Spread Foundation
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Spread Foundation
Home Civil Engineering Tips What is Spread Footing | Design of Spread Footings | 8 Types of Spread Foundation | Spread Footing Advantages and Disadvantages
Spread footings are utilised in the construction of the building to provide a stable foundation for the columns and walls . The load from the structure is delivered and transmitted to the ground via spread footing.
A shallow foundation is known as a spread foundation. Spread foundations are often employed as a building’s foundation. The spread foundation extends or spreads to the structure’s individual support curves. Open excavation is commonly used to build spread foundations.
This sort of footing can come in a variety of shapes, including round, square, and rectangular. To transfer the building’s high load, this form of footing is sometimes done utilising a step approach.
In the case of a bridge, the spread foundation enlarges at the bottom to provide individual support to the column or bridge abutment. Many of the varieties of spread foundation can be found under shallow foundation.
Simple or immersed wall footings are available. Light loads are carried by simple footings, while large loads are carried by stepped footings.
A simple footing has only one projection on each side of the wall, whereas stepped footings have many projections on each side.
The most frequent type of shallow foundation built around the world is isolated footings, often known as column footings. Furthermore, as compared to other types of foundations , shallow foundations are easier to design and construct due to their simplicity.
When two columns are close to each other and their individual footings overlap, a combined footing is necessary. This footing supports two columns and can be trapezoidal or rectangular.
A strap footing is a part of the foundation of a building . It’s a sort of composite footing that connects two or more column footings with a concrete beam . A strap beam is a name for this type of beam.
A strap footing is made up of two or more separate footings that are joined by a strap beam. It’s also known as a cantilever foundation or a pump handle foundation.
The foundation that supports more than two columns is known as a continuous footing. The footing is similar to a wall’s strip footing . When the weights are heavy, the loads from the individual columns are delivered either directly to the footing slab or through a longitudinal beam running lengthwise.
This type of foundation is appropriate for earthquake-prone areas and prevents differential settlement.
More than two columns can be supported by this form of footing. The load from each individual column is conveyed directly to the footing with this form of footing. Continuous footing is when the column’s footings are connected to each other by a longitudinal beam.
In earthquake-prone areas, continuous footing is commonly used. During a natural disaster, this style of footing can be a compromise against mobility. Its purpose is to keep the constitutional movement from being jeopardised and to maintain stability.
Inverted Arch Footing was once utilised as a foundation for multistory structures. This form of foundation is employed when the soil’s carrying capacity is low and the structure’s load is carried to the footing via the walls. Where there is a concern, deep excavation can also be done.
Grillage foundations are utilised to replace columns and piers with heavy structural loads on bearing capacity that is poor or non-existent.
As a shallow base, the grillage foundation is extremely beneficial. In which the footing is made up of more than two layers of beam. Which is used to distribute the load across a large area on the ground.
A raft foundation, also known as a mat foundation , is essentially a continuous slab sitting on the earth that runs the length of the building’s footprint, supporting it and distributing its weight to the ground.
Raft foundation is a combined footing that covers the entire region immediately beneath a structure and supports all of the walls and columns.
The depth of footing should be sufficient to effectively resist punching shear and direct shear transferred by column load. In addition, the footing’s reinforcement should be built to withstand bending moments.
The dowels provided at the column-footing contact should suffice as a load transfer mechanism for the column. Spread footings are made of concrete and reinforced with steel to offer additional support.
When compared to spot footers, the load transferred by spread footing is distributed over a larger area and has a lower chance of failure.
Spread footings can be designed using a design technique, software, or by manually applying design formulas.
Design Procedure is listed in a step by step manner,
Step 1– At the foundation level, determine the structural loads acting on the structure and various member sizes.
Step 2- After collecting all geotechnical data, the proposed footings are placed on factual and interpretive ground (geotechnical profile).
Step 3- After establishing the geotechnical profile, the depth and location of all foundation elements must be determined.
Step 4- Next, determine the carrying capacity of the soil on which the footing will be laid.
Step 5– Possible ground settlements in the form of total and differential settlement should be determined, and the impacts should be checked at 2B depths.
Step 6– To determine concrete strength, an appropriate grade of concrete is chosen.
Step 8- Determine the dimensions of the required footing.
Step 9– Determine the thickness of the footing (T or D).
Step 10– The size of the footing must be determined, including the diameter of the bar, the number of bars, and the distance between them.
Step 11- Design the connection between the superstructure and the foundation
Step 12– The structure-soil system is checked for uplift and stability against sliding and overturning in the final step of the operation.
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Spread footings are used in the building to form a strong base to support columns and walls. Spread footing is used to deliver and transmit the load coming from the structure to the ground.
