Brief overview of balanced footings and cantilever footings

A balanced footing is also known as strap footings which include two separate footings associated with a strap beam. Balanced footing is vital when one of the footings contains restricted space for bearing, or if any restricted is made for transmitting the pressure from footing to the ground.

It may happen if any variance occurs in soil bearing capacity, or loose soil under one of the footing. The load in a balanced footing is shifted from one footing to the other thru cantilever action, so it is also defined as a cantilever footing.

There is a difference between balanced footing and a combined footing. A combined footing contains a single footing whereas a balanced or cantilever footing contains two individual footings which are attached through a strap beam. The strap beam transmits the loads from exterior footing to the primary footing thru cantilever action.

A balanced footing and cantilever footing is utilized in the following conditions :-

1. If there is not enough place in the foundation to transmit the loads securely to the ground, as for example footing is developed on the property line. The another case is when the soil supporting the foundation does not include sufficient bearing capacity to transmit the loads securely and area of the footing is extended outside the limit.

2. When the new structure is constructed adjacent to the current building or structure, and the current building fails to support the loads from new structure, then new column and footing should be arranged adjacent to the current structure, but there remains restricted space for footing.

3. Combined footings are utilized for supporting two columns, but if the distance among the two columns is very big, it turns out to be uneconomic because of enormous bending moment to support two columns on a solo footing, then it is recommended to apply a strap beam for relating such footings.

In balanced footings, center of gravity of loads and footings is matched so as to the pressure on the foundation is constant. For cantilever footing, as one of the columns is supported with a cantilever action, the center of gravity of loads and footings is not matched and so pressure on foundations is not identical.

To design both the balanced footings and cantilever footings, equalize the loads on the footings by inserting dead loads on the internal column. If the fulcrum is adjacent to the internal column, the internal footing becomes very weighty. The strap beam that associates the two footings, are designed on the basis of bending moments because of external and internal column loads and as a cantilever for cantilever footings.

Ref: theconstructor.org