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What Is Spread Footing?
Spread Footings are generally used to function under each column of buildings and bridge piers in bridges. These footings are cost-effective and economical in construction. It is favorable for any soil with adequate load-bearing capacity.
Codes and specifications of structural design assume linear soil pressure distribution for the design of spread footings. Which makes designing spread footing a much simpler task.
Spread Footing can be constructed in many shapes and sizes such as circular, square, or rectangular. This footing can actually act as an inverted cantilever with loads provided in an upward direction.
Spread Foundation
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.
Read More: Grillage Foundation | Types of Grillage Foundation | Steel Grillage Foundation | Grillage Footing
Types of Spread Foundation
Following are the types of foundations categorized under Spread Foundation,
- Wall Footing
- Isolated Footing or Column Footing
- Combined Footing
- Strap Footing or Cantilever Footing
- Continuous Footing
- Inverted Arch Footing
- Grillage Footing
- Raft Foundation or Mat Foundation.
1. Wall Footing
Wall Footing is also known as strip footing is a continuous strip of concrete that could spread the load of the structure to a wide area of soil. It is a shallow foundation designed either in plain concrete or in reinforced concrete depending on the vertical loads.
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 as can be done using stone, brick, plain concrete, or reinforced concrete in an efficient way.
Conditions suitable for the laying of wall footing are that it is constructed where load transmitted is of small magnitude and the underlying soil layer is of dense sand and gravels. 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 knew 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. However, it must be designed in order to avoid exceeding bearing capacity and subsequently provide safety against overturning or sliding while preventing the ground from settling.
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 most 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 as combined footing. The main role of the footing is to distribute uniform pressure of 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 one which bears 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
Footing supporting more than two columns, in which loads from the individual columns are transferred either directly to the footing slab, or through a longitudinal beam running longitudinally when the loads are heavy, can be termed as Continuous Footing.
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.
Its design is analogous to strip footing used to support walls. The load transfers are transferred to beams and then from beams to ground. Also, it can support the RCC wall and long masonry wall effectively throughout the length.
6. Inverted Arch Footing
In older times, multistoried buildings were often provided with Inverted Arch Footing/Foundation, for the 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.
In 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 than 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 the 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.
Raft foundation is an alternative option to the trench fills or strip foundation. 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.
Read More: Raft Foundation | Raft Footing | Types of Raft Foundation | Mat Foundation | Raft Foundation Detail
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 the 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.
Design Procedure
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, the determining of the size i.e. diameter of the bar, number of bars, and spacing between them is required.
Step11– 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. In which by giving the loading conditions, properties of the soil, and the footing’s material properties are provided, and in turn, the spreadsheet can give the dimensions of the footing and the maximum moment and shear acting upon it using specific formulas.
Input Required
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 spread footing,
A=Qt/q
Where,
- Qt- The total load acting on the spread footing
- q- 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 variety of shapes, such as rectangular, square, circular, etc. and are 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 hence affordable.
- The construction process can be carried out using labors and no experts are required.
- Materials used are local and are easily available.
- Since the construction process is simple there is verylow risk of failure to structure because of its simplicity.
- Once the building starts settling there are chances of 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 into 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 pullout.
- 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.
Shallow Spread Footings
In the case where the depth of foundation is smaller than that of the breadth of foundation then it is referred to as Shallow or Stepped Foundation.
These types of foundations are basically the ones that can transfer building loads to the ground not too deep from the ground level, which has a minimum depth of 800 mm and a maximum of about 4 meters.
If the bearing capacity of soil on which the structure is required to be constructed is adequate then Shallow Spread Footings are the best choice in such circumstances.
Spread Footing with Pier
A pier foundation can be explained as an assembly of sizeable diameter cylindrical columns capable of holding up the superstructure and convey a specific amount of super-imposed load to the firm soil strata below.
Technology of Spread Footing with Pier
For laying Spread Footing Pier, the square or rectangular borehole is dogged and formwork is made ready in the pit which settles the shape of the foundation to be constructed. This is followed by installing a reinforcement cage and then concrete is poured.
After the hardening of concrete, the formwork is removed and canopied with a pillar.
Pier Foundation is laid depending on the following conditions,
- When upper layer of soil consist of decomposed rocks followed by hard rocks in underlying layer.
- When clay present in the foundation area offers resistance while driving a bearing pile.
- If the construction of structure is to be carried out on slopping plane.
- The soil on which foundation is to be laid is capable of bearing the load of structure.
Spacing of Pier- Pier foundations is generally constructed1 to 1.5 feet high from the bottom to resist the moisture.
FAQs
Spread Footing
Footing which is commonly used in residential buildings, having a wider bottom portion than the load-bearing foundation walls it supports can be termed as the Spread Footing. Spread Footings are generally used to function under each column of buildings and bridge piers in bridges
Spread Footing Design
Spread footings are specifically usually designed to use a factor of 3 ksi < f’c < 4 ksi, however, modern structural members require concrete in between the range of 4 ksi < f’c < 8 ksi. Members with higher concrete strength help in reducing the member size by a significant value.
What Is a Spread Footing?
Footing which is commonly used in residential buildings, having a wider bottom portion than the load-bearing foundation walls it supports can be termed as the Spread Footing. Spread Footings are generally used to function under each column of buildings and bridge piers in bridges
Concrete Spread Footings
Spread Footings are generally used to function under each column of buildings and bridge piers in bridges, hence in order to provide additional support, spread footings are developed with concrete & reinforced with steel.
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 convey a specific amount of super-imposed load to the firm soil strata below.
Shallow Spread Footings
In the circumstances where the bearing capacity of soil on which the structure is required to be constructed is adequate then Shallow Spread Footings are the best choice.
Advantages of Spread Footing
Spread Footing Advantages are as following,
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 hence affordable.
5. The construction process can be carried out using labors and no experts are required.
Spread Foundation
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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- Grillage Foundation | Types of Grillage Foundation | Steel Grillage Foundation | Grillage Footing
- Raft Foundation | Raft Footing | Types of Raft Foundation | Mat Foundation | Raft Foundation Detail
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