Uplift capacity of the Footing

Greetings!

Footings are sometimes subjected to uplift or tension forces, few examples are:

1. Legs of the elevated water tank
2. Anchorages to the anchor of the footings.
3. Footings of the transmission line towers.

Bala(1961) developed the expressions along with the following procedure for their use:

• A footing may be considered as shallow or deep if    Df/B < H/Blimiting.  Which depends upon angle of internal friction. 

             Angle of Internal friction (Degrees)              H/Blimiting

                                     20                                        2.5

                                     25                                         3.0

                                     30                                         4.0

                                     35                                         5.0

                                     40                                         7.0

                                     45                                         8.0

• For shallow footing, i.e. Df/B < H/Blimiting

1.  For Circular Footings  
   
    
2. For Rectangular Footings

• For Deep Footing Df/B > H/Blimiting

1.  For Circular Footings 
   
2. For Rectangular Footings
   

Where,

                 Sf= Shape Factor = 1+m.Df/B - For shallow

                                             = 1+ m.H/B  - For Deep

                 K = Co-efficient of earth pressure at rest.

The value of m: 

          Angle of friction       H/Blimiting              m

                      20                       2.5                0.05

                      25                       3.0                0.10

                      30                       4.0                0.15

A factor of safety of 2.0 to 2.5 may be adopted to find the allowable bearing capacity.

If soil is very poor, Pu may be taken equal to W. In this case FOS of 1.5 may be taken to determine the pull capacity Pa.

Thanks for your kind visit!