The Cantilever or strap footing is used to connect an eccentrically loading column to inside a column.
The strap is used to transmit the moment caused by an eccentricity to the interior columnar foot so that a uniform soil pressure is produced under both footings.
If the distance between the columns is massive or the allowable soil pressure is comparatively large, strap footing could also be used as an alternative of rectangular or trapezoidal joint footing where the extra footing space is not required.
The two types of cantilever footing with the strap:
- Cantilever footing with uniform strap.
- Cantilever footings with non-uniform strap.
This is a special type of footing that is used for two columns provided by two separate footings that are connected by a rigid beam called a “strap beam”.
basic ideas for strap footing design:
1. The strap must be rigid, this stiffness is necessary to control the rotation of the exterior footing.
2. Footing should be proportioned to approximately equal soil pressure and differential settling is reduced due to large spacing.
3. The strap should be out of contact with the soil so that soil reactions do not occur, it is common to neglect the strap weight in the design.
Strap should be sufficiently connected to each the column and the footing so that the dowels can be used, such that the footing and the strap act as a unit, the footing is subjected to one-way bending.
The cantilever beam is reinforced between the columns with the main reinforcement at the top and the bottom of the interior footing.
Design of strap footing:
Cantilevered footings are special types of footings that are used for two columns, it is designed by balancing the load on footing of the building.
In the case of cantilevered footings, the 2 bending designs combine base bending moments because of the external and internal loads of the strap beam-column.
Assumptions for the strap or cantilevered footing:
- The strap is considered as a pure flexural member and it does not carry any soil reaction.
- The strap is an infinitely inflexible member.
- Strap beam function is to tie two different notches with each other, the strap beam transfers the load through cantilever action.
- The building’s column, which is located at the boundary of the property line, has an eccentric loading footing.
- The houses are very contiguous to each other, and very less and congested spaces are available, due to which the outer walls are very close to each other.
- In this situation, very limited space is available, due to which isolated legs cannot be placed.
- The column at the end of the property line is eccentrically loaded.
- In such a situation, strap footing are best suited, which gives structural stability to the structures without compromising the aesthetic view of the structures.
- The main purpose of providing cantilevered footing is to counteract the asymmetrical loads on the structure.
Cantilever Action in the Structures:
Structural components of buildings are supported at both ends for proper support to the load on the structure.
Cantilever structural elements are used, which are supported at one end and free at the other end.
The load transferred in the cantilever footings from one foot to another foot through cantilever action.
In cantilevered footing and structure, the load is designed at the end so that it will resist bending moments and shear stresses.
When a particular column is located near the boundary of the property line, it is necessary that square or rectangular footing concentrically located under the column, which extends into the adjoining property.
In the case of cantilever footings, it is necessary that the center of gravity of the load and the foundation must coincide with each other to transfer the load evenly.
Situations Where strap footing Are Used:
- Cantilever footings are preferred when sufficient area is not available to move the foundation to underground terrain.
- Cantilever footings are also used when the soil does not have good bearing capacity to transfer the load coming from the structure to the ground, and the size of footings can be increased outside the range.
- The cantilever is used when the column is constructed near the boundary of the plot and it cannot be extended further.
- When the distance between two successive columns could be very large, and it is not attainable to offer a combined footing, cantilever footing is used.
A cantilever footing is frequently used in conjunction with columns that are located along the property or lot line of a building.