Design of Shallow Foundation (Isolated Footings)

1) AIM & INTRODUCTION: design of square footing for a column size of 400 x 400 using ETABS.

PROCEDURE:

STEPS:

• calculation of gross soil pressure for footing area
• calculation of net soil pressure for flexure and shear design
• intial depth of footing calculation
• flexural reinforcement calculation
• one way shear check
• two way shear check

Gross soil pressure & footing size:

unfactored load with surcharge + self weight of footing /allowable soil pressure

=(2000/1.5x1.1)/150 = 9.78m^2

Assuming we have square footing

`(9.78)`=3.12 meter side footing

consider 3.2 meter sided isolated footing

Net soil pressure & Intial footing depth:

factored load/actual area of footing = 2000/1000/(3.2x3.2)

Pn=195.3 kn/m^3

assume depth of footing 500mm and checking it for bearing stress as per IS 456-2000 clause 34.4

(footing base area /isolated )^1/2<-2

permissible bearing stress 0.45 x Fck =11.25n/mm2

permissible bearing stress multiplied with area factor = 11.25x2 =22.5 N/mm2

Actual bearing stress at column base = (2000/1000)/(400x400) =12.5 N/mm2

12.5 N/mm2 actual stress is less than permissible 22.5 N/mm2 .Hence okay

we can proceed with 500m depth footing for further design checks

if we assume 50mm cover for footing ,the effective depth (d) shall be taken as 450mm

Flexural reinforcement:

conversion of net pressure into uniformly distributed load = 195.3 x 3.2 = 625 kn/m

Bending moment due to UDL over a cantilever section = wl2/2=625x1.6x1.6/2

M = 800KNm

Reinforcement calculation as per IS456-2000 Annex-G

Mu=0.87fyAstd(1-Astfy/bdfck)

Assume fy=500n/mm2,In the result Ast Required = 4350 mm2

If we provide 16mm dia rebar at 125mm spacing , the actual rebar provided = 3200x125x201

Ast provided = 5146 mm2

hence okay

One way shear stress check:

shear force in the one way area = 0.95x3.2x195.3=593.7 Kn

design shear stress at one way cut face Tv=593.7x1000/(3200x450)

=0.412 N/mm2

shear stress capacity of section as per IS456-2000 Table 19(include rebar) = 0.425N/mm2

Design shear stress is less than shear capacity .Hence okay.

Two way shear stress check:

shear force in the two way area = (3.2x3.2)-(.85x.85)x1953 =1859 Kn

design shear stress at two way cut face Tv=18597x1000/(3400x450)

=1.215 N/mm2

shear stress capacity of section as per IS456-2000 clause 31.6.3.1 = 0.25 x (fck)^1/2 =1.25 n/mm2

Design shear stress is less than shear capacity .Hence okay.

DEVELOPMENT LENGTH Ld=O6S/4Tbd=20*.55*500/4*1.4=98.2m

RESULT:designed of square footing for a column size of 400 x 400 using ETABS succesfully

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