Design of Shallow Foundation (Isolated Footings)

Design a square footing for a column size of 400x400. The compression axial load for the load combination of (1.5 DL + 1.5 LL) is 2000 KN. The safe soil bearing capacity is 150 KN/m2 at a depth of 2 metres below E.G.L. Participants are free to go for either a tapered or stepped footing. Besides the total axial load, also account for the self-weight of the footing and soil above it. Assume M25 concrete grade. Do the following checks:

• If the gross bearing pressure under the footing is within safe bearing capacity
• Provide sufficient depth and longitudinal steel for resisting bending and shear
• Sufficient anchorage for column steel into the footing
• If the bearing pressure at column-footing joint is within 0.45 times the maximum concrete compressive strength

Provide a simple sketch showing the size of the footing, maximum and minimum depth of the section, flexural reinforcement in both directions and anchorage of column steel.

Please do go through the solved examples in the document provided with the challenge.

Ans:-

• Gross soil pressure and Footing area:

=Un factored load with surcharge + Self weight of footing/ allowable soil pressure

=(2000/1.5x1.1)/150

=9.78m2

meter sides footing

Consider 3.2m sides footing

• Net soil pressure & initial footing depth

Factored load/actual area of footing

=2000/(3.2x3.2)

P_n=195.3Kn/m²

Assume depth of footing 500mm and checking it for bearing stress as per IS456-2000 clause 34.4

≤2

• Net soil pressure & initial footing depth:

Permissible bearing stress 0.45 x fck = 11.25n/mm2

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

Actual bearing stress at column base

N/mm2

12.5n/mm2 actual stress is less than permissible 22.5n/mm2 hence okay

We can proceed with 500mm depth footing for further design checks

If we assume 50mm cover for footing, the effective depth shall be taken as 450mm

• Flexural reinforcement:

Conversion of net pressure into UDL

=195.3x3.2=625kn/m

Bending moment due to UDL over a cantilever section

R/f calculation as per IS 456-2000 annex-G

M_u=

Assume Fy=500n/mm2

Ast=4350mm2

If we provided 16mm dia rebar at 125mm spacing, the actual rebar provided

.=32000/125x201

=5146mm2…. Ast provided

• One way shear check

• Shear force in the one way shaded area

=0.95x3.2x195.3

=593.7kn

• Design shear stress at One way cut face

=593.7x1000/(3200x450)

=0.412n/mm2

• Shear stress capacity of section as per IS456-2000 Table-19

=0.425n/mm2

• Design shear stress is less than shear capacity …. Hence Okay

• Two way shear stress check:

• Shear force in the two way shaded area

=(3.2x3.2)-(0.85x0.85)x195.3

=1859 kn

• Design shear stress at two way cut face

=1859x1000/3400x450

=1.215n/mm2

• Shear stress capacity of section as per is 456-2000 clause 31.6.3.1

=0.25x√Fck=1.25 n/mm2

• Design shear stress is less than shear capacity .. Hence Okay

C/s rainforcement detail