Design of Shallow Foundation (Isolated Footings)

1. Ans ,

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. Do the following checks and design the building.

INTRODUCTION :

• Draw the Model with column size of 400 * 400 for design of the building.
• Define frame elements and create the column size and assign the loads etc..
• Given datas are Fck = 25 , Fy =500 , Factored load = 2000 kn ,

• Gross soil pressure
• = unfactored load + soil pressure / Allowable soil pressure = 2000 *1.1/(1.5*150) = 9.78 m
• Assuming square footing = (9.78)-² = 3.2 m

• Net soil pressure & inital footing depth
• factored load / area of footing = 2000 /3.2*3.2 = 195.3 kn/m²
• Assuming depth of footing = 500 mm
• Cover = 50 mm
• Effective depth d = 450 mm
• As per is - 456 2000
• footing base bearing area / load area = 2.4* 2.4 / .4*.4 -< 2
• Premissbile bearing stress = 0.45 * Fck
• Permissible bearing stress with multiplying with area factor = 0.45 fck * 2 = 22.5 kn/mm2
• Actual bearing stress at column
• = factored load / area of column = 2000 / 0.4*0.4 = 12.5
• Actual bearing stress is less than the permissible bearing strees
• Hence safe.
• Flexural reinforcement
• Converting net pressure in to unifrom distrubted load = 195.3 *3.2 = 624.96 kn/m
• For bending moment at one side of the footing 1.6:
• wl²/2 = 624.96 * 1.6 * 1.6 /2
• = 800 kn/m

Reinforcement calculation as per IS 456 2000 Annex G

Mu= 0.87 Fy Ast d(1- Ast*Fy/bdFck)

Ast = 4350 mm²

Assuming 20 mm dia with 200 spacing = 3200 *314/ 200 = 5024 mm²

• One way shear check :
• Shear force at one way check
• 0.95 * 3.2 *195 = 592.8

Desing shear check at the shear cut at one way

= load / area = 592.8/3.2*.45 = 368

Shear capacity of one way shear section as per IS 456 2000 table 19

= .425 n/mm²

The design shear check is less the shera capacity of the one way shear hence safe.

Two way shear check :

Shear force on the two way check :

= 3.2*3.2- (.8*.8)* 195 = 1875 kn

Design shear check of the two way shear cut area

= Load / area = 1875 / 3.2 *.45 = 1.3 n/mm²

Shear capacity of the two way shear check at cut area check As per IS 456 - 2000

= .25 root of Fck = 1.25 n/mm²

From the above design shear is less the shear capacity hence safe.

RESULT :

From the above Steps the gross, net soil perssure , flexural reinforcemnet , one way and two way shear stress is calculated of the islolated footing using IS - 456 200 Hence proved.