Spread Footing

Spread Footing

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Soil pressures and deformation of an isolated spread footing

Reinforcement of an isolated spread footing

3D view of spread footing reinforced with both upper and lower rebar grid

Detail of spread footing reinforced with both upper and lower rebar grid

Controlling punching shear, with the use of stirrup cages

Spread footing punching shear mechanism

Spread footing with punching shear reinforcement

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The spread footing behaves like an inverted cantilever with loads applied in the upward direction. As a rule, a spread footing is a quite rigid element therefore, the applied soil stresses are almost linear and in case of a symmetric (with respect to the pedestal) footing, they are orthogonal. These soil pressures are the loads carried by the footing that behaves like a slab and is deformed by the way shown at the figure.

The real deformation is in the order of a millimeter and although it is not visible to the human eye, it always has that same form. The reinforcement is placed at the lower surface of the footing both along the x and y axis.
The footing is reinforced with an orthogonal rebar grid that may have bars of different diameter and spacing in either direction. Bending the rebars ends helps in the proper anchorage of the reinforcement.
The reinforcement is the same both for flexible and rigid spread footings either being centrically or eccentrically constructed.
In certain occasions, the footing may also have an upper reinforcement rebar grid.
Punching shear stresses are similar to the shear stresses (diagonal flexure) and they appear along the perimeter of the column.
Punching is a form of column sliding along the side of a cone that forms a 35° angle with the horizontal axis, as shown at the opposite figure. The first stirrup leg must be placed at a maximum distance of 0.5d away from the face of the pedestal and the last stirrup leg at a maximum distance of 1.25d.
In this specific spread footing, punching shear is controlled by stirrup cages (figure a) however, it could be also controlled with bundles of properly bent-up rebars (figure b), or with special industrial elements (figure c).
Buildinghow Team integrates all Earthquake Resistant Building Technologies. The team consists of experienced structural engineers and IT experts, led by Apostolos Konstandinidis.
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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 spread footing foundation. It is constructed for special locations.
Grillage type of structure is constructed to support the superstructure. The grillage may construct from steel or reinforced concrete beams.
In addition, in single-story structures, beams are placed on the ground similar arrangement to a grillage to support the walls. Having good soil conditions is a must to construct a spread foundation of this nature.
This is not common in the modered construction of spread foundation. These types of construction were done in old days. Most ancient structures have higher weight had been built of these types of foundations.
The inverted arch represents the strip-type footing. The function of the arch has been used to stiffness the foundation and to improve the load transfer.
The inverted arch construction has been initiated even before the invention of concrete. Therefore, the main construction material of inverted arch footings was brick or stones.
This would be very critical for foundations having smaller axial loads. The upward movement will cause cracks in the structures. Especially this will affect the nonstructural elements such as brick walls.
To overcome these issues, in small buildings, tie beams shall be designed for these upward movements. Strengthening the tie beams would minimize the effect on the nonstructural elements.
The designer shall be aware of the amount of settlement that causes in the foundations and take necessary action to avoid such settlement or design the foundation and superstructure for the expected settlement.
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The spread footing is used to support the column and walls and also transmit and distribute the load coming on the structure to the soil below it.
With loads applied in the upward direction, this footing behaves like an inverted cantilever, and this type of footing is quite rigid element & they are orthogonal in case of symmetric footing.
This type of footing is a circular, square, or rectangular slab of uniform thickness and to spread the load over a large area sometimes it is stepped or haunched.
A spread footing base is wider than a typical load-bearing wall foundation and spreads the weight from the building structure above more area and provides recommended stability.
Where the bearing soil layer is within 10 feet from the ground surface, this type of footing is used for individual columns, walls, and bridge piers.
If there is any possibility of a ground flow of water above bearing layer of soil which may result in scour or liquefaction, these should not be used on such soils.
Using the following formula, the base area of the spread footing is calculated;
The total load coming on the spread footing is Qt &
q is the base area of spread footing.
There are four types of this footing as given below;
It is a type of shallow foundation typically used for shallow establishments to convey and spread concentrated burdens by pillars or columns and used for ordinary buildings.
Directly at the base of the segment isolated footing comprises a foundation and transfers the loads from the column to the soil and it might be rectangular, roundabout, or square.
By dividing the total load at the column base by the allowable bearing capacity of the soil the size of the footing can be roughly calculated.
A combined footing is required when two columns are close to each other and their individual footings overlap and this footing supports two columns and it may be trapezoidal or rectangular.
Strip footings are used to distribute loads of structural or non- structural load-bearing walls to the ground.
Strip footing distributes loads in such a way that the load-bearing limit of the soil is not outperformed and the width of the wall foundation is usually 2 to 3 times greater than the width of the wall.
Strip footings are economical when the small magnitude loads to be transmitted or when it is placed on dense gravel and sand.
A mat foundation supports several columns and walls under the entire structure and is a large slab. A single footing for all columns is provided if several columns overlap each other and are called mat footing.
To reduce the differential settlements on non-homogeneous soils mat foundations are used.
For providing the spread footing as a foundation in the building, there are the following advantages such as;
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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–
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