Balanced and Cantilever Footings
A type of footing known as a balanced footing, or strap footing, consists of two individual footings that are connected by a strap beam. This type of footing is typically used when one of the footings has limited space for bearing or if there are restrictions in place that prevent pressure from being transferred from the footing to the ground. This can occur when there is a difference in soil bearing capacity or if loose soil is present beneath one of the footings.
Loads in a balanced footing are transferred from one footing to the other through the use of a cantilever action. This is why it is also referred to as a cantilever footing. It is important to note that a balanced footing is distinct from a combined footing, which has a single footing rather than two separate ones. The strap beam in a balanced footing serves to transfer loads from the external footing to the main footing using cantilever action.
When to Use Balanced and Cantilever Footings
Balanced footings and cantilever footings are commonly used in construction when certain conditions are present. One such circumstance is when the foundation does not have enough area to safely transfer the loads to the ground, such as in the case of a footing built on a property line. Additionally, these types of footings are used when the soil supporting the foundation does not have sufficient bearing capacity to transfer the loads safely, and the area of the footing cannot be increased beyond a certain limit.
Another situation where balanced and cantilever footings are used is when a new structure is being built near an existing building or structure that cannot support the loads from the new structure. In this case, a new column and footing must be provided near the existing structure, but there may be limited space for the footing.
Combined footings are also sometimes used to support two columns, but when the distance between the columns is significant, it may become uneconomical to use a single footing to support both columns due to the large bending moment. In such cases, a strap beam can be used to connect the footings.
In the case of a balanced footing, it is essential that the center of gravity of the loads and footings coincide so that the pressure on the foundation is uniform. However, in a cantilever footing, where one of the columns is supported by a cantilever action, the center of gravity of the loads and footings may not coincide, resulting in non-uniform pressure on the foundation.
Fig: Types of Balanced Footings
Design of Balanced and Cantilever Footings
When designing balanced footings and cantilever footings, the goal is to balance the loads on the footings. To achieve this, dead loads are added to the internal column. However, if the fulcrum is located close to the internal column, the internal footing may need to be quite heavy.
To connect the two footings, a strap beam is used. The design of this beam takes into account the bending moments that result from both external and internal column loads. In the case of cantilever footings, the strap beam is designed as a cantilever.