
RESULTS
- provide a 3.2*3.2m footingwith a depth of 500mm
- provide reinforcement of 16mm dia bars in both direction
- provide anchorage for colomn and footing at 750mm