Footing Design Calculations - Shallow Foundation

Determination,
  • Area of Footing (a)
  • Depth of Footing (d)
  • Short form of Footing Designs

Footing Designs Calculation - Shallow Foundation:

Footing Designs Calculation
Footing Designs Calculation

Bearing Capacities of the Shallow Foundation from SPT:

The bearing capacities of the shallow foundation particularly for top layer of cohesive soil may be estimated from the SPT values, as suggested by terzaghi, according to following:

Bearing capacities of the shallow foundation from SPT
Bearing capacities of the shallow foundation from SPT

Bearing capacities of the shallow foundation from SPT (Easiest form)
Bearing capacities of the shallow foundation from SPT (Easiest form)

Allowable bearing capacities of various foundation beds, (tsf)
Allowable bearing capacities of various foundation beds, (tsf)
Remark:Soils unable to carry 2000 psf generally requires pilling. 


Design procedure of Isometric Footing:

  • Step 1: Determination of area of the Footing

Assuming, P = 225k ; qu = 2.5 ksf

So, the area of the footing = (Total Load)/(Bearing Capacity).

= (Load of footing)/(Bearing Capacity).

= (P * 1.1)/qu.

= (225* 1.1)/(2.5)

= 99 ft  ≈ 100 ft.

[Assuming, self-wt. of the footing 10%]

Total load = P+(10/100)*P   = P(1+0.1)

B = √100 = 10 ft.

So, Size of the footing is “10 ft X 10 ft”

Area of the Footing is 100 Sq Ft.




  • Step 2: Determination of Depth of the footing 

            Punching Shear Check


Area of punching shear
Area of punching shear (Plan)
Area of punching shear
Area of punching shear (Elevation)

Perimeter, P0(for square footing) = {(d +b) + (d +b)} * 2

= 2 (d +b) * 2

=4 (d +b)  

P0 (for rectangular footing) = 2(b_1 +b_2) + 4d

So the resisting punching load = P0.d. 100⁄1000  



= 4 (d +b) * d * 100⁄1000


= 4 (18 +15) * d * 100⁄1000

 = 237.6k > 225k


Some loads can be deducted from the actual load to reduce the depth of the footing which brings economy by ensuring safety.

Net pressure = Lad/(Area of the footing)  = 225/(10 ft * 10 ft) = 2.25 ksf

Now, the modified pressure ={(102 + 2.752)} * 2.25 = 208 ksf

[Assuming, d = 18 inch; (18+15) = 33 inch = 2.75 ft]

Now,  P'/P = 0.92≈0.90

So, the load can be taken = 0.9 * P = 0.9 * 225 = 202.5 ≈ 205k


The short formula for determining the depth is 



80<(Modified Load)/(P' * d)<90 = 80 <(205 * 1000)/( 4(b+d)  * d)<90


Short form for designing the footing:


Determine width,        B = √((225 X 1.1)/(2.5)) ≈ 10 ft

And then trial, 

            For square footing,                   80 <(225 * 1000 * 0.9)/( 4(b+d)  * d)<90

            For rectangular footing,         80 <(225 * 1000 * 0.9)/({2(b_1  + b_2 )+ 4d}* d)<90


 Determination of Moments:

Determination of Moments
You also know pile cap design calculations details, Thank you.

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