Spread footing is constructed to create a stable foundation for columns and walls and it is used to deliver and transfer the load of the structure to the earth.
The load-bearing foundation base is made slightly broader than the base for the spread footing.
This is also known as a stepped spread foundation.
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What is Spread Footing?
For each construction column and the piers of bridges, spread footings are typically used.
These footings are practical and economical for building.
It can be built in a variety of sizes and shapes like square, circular, and rectangular.
When stresses are applied upward, this footing can really operate as an inverted cantilever.
Purpose Of Spread Footing:
- This designing of spread footing considerably easier.
- To support a foundation or series of piers beneath a building, spread footings are utilized.
- Spread footings are constructed using concrete and steel reinforcement to add additional stability.
Types of Spread Footing:
1. Wall Spread Footing:
Wall footing is also known as strip footing.
These continuous concrete wall foundation strips might disperse the weight of the building over a large region of soil.
One shallow style of footing may be constructed using either standard cement concrete or reinforced concrete.
Wall footings in the form of a pad or spread-and-strip footings are constructed to support both structural and non-structural walls.
These footings transmit and distribute loads to the soil in a way that doesn’t go beyond the earth can bear.
The construction of a wall footing requires the load imparted be small and that the underlying soil layer is composed primarily of compact sand and gravel.
2. Column Footing or Isolated Footing:
To carry the load imposed by the columns and transfer it uniformly to the soil below.
Isolated footing is also known as column footing, pad footing, or isolated spread footing.
It could also be made of reinforced or non-reinforced material, similar to wall footing.
They are simple, inexpensive, and easy to build, also require less excavation work.
It is used when there isn’t a significant change in the geotechnical characteristics of the soil around the foundation.
It is also the kind of foundation that is most profitable financially.
There are three types of isolated footings:
- Stepped Footing.
- Simple Spread Footing.
- Sloped Footing.
3. Combined Footing:
This footing can be rectangular or trapezoidal that supports two columns when they are close to one another and their individual footings overlap.
According to the aforementioned goal, it is constructed such that the footing area’s centre of gravity matches the centres of two or more supporting columns.
Assumed to be stiff and resting on uniform soil, the combined footing is often composed of reinforcing concrete.
The best circumstances for the construction of combined footing are poor load-bearing capacity soil and the need for extra space for individual footing.
4. Continuous Footing:
Typically, continuous footing consists of more than two columns, with the stresses from each column being distributed either directly to the footing slab or through a horizontal (longitudinal) beam that runs perpendicular to the footing.
A continuous spread footing is utilized when a big load is dispersed across a large region.
For earthquake-prone areas where uneven or differential settlement may happen during disasters, the continuous footing is a good fit.
It is intended to stop the structure from settling in different places.
5. Inverted Arch Footing:
To prevent the arch thrust from tending to tear the pier connection, end piles in inverted arch footing must be properly strengthened by buttresses.
This is one of the key problems with inverted arch footing.
In the past, deep excavations were a worry for multistorey buildings, thus in circumstances where the soil’s carrying capacity was exceptionally low and the weight was focused on the walls, inverted arch footings or foundations were frequently used.
6. Strap Footing:
A concrete beam is used to secure two or more column footings together.
In order to achieve stability, it serves as a medium to disperse the weight of either highly or eccentrically loaded column footings to nearby footings.
A conjunctive component known as a strap footing is utilized with columns that are situated alongside a building’s property or storyline.
In order to transfer the moment produced by an eccentricity to the interior columnar foot, it joins an eccentrically loaded column to another column that is well inside the foundation region.
7. Grillage Footing:
This shallow foundation distributes the load over a larger area of wet or loose soil beneath by using one, two, or more tiers of beams superimposed over a layer of concrete.
It is appropriate for heavy scaffolds and building columns and piers.
The grillage foundation is also appropriate for shallow foundations that are positioned over wet or loose soil to support buildings.
Grillage foundations have the ability to support and disperse significant loads from the structure over wide areas.
8. Raft Foundation:
The raft foundation can lessen the pressure on a particular section of the ground by dispersing the weight of the building over a broader area than other foundations.
Since of its ability to disperse the weight of the beard, it has been classified as a spread footing.
An alternative to strip foundation and trench fills is a raft foundation.
Engineers advise the use of raft foundations in the event that trench fill and strip foundations are difficult to build.
Spread Foundation Design:
- The first step entails figuring out the various member sizes and the acting structural loads at the foundation level.
- The suggested footings are set on the factual and interpretive ground in the following step, which also involves gathering all the geotechnical data (geotechnical profile).
- Determine the depth and placement of each foundation component after the geotechnical profile has been defined.
- The following step involves figuring out the soil’s bearing capacity for the footing that will be placed.
- Determine any potential ground settlements in the form of total settlement and differential settlement, and assess the consequences at 2B depths.
- The strength of the concrete is determined by selecting a suitable grade of concrete.
- Steel Grade is chosen.
- Establish the necessary footing dimensions.
- Determine whether the footing Thickness is T or D.
- To reinforce the footing, it is necessary to determine the diameter of the bar, the number of bars, and the distance between them.
- Plan the foundation’s and the superstructure’s interface.
- The stability against sliding and overturning as well as uplift of the structure-soil system are investigated.
Formula for Calculating the Spread Footing Area:
The formula is used to calculate the spread footing’s bottom.
Qt: The overall load on the spread footing.
q- A base area of spread footing.
Advantages of Spread Footing:
- The spread footing increases stability by spreading out the weight from the structure across a larger area of the soil beneath it.
- It is much less likely to fail as compared to other types of footing.
- This footing prolongs the life of the buildings by reducing structural damage.
- These footings are continuously employed to sustain the structure.
- The building structures make it simple to install these footings.
- Spread footing has a wider bottom component than a load-bearing foundation, which distributes the structure’s weight for greater stability across a wider area.
- Spread footing is significantly less likely to fail than other types of footing.
- Concrete and steel reinforcement was used in the construction of these footings to provide additional strength.
Disadvantages Of Spread Footing:
- Spread footing cannot be used for all types of soil and it is only acceptable for specific soil structures.
- Torsion, moment, and withdrawal are constant threats.
- The main problem with this type of foundation is a settlement.
- The spread footing kind of foundation makes the building sloppy over irregular ground surfaces.
Also read: Strap Footing | Stepped Footing
The spread footing was shaped and constructed to support superstructure components including steel or concrete columns, walls, and other structures.
It is the most typical design employed to prevent a structure from settling into the ground.
Its construction costs are lower than those of other foundations, and its quality assurance and quality control procedures are simpler.