- 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,

**(for square footing) =***P*_{0}**{(d +b) + (d +b)} * 2**

**= 2 (d +b) * 2**

**=4 (d +b)**

**(for rectangular footing) =**

*P*_{0}**2(b_1 +b_2) + 4d**

So the resisting punching load =

*P*_{0}.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 ={(10

^{2}+ 2.75^{2})} * 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:

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

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