The spread foundation is a type of shallow foundation. Spread foundations are commonly used as the foundation of a building. The spread foundation extends or spreads to the individual support curves for the structure. Spread foundations are usually constructed by open excavation.
The shape of this type of footing can sometimes be round, square, or rectangular. This type of footing is sometimes done with a step method to transfer the high load of the building.
The base of the spread footing is made slightly wider than the base for the load-bearing foundation. This type of spread footing is also called a stepped spread foundation.
Spread footing foundations are widely used in residential buildings. This type of foundation “spreads” the weight of the structure to spread the load coming through the load-bearing over a wide area and for greater stability of the building.
The design and layout of spread footings are controlled due to some adverse conditions. There may be some conditions such as soft soil beneath the structure or a change in soil volume due to frost penetration.
In the case of bridges, a spread foundation is created to support the column individually. The Spread Foundation has been given some of its subtypes as follows.
Wall footing can also be defined as strip footing. This type of footing is designed to transfer the load from the building to a large area of land through a continuous strip of concrete. Depending on the load-bearing wall this footing is formed by increasing the thickness of the plain concrete or reinforced concrete.
Wall footings are usually made in houses as pads or spreads and strip footings. Which is used to give a strong base to structural and non-structural walls by transmitting loads to the structure.
It is very easy to construct wall footing using materials like stone, brick, plain concrete, or reinforced concrete.
Some conditions should be kept in mind for the construction of wall footing. Wall footing is suitable in such situations when the load coming from the load-bearing walls is of low intensity and the soil is composed of thick sand and gravel. As a result, small buildings around us use this type of wall footing.
Isolated footing is also commonly called pad footing, column footing, or isolated spread footing. This type of footing is used to carry a load of stricture through the column and to transfer it safely to the ground. Usually, this type of footing can also have reinforced material.
Isolated footing is economical for general buildings, easy to construct.
In the construction industry, isolated footing can be used in the most commonly used foundations to support a single column when the span distance between two columns is greater. This type of foundation is used economically when the geotechnical properties of the soil do not change.
Isolated footing is divided into 3 subtypes
In this type of footing, a single footing is formed for two or more columns. Combine footing can be formed to evenly transfer the load of the structure through this column to the ground. The main purpose of this type of footing is to distribute the load of the building on the ground below.
This type of basic design is done regardless of the load on the column.
Combine footing is formed when the footing of the column is close to each other and the footing of the column overlaps with each other.
The choice of combined footing depends on the depth of the bearing strata, soil conditions, and the type of superstructure.
The shape of combined footing can be rectangular, trapezoidal, or T-shaped as required.
Strap footing is composite footing used as the foundation of a building. This type of footing is formed by joining two or more column footings by a concrete beam. This beam is known as a strap beam. Its use helps to distribute the weight of heavy or eccentric load column footing coming from the structure into adjacent footings.
Strap footing is used especially when located along a building property or plot line.
This type of footing prevents overturning of the footing by connecting it to nearby footings through strap beams.
This type of footing supports more than two columns. In this type of footing, the load coming from the individual column is transferred directly to the footing. When the footing of the column is connected to each other by a longitudinal beam, it is called continuous footing.
Continuous footing is widely used for earthquake-affected areas. This type of footing can be a compromise against movement during a natural disaster. It is designed to prevent compromise of the constitutional movement and to provide stability.
In the old days, Inverted Arch Footing was used as a foundation in multistory buildings. This type of foundation is used where the bearing capacity of the soil is very weak and the load of the structure is transferred to the footing through the walls. Also, deep excavation can be done where there is a concern.
In modern times, due to advances in strong construction methods such as reinforced cement concrete, the inverted arch footing is formed in very rare cases of use. Inverted Arch Footing is a suitable way to compromise with the depth of foundation in soft soil.
Grillage foundations are used to replace columns and piers with heavy structural loads on weak or low bearing capacity.
The grillage foundation is very useful as a shallow foundation. In which footing is formed consisting of more than two layers of the beam. Which is used to spread the load over it over a wide area on the ground.
Depending on the material used for this type of foundation, it can be divided into two categories: a) Steel grillage foundation b) Timber grillage foundation.
Raft foundations are known as a type of shallow foundations. This type of foundation is able to spread the load coming from the building over a larger area than other foundations. Therefore it is classified under spread foundation due to load transfer over a large area.
The raft foundation is a continuous type of slab resting over a large area. It covers the entire area of the building to support the building.
In modern times spread footing can be created by design process using software by calculating the load for the foundation of the building or by manually applying designing formulas.
The various steps of the design process are explained as follows,
Spread footing can be calculated for the area using the formula
This type of foundation transfers the load of the building to the ground level over a large area on the ground. Which are not deeper than the ground. The minimum depth of spread footing is kept at 800mm.
Spread footings as a shallow foundation are the best choice in cases where it is necessary to build on a building with the low bearing capacity of the soil.
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Structural loads, structural analysis and structural design are simply explained with the worked example for easiness of understanding. Element designs with notes and discussions have added to get comprehensive knowledge. Also, construction materials, shoring system design, water retaining structures, crack width calculations, etc. have discussed in addition to other aspects.
Spread footing is the most common type of foundation constructed in the world as a shallow foundation especially in the buildings and their related construction works.
Spread footing is also called a spread foundation or spread footing foundation.
It is a type of foundation designed and constructed to support superstructure elements such as steel or concrete columns, walls, etc. It is the most common type of construction done to rest the structure on the ground.
It has low construction cost and the quality assurance and quality control process is more convenient when compared to the other foundations such as cast-in-situ bored piles.
Types of foundations can be listed as follows.
The simplest type of foundation is constructed in the building construction. The wall foundation may be a concrete foundation or random rubble foundation.
Mostly, wall foundations are random rubble foundations unless there are no special loads applied on the foundation. There will be a reinforced concrete tie beam on the top of the wall foundation in order to minimize the settlement of the foundation due to the variation of loads or ground conditions.
Wall foundations are designed for line loads. Discontinuation of the wall generates additional stresses on the foundation. For a single-story structure, that stress is negligible.
Isolated footing is also known as column footings is the most common type of shallow foundation constructed in the world. Further, due to the simplicity of the shallow foundations, design, and construction is simpler when compared to the other type of foundations.
The design of this type of spread footing is that complicated. Only there are several steps to be followed.
A = F / σ where, A – area of the footing, F – Column load (serviceability limit state) and σ – allowable bearing capacity
σ U = P / A were, σ U – Ulitmate pressure under the footing, P – ultimate axial load
There are types of isolated spread footing foundation types such as
One of the most difficult types of spread footing to design is the strap footing. This type of spread foundation is construed at the boundary of the land.
When there are limited spaces, we construct the buildings at the edge of the boundary to get the maximum use of the land. When the load is applied on the edge of the footing, there will be an eccentric in the loading. This needs to be addressed during the design and construction.
We construct the beam called the strap beam to balance the induced eccentricity in the footing to a certain extent. It makes the pressure in the foundation uninformed to a certain extent. It will reduce the settlement of the footing too.
There are different types of strap spread footing foundations.
Raft Foundation is also called the mat foundation.
Mat foundation is a spread footing constructed on the ground. It has a uniform thickness and there may be an increase in the thickness of the foundation at column locations to improve the bending and shear capacity.
The last option of the spread footing foundations is the mat foundation. With the increase of the column axial loads, required are of the foundation increase. It will be difficult to construct the structure on the pad footings. Thus, when the number of floors of the building increases, the raft foundation option is selected.
In general, the cost of construction of raft foundation is less than cast in situ bored piles. Pile foundations incur higher costs for drilling, concreting, etc. In addition, the construction of ground beams and pile caps increases the cost.
The design of mat foundations is somewhat challenging when compared to the isolated pad footings. Further, manual design calculations are also complicated. Therefore, the common practice is to uses computer aid software to analyze the foundation. The following factors could be considered during the design.
Subgrade reaction = Safety Factor x 40 x Bearing Capacity . – From Bowel’s Foundation Book.
This calculation is done assuming a 25mm settlement in the foundation. If the expected settlement is more, the above equation could be modified. The safety factor is the one considered in the calculation of the allowable bearing capacity. That shall be used in the equation.
Now we can model the raft foundation. Generally, the foundation is a model with shell elements. However, if the thickness is larger, it could be modeled as plate elements.
When the column load increases, the area of the pad footings are increases. A higher area of spread foundation is required. Then we increase the area by placing strip footing combining columns in a single row.
An increase in the bending and shear forces can not beared by the strip footing as has a uniform thickness. Further, an increase in the column load leads to an increase in the thickness of the footing.
Therefore, a beam is added at the middle of the strip footing and then it becomes the Inverted T type foundation.
The design of Inverted T-type foundations is not that complicated. However, the use of computing aid software will make the analysis more accurate and easier. Soil can be modeled as a subgrade reaction.
The method of modeling will decide the type of spring or the subgrade reaction to being applied in the model. If footing is modeled as a line element by calculating overall stiffness, we have to apply line springs. If we model the strip footing as a slab(shell) and beam as line elements, we can use area springs to model the foundation.
Grillage footings are not that common type of
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