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Footing Design Calculations And Shallow Foundation

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

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 (Easiest form)

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
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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.
= (2251.1)/(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 (Plan)
                                                                         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
[Assumingd = 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:

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About Raja Junaid Iqbal

Hi My name is Raja Junaid Iqbal and i am Land Surveyor by Profession but I've specialized in the field of Q,S and land development. As Professional Surveyor I engaged my self in all over the Gulf since 14 years.

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