The Cantilever or strap footing is used to connect an eccentrically loading column to inside a column, while 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 types of footing that is used for two columns provided by two separate footings that are connected by a rigid beam called a “strap beam”.
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basic ideas for strap footing design:
1. The strap must be rigid, while stiffness is necessary to control the rotation of the exterior footing.
2. Footing should be proportioned to approximately equal to soil pressure and differential settling is reduced due to large spacing.
3. The strap should not be in contact with the soil so that soil reactions do not occur, it is common to neglect the strap weight in the design.
The strap should be sufficiently connected to each 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 the 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 which does not carry any soil reaction.
- The strap is an infinitely inflexible member.
- The strap beam function is used to tie two different notches with each other, it 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, 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.
- Strap footing is best suited for structural stability 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.
- These 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 possible to offer a combined footing or cantilever footing.
Strap footing Advantages and disadvantages:
- This helps to distribute the load uniformly and transfers the moment to the adjacent footing.
- This prevents the column from bending.
- The employment of strap footing might be excusable where the gap between columns is long and regular combined footing is impractical because of the large-scale excavations required.
- Strap footing requires more concentration, while it is constructed because it has a tough design.
- This will become comparatively extra time-consuming because the calculation of stress-bearing, pressure distribution, moments, forces, and shears can become difficult.
- It can demand specialist workers for its construction.
Also read: Difference between Footing and Foundation