What is Spread Footing Foundation?
A shallow foundation that is used to support individual columns or piers is known as a Spread footing foundation. It has got a wide base also known as footing, that divides the load of the column over a larger area of soil.
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The weight and properties of the column determine its depth and width. To increase durability and strength, a spread footing foundation is usually constructed from concrete and reinforced with concrete.
They are commonly used in areas where the loads are relatively lighter and the soil is stable, like residential and small buildings. They are more inexpensive and easier to construct in comparison to other types of foundations.
This spread footing foundation is not good for constructing buildings with heavy loads or unstable soil conditions. In the case of building with heavy loads or unstable soil conditions, deep foundations such as pile foundations or drilled pier foundations may be required.
To ensure their continued stability and prevent settlement or other forms of damage, the spread footing foundation requires regular maintenance. Overall, this type of foundation is popular and widely because of its simplicity, cost-effectiveness, and ease of construction.
Design steps of Spread Footing Foundation
a) Determining the load: We need to calculate the total load that the foundation requires to support, including the weight of the building, contents, and people.
b) Determining the soil bearing capacity: We have to conduct a soil investigation so that we can determine the type of soil and its bearing capacity. We know that bearing capacity is the maximum load that the soil can support without excessive settlement or failure.
c) Determining the footing size: For determining the footing size, we need to calculate the required size of the footing based on the load and soil bearing capacity. The size of the footing should be large enough to distribute the load equally over a sufficient area of soil to prevent settlement.
d) Determine the footing depth: We have to determine the depth of the footing by considering the frost depth, soil type, and any other local requirements.
e) Reinforcement design: We can determine the required reinforcement for the footing based on the load, footing size, and soil conditions. Reinforcement is typically provided in the form of steel bars or welded wire mesh.
f) Design the concrete mix: The concrete mix should have sufficient strength and durability to withstand the loads and conditions. We determine the required strength of the concrete and design a mix that meets the specifications.
g) Prepare construction drawings: We need to prepare detailed construction drawings that show the shape, size, reinforcement, and concrete specifications for the spread footing.
h) Obtain permits: We should remember to obtain necessary permits or approvals from the local building department before starting construction.
i) Excavation and preparation: We need to excavate the site up to the required depth and prepare the subgrade with a suitable base material.
j) Pour and place concrete: We need to pour and place the concrete for the footing in accordance with the drawing of the construction and specifications.
k) Install reinforcement: We should install the reinforcement according to the drawing of the construction and specifications.
l) Cure and finish: We should allow the concrete to cure and finish the surface to the desired texture and appearance as in the drawing.
m) Backfill and site grading: Backfill around the footing and grading of the site to the desired slope and drainage pattern should be done.
n) Conduct quality control and testing: We have to conduct quality control checks and testing to ensure that the foundation meets the proper design requirements and specifications.
o) Complete documentation: At last, we have to prepare complete documentation, including inspection reports, testing results, and as-built drawings, for the project recording.
Characteristics of spread footing foundation
a) Shallow foundation: It is a type of shallow foundation that is used to support individual columns or piers.
b) Wide base: It has got a wide base (or “footing”) that distributes the load of the column over a larger area of soil.
c) Load distribution: To reduce the pressure on the soil and prevent excessive settlement, the base is made wide which helps to distribute the load of the structure over a larger area of soil.
d) Concrete construction: To increase the strength and durability of spread footing, they are made up of concrete and reinforced with steel bars.
e) Simple design: The design is relatively simple so that it can be easily calculated and constructed.
f) Suitable for light loads: These spread footing foundations are commonly used in residential areas and small commercial buildings where the loads are relatively lower and the soil is much more stable.
g) Inexpensive: They are not more expensive in comparison to other types of foundations, such as deep foundations.
h) Easy to construct: Spread footing foundations do not require specialized equipment or skills so they are easy to construct.
i) Requires regular maintenance: For continued stability and prevention from the settlement or other forms of damage, regular maintenance is required.
j) Limited to stable soil: Deep foundations such as pile foundations or drilled pier foundations may be required in the case of buildings with heavy loads or unstable soil conditions.
Advantages and disadvantages of spread footing foundation
Cost-effective: These are less expensive in comparison to other types of foundations, which makes it a cost-effective option for small to medium-sized buildings.
Simple design: The overall construction time and cost is reduced because the design of spread footing foundations is simple and can be easily calculated and constructed.
Suitable for stable soil: They are ideal for supporting light to moderate loads and are commonly used in stable soil conditions.
Easy to construct: It is accessible to a wide range of constructors and builders because they are easy to construct and do not require specialized equipment or skills.
Prevents excessive settlement: Spread footing foundation helps to distribute the load of the structure over a larger area of soil because it has a wide base which reduces the pressure on the soil and prevents excessive set.
B) Disadvantages of spread footing foundation
Limited to stable soil: It is not suitable for buildings with heavy loads or unstable soil conditions. Deep foundations such as pile foundations or drilled pier foundations may be required in such cases.
Limited depth: The stability of the soil limits the death of the spread footing foundation, which means that it may not be suitable for buildings with deep or heavy loads.
Limited to small to medium-sized buildings: These are generally suitable for small to medium-sized buildings only and may not be suitable for larger structures.
May require maintenance: To ensure their continued stability and prevent settlement or other forms of damage, spread footing foundations require regular maintenance.
Limited to certain soil types: It may not be suitable for all types of soil, and a thorough soil investigation is very important to determine the soil type and its suitability for a spread footing foundation.
Numerical to design spread footing foundation according to IS Code
Here with the help of this example, we are going to learn, how to design a spread footing foundation according to the Indian Standard Code (IS 456:2000):
Load on column = 300 kN
Soil safe bearing capacity = 150 kN/m²
Reinforcement steel = Fe 415
Concrete grade = M20
Step 1: Determine the size of the footing
Let us assume the footing is square, and we can use the formula: A = P / (q * FOS)
where A is the area of the footing, P is the load on the column, q is the safe bearing capacity of the soil, and FOS is the factor of safety (typically 2.5 to 3).
After plugging in the values, we get: A = 300 / (150 * 2.5) = 0.8 m²
By assuming a square footing, the side length is then: √0.8 = 0.89 m
Hence, the size of the footing is 0.89 m x 0.89 m
Step 2: Calculate the depth of the footing
Let us assume a minimum depth of 150 mm for the footing, we can calculate the required depth as: D = A / B
where D is the depth of the footing, A is the area of the footing, and B is the base area of the footing.
By plugging in the values, we get: D = 0.8 / (0.89 * 0.89) = 1.0 m
Hence, the depth of the footing is 1.0 m
Step 3: Determine the reinforcement required
Again, here we are assuming 4 bars of 16 mm diameter are required, we can calculate the area of steel required using the formula:
As=(0.5*P*e)/(0.87 * fy * d)
where As is the area of steel required, P is the load on the column, e is the eccentricity of the load, fy is the yield strength of the steel, and d is the effective depth of the footing.
Here by assuming an eccentricity of 150 mm, we can calculate e as: e = (0.5*0.89) – 0.15 = 0.29 m
Now again plugging in the values, we get:
As = (0.5 * 300 * 0.29) / (0.87 * 415 * 0.85) = 0.0098 m²
Hence, the area of steel required is 0.0098 m²
By using 4 bars of 16 mm diameter, the total area of steel is 0.01 m², which is greater than the required area.
Step 4: Check for deflection and cracking
Here in this step, we can use the formula: δmax = (S * L) / (3000 * EI)
where δmax is the maximum permissible deflection, S is the safe bearing capacity of the soil, L is the length of the footing, E is the modulus of elasticity of concrete, and I is the moment of inertia of the footing.
By assuming a length of 0.89 m and a thickness of 1.0 m, we can calculate I as: I = (1/12) * 0.89 * (1.0^3) = 0.074 m^4
And plugging in the values, we get: δmax = (150 * 0.89) / (3000 * 2 * 10^5 * 0.074) = 0.003 m
The maximum permissible deflection for residential buildings is typically 10 mm, which is greater than the calculated value of 0.003 m, so deflection is not a concern.
Cracking can also be checked by using the given formula: As / (b * d) >= 0.12 * √fck
where b is the width of the footing, d is the effective depth of the footing, and fck is the characteristic compressive strength of concrete.
By assuming a width of 0.89 m, we can calculate the cracking load as:
Pcr = 0.12 * √(20) * (0.89 * 1.0) * 1000 = 283.6 kN
Until the load on the column is less than the cracking load, cracking is not a concern.
Step 5: Detail the reinforcement
The footing will have got 4 bars of 16 mm diameter in both directions
From the bottom and sides of the footing, the reinforcement should be kept at a cover of 50 mm
The bars should be placed such that the clear spacing between bars is 200 mm.
The bars should be bent up at 45° to 60° from the vertical and anchored to the column.
This is an example of designing a spread footing foundation according to the Indian Standard Code. Based on specific project requirements and local building codes actual design may vary.