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

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

Assuming, P = 225^k ; q_u = 2.5 ksf

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

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

= (P * 1.1)/q_u.

= (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 (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

 = 237.6^k > 225^k

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 ≈ 205^k

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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