What Is Spread Footing?
Spread footing, also known as continuous footing, is a type of foundation commonly used in construction. It is a reinforced concrete slab that extends under the entire perimeter of a structure, distributing the weight of the building evenly over the soil.
Spread footings are typically used in low-rise buildings where the loads are relatively light and the soil can adequately support the weight of the structure. They are also commonly used in residential construction, such as for homes, garages, and sheds.
The size and depth of a spread footing depend on a variety of factors, including the weight of the structure, the type of soil, and the climate and environmental conditions of the area.
Engineers use soil tests and other data to determine the necessary size and depth of the footing.
One of the advantages of spread footings is that they distribute the weight of the structure over a larger area, reducing the pressure on the soil and preventing settling or sinking.
They are also relatively simple to construct and can be adapted to a variety of soil types and building configurations.
Overall, spread footings are a reliable and cost-effective foundation option for a wide range of construction projects.
By ensuring that the size and depth of the footing are appropriate for the specific conditions of the site, engineers can help ensure that the finished structure is stable, durable, and safe.
The Spread foundation enlarges at the bottom in order to provide individual support to the column or bridge abutment in case of a bridge.
Spread foundation can be categorized under shallow foundation with many of its subtypes.
Types of Spread Foundation
Following are the types of foundations categorized under Spread Foundation,
1. Wall Spread Footing
This is one shallow type of footing that can be constructed as plain cement concrete footing or reinforced concrete footing.
Wall footings are constructed in the form of a pad or spread and strip footings, used to support structural and nonstructural walls by transmitting and distributing the loads to the soil, in such a way that the load-bearing capacity of the soil does not exceed.
Construction of Wall footing is very flexible and can be done using stone, brick, plain concrete, or reinforced concrete in an efficient way.
Such conditions are best suited for small buildings; hence we can see small buildings with wall footing.
2. Isolated Footing or Column Footing
Isolated Footing known as Column Footing, Pad Footing, or Isolated Spread Footing is used to carry the imposed by the columns and transfer evenly to the soil beneath. As that of wall footing, it could also consist of either reinforced or non-reinforced material.
Isolated footings are economical, easy to construct, require less excavation operation, and are easy to construct.
It is carefully designed considering soil bearing capacity and is safe from any sliding and overturning effect. It also properly resists ground settlement under the earth.
In the construction industry, an isolated footing is among the most used foundation used to support single columns when there is a long distance between columns.
It is applied when the geotechnical properties of the soil do not drastically change in the foundation area. It is also a financially beneficial kind of foundation.
An isolated footing can further be classified into 3 subtypes,
- Stepped footing
- Simple spread footing
- Sloped footing
3. Combined Footing
Footing supporting two or more columns and transferring their imposed load evenly to the soil beneath can be termed combined footing.
The main role of the footing is to distribute the uniform pressure of the structure to the ground beneath.
Its construction depending on the above purpose is done such that the center of gravity of the footing area should be equal to the center of gravity of the two columns or more column footing is supported.
The combined footing is generally made of reinforcement concrete as it is assumed to be rigid and resting on homogeneous soil.
Soil with low load-bearing capacity and individual footing required more area, which are the ideal conditions for the construction of combined footing.
4. Strap Footing
Strap Footing Consists of two or more column footings strapped together by a concrete beam. It functions as a medium to distribute the weight of either heavily or eccentrically loaded column footings to adjacent footings to obtain stability.
A strap footing is a conjunctive element used with columns that are located along with a building’s property or plotline.
It connects an eccentrically loaded column to a column that is well inside the foundation area, so as to transmit the moment caused by an eccentricity to the interior columnar foot.
This produces uniform pressure under footings that are eccentrically loaded and bear transferred load. Similarly, an eccentric load on a portion of the footing, causing it to tilt to one side is restrained by the strap beam.
5. Continuous Footing
Continuous footing usually has more than 2 columns, in which loads of each column are transferred to the footing slab directly, or through a horizontal (longitudinal) beam running parallel to the footing.
In case when a heavy load is spread over a large area, a continuous spread footing is used. Continuous footing is well suited for Earthquake prone zones, where uneven or differential settlement might occur during calamities. It is designed to prevent differential settlement of the structure.
6. Inverted Arch Footing
In older times, multistoried buildings were often provided with Inverted Arch Footing/Foundation, for cases where the bearing capacity of the soil is extremely poor and also the load of the structure is concentrated over the walls, where again deep excavations are of concern.
In modern times, because of advancements in reinforced cement concrete construction practices, the use of Inverted Arch Footing is done in very rare cases. However, it has the advantage of compromising the depth of foundation in soft soils.
Inverted Arch Footing end piles are needed to be specially strengthened by buttresses in order to avoid the arch thrust tending to rapture the pier junction. This is one of the major drawbacks of Inverted Arch Footing.
7. Grillage Footing
Grillage Footing/Foundation is a shallow type of foundation that consists of one, two, or more two tiers of beams superimposed on a layer of concrete to disperse load over a wider area of wet or loose soil beneath. It is suitable for heavy structure columns piers and scaffolds.
The grillage foundation is also suitable when the foundation of a structure is a shallow type of foundation that too laid over wet or loose soil.
Grillage foundation has characteristics property to sustain & transfer heavy loads from the structure to large areas.
8. Raft Foundation
Raft Foundation is also a type of Shallow Foundation, capable of spreading the load of the building over a larger area than other foundations to lower the pressure of a specific portion of the ground.
Hence it has been categorized under spread foundation because of its capabilities to spread the beard load.
A Raft foundation is an alternative option to trench fills or strip foundations. In case there is difficulty in laying trench fill and strip foundation, engineers suggest the option of raft foundation.
It is structurally a reinforced concrete slab spread over a whole foundation area, which looks similar to that of a raft floating on water. Hence termed as a raft foundation.
Concrete Spread Footing
For efficiently resisting punching shear and direct shear transferred by column load the depth of footing should be enough.
Also, the reinforcement provided within the footing should be designed to resist bending moments.
The dowels provided at the column-footing interface should be well sufficient to provide a column load transfer mechanism.
In order to provide additional support, spread footings are developed with concrete & reinforced with steel.
Load transferred by spread footing is over a sizeable area, there is a little risk of failure compared to spot footers.
Design of Spread Footing
Designing of Spread Footing can be done following design procedure, using software, or manually applying designing formulas.
The design Procedure is listed in a step-by-step manner,
Step 1– The first step involves determining the structural loads acting and various member sizes at the foundation level.
Step 2-In the next step all the geotechnical data is collected and the proposed footings are set on the factual and interpretive ground (geotechnical profile).
Step3-After setting the geotechnical profile one should determine the depth and location of all foundation elements.
Step 4-Next step proceeds with determining the bearing capacity of soil on which footing is to be laid.
Step 5– Possible settlements of ground in the form of total settlement and differential settlement should be determined and checks of the effects at 2B depths are performed.
Step 6– Concrete of a suitable grade is selected to determine concrete strength.
Step 7-Steel Grade is selected.
Step 8– Determine the required footing dimensions.
Step 9– Estimate the footing Thickness T or D.
Step10– For reinforcing the footing, determining the size i.e. diameter of the bar, number of bars, and spacing between them is required.
Step 11– Design the connection between the superstructure and the foundation.
Step 12–The last step of the procedure is to check uplift and stability against sliding and overturning of the structure-soil system.
Design of Spread Footing Using Software
Shallow Foundations are simple and economical to build, hence are the most commonly used type of foundations.
Spread footing is also a shallow footing and fortunately for the same, the average price of software for the design and analysis of shallow foundations is about $ 400.
For example, a simpler software to design Spread Footing is Microsoft’s® Excel spreadsheet. By giving the loading conditions, properties of the soil, and the footing’s material properties are provided, in turn, the spreadsheet can give the dimensions of the footing and the maximum moment and shear acting upon it using specific formulas.
1. Physical Layout: The required ratio of length to width and the estimated thickness of the footing.
2. Material Properties: Unit Weight of Concrete, and Allowable Bearing Capacity of the soil.
3. Loading Conditions: Dead Load, Live Load, Moments about the x-axis and y-axis, and Dead Load Imposed on Footing.
The Formula For Determining Area of Spread Footing
The formula is used to determine the bottom of the spread footing,
- Qt- The total load acting on the spread footing
- q- A base area of spread footing
Advantages of Spread Footing
Spread footing has the following advantages,
- Spread Footing is one of the simplest types of footing hence easy to construct.
- It is available in a variety of shapes, such as rectangular, square, circular, etc, and is used as per requirement.
- The resulting pressure on the supporting soil does not exceed the soil’s allowable bearing because of its capability to can distribute building loads over a large area.
- The method of constructing spread footing is cost-effective and hence affordable.
- The construction process can be carried out using labor and no experts are required.
- Materials used are local and are easily available.
- Since the construction process is simple there is a very low risk of failure to structure because of its simplicity.
- Once the building starts settling there are chances of the formation of cracks; such crack formation can be reduced using spread footing.
- Damage due to frost heaving can be reduced.
- Differential settlement can be kept in check.
Disadvantages of Spread Footing
- Spread footing is limited to some soil structures only and cannot be used for every form of soil.
- This type of foundation is constantly subjected to torsion, moment, and pull-out.
- Settlement is a major problem regarding this type of foundation.
- Irregular ground surfaces make the structure sloppy once the spread footing type of foundation is laid.
What Is a Spread Footing?
Spread Footing with Pier
A pier footing can be explained as an assembly of sizeable diameter cylindrical columns capable of holding up the superstructure and conveying a specific amount of super-imposed load to the firm soil strata below.
Shallow Spread Footings
In circumstances where the bearing capacity of soil on which the structure is required to be constructed is adequate then Shallow Spread footing is the best choice.
Advantages of Spread Footing
Spread Footing Advantages are as follows,
1. Spread Footing is one of the simplest types of footing hence easy to construct.
2. It is available in a variety of shapes, such as rectangular, square, circular, etc., and is used as per requirement.
3. The resulting pressure on the supporting soil does not exceed the soil’s allowable bearing because of its capability to can distribute building loads over a large area.
4. The method of constructing spread footing is cost-effective and hence affordable.
5. The construction process can be carried out using labor and no experts are required.
The Spread foundation enlarges at the bottom in order to provide individual support to the column or bridge abutment in case of a bridge. Spread foundation can be categorized under shallow foundation with many of its subtypes.
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