What Is Spread Footing | Types, Design, Advantages & Disadvantages

What Is Spread Footing?

The spread footing, a key crucial element in foundation types, is utilized to support the column & walls and additionally to convey & disseminate the load coming to the structure to the soil below it.

With loads provided within the upward direction, this footing actually acts like an inverted cantilever, and this sort of footing is typically a rigid element & they’re orthogonal just in case of symmetric footing.

This type of footing may be a circular, square, or rectangular slab of uniform thickness and to spread the load over a sizeable area sometimes it’s stepped.

The base of spread footing is slightly wider than load-bearing foundation base. This is also called a stepped spread foundation.

During this sort of footing, a base foundation is made which is an RCC member. This wider bottom disseminates the burden over a good area, adding stability to the building.

The spread footings, an example of footing concrete, are constructed with concrete & steel and, because of their design, they’re less likely to fail than other spot footers.

Also, Read: How to Monolithic Construction

Types of 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.

A wall footing or strip footing is a continuous strip of concrete that serves to spread the weight of a load-bearing wall across an area of soil. It is the component of a shallow foundation. Wall footings carrying direct vertical loads might be designed either in plain concrete or in reinforced concrete.

Wall footing is often either simple or steeped. Wall footing may be further categorized into two types:

1. Simple wall footing.
2. Stepped wall footing. Click this link for more about “Strpped Footing”

2. Isolated Footing.

Isolated footing is the single or individual footing which transfers load to the underground soil. It is provided when a single column is to be provided. A foundation is the part of the substructure which makes a direct contact or rested inside the ground.

This type of footing is employed for a single column. This isolated footing is further classified into 3 types:-

1. Stepped footing.
2. Simple spread footing.
3. Sloped footing.

3. Combined Footing.

When individual footing has to accommodate two or in exceptional case more than Two columns is known as the combined footing or When two or three columns are located very near to each other, we provide a single substructure for them and the footing is known as a combined footing.

When two or more columns are available in a row then this kind of footing is made. In this, there are two kinds of footing:-

• Rectangular-shaped combined footing. Columns are uniform or identical.
• Trapezoidal-shaped combined footing. Columns aren’t equal or same & have space limitations.

4. Strap Footing.

A strap footing is a component of a building’s foundation. It is a type of combined footing, consisting of two or more column footings connected by a concrete beam. This type of beam is called a strap beam

Strap footing is a combination of two or more individual footings connected strap beam. It’s additionally sometimes called as cantilever footing or pump handle foundation.

5. Continuous Footing.

A continuous footing is the one which supports more than two columns. The footing is analogous to the strip footing for wall. The 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.

This sort of foundation is suitable within the earthquake area and also prevents differential settlement.

6. Inverted Arch Footing.

An inverted arch or invert is a civil engineering structure in the form of an inverted arch, inverted in comparison to the usual arch footing.

In the simplest case, the arches spread the downward loads of viaduct piers into a wider ground area, just like an inverted arch bridge.

This type of foundation is used in places where the bearing capacity of the soil is extremely poor, and the load of the structure is concentrated over the walls. It is also used where deep excavations are not possible.

7. Grillage Footing.

A foundation consisting of one, two or more layers of beams (typically steel) superimposed on a concrete layer to disperse the load over a large area is a foundation for grilling. This type of foundation is generally used for pillars and column scaffolds with a heavy structure.

A grillage footing is utilized to transmit ponderous loads from the steel column to the soils having low bearing power.

8. Raft Foundation.

A raft foundation, also called a mat foundation, is essentially a continuous slab resting on the soil that extends over the entire footprint of the building, thereby supporting the building and transferring its weight to the ground.

A raft foundation is actually a merged footing that covers the whole area just underneath a structure & holds all the walls & columns.

In this foundation, an R.C.C. slab is provided with a beam or without the beam of suitable thickness.

Concrete of Spread Footing:

The footing depth should be deep enough to resisting punching shear and direct shear transferred from column load. The reinforcement within the footing should be designed to resist bending moments.

The dowels at the column-footing interface should be sufficient to transfer column loads. To feature additional support, spread footings are developed with concrete & reinforced with steel.

Since spread footing transfers the load of the building over a sizeable area, there is a little risk of failure compare to spot footers.

Where moment framed structure is employed, spot footings will be reinforced to resist moment forces generated by wind load or earthquake load.

Also Read: Building Estimation Excel Sheet

Spread Footing Design:

Follows the steps for designing of Spread footing:

1. Determine Structural Loads and Sizes: At first, in spread footing design, we need to determine the structural loads and sizes of different members at the foundation level.
2. Collect Data and Set the Proposed Footing: Then, we need to collect data and set the proposed footing.
3. Determine Depth and Location: After that, we determine the depth and location of the footing element.
4. Determine Bearing Capacity of Soil:
   • Soil Types and Suitability:
     • Clay Soil: Clay soil has low bearing capacity and may require deeper or larger spread footings to avoid excessive settlement.
     • Sandy Soil: Sandy soils are generally