Design of Shallow Foundation (Isolated Footings)

AIM:

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.

INTRODUCTION:

• 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.

PROCEDURE:

Steps involved in footing design:

• 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 and footing area:

• unfactored load with surcharge + self weight of footing /allowable soil pressure.
• =(2000/1.5 x 1.1)/150=9.78m2 (assuming we have square footing),
• one side footing length=3.12m
• taken as 3.2m isolated footing.

Net soil pressure and intial footing depth:

• factored load/actual area of footing
• 2000*1000/(3.2*3.2)
• Pn=195.3KN/m2
• Assume depth of footing 500mm and checking it for bearing stress as per IS-456-2000
• square root(footing base bearing area/loaded area )
• permissible bearing stress 0.45 x Fck = 11.25N/mm2
• permissible bearing stress multiplied with area factor 11.25 x 2 =22.5N/mm2
• Actual bearing stress at the column base
• (2000x1000)/(400x400) = 12.5N/mm2
• 12.5N/mm2 actual stress is less than permissible 22.5N/mm2.heance ok.
• we can proceed with 500mm depth footing for further design check.
• if we assume 50mm cover for footing,the effective depth(d) shall be taken as 450mm

Flexural Reinforcement :

• Conversion of net pressure into uniformity distributed load = 195.3*3.2=625KN/M
• Bending momment due to UDL over a cantilever section = wL2/2 =625 * 1.6 *1.6/2
• M=800KNm
• Reinfocement calculation as per IS -456-2000Annex-G,
• Assume Fy=500N/mm2,in result Ast required = 4350mm2
• if we provide 16mm dia rebar at 125mm spacing,the actual rebar provided=3200/125*201 Ast provided = 5146mm2.

one way shear stress check:

• shear force in the one way shaded area = 0.95 x 3.2 x 195.3 =593.7 KN.
• Design shear stress at one way cut face Tv = 593.7 x 1000 /(3200x450) = 0.412 N/mm2
• shear stress capacity of section as per IS-456-2000 = 0.425 N/mm2.
• Design shear stress is less than shear capacity Heance okay.

Two-way shear stress check:

• shear force in the one way shaded area = (3.2x3.2)-(0.85x0.85) x 195.3= 1859KN
• Design shear stress at one way cut face Tv = 1859x1000/(3400X450)=1.215 N/mm2
• shear stress capacity of section as per IS-456-2000 = 1.25 N/mm2.
• Design shear stress is less than shear capacity Heance okay.

Reinforcement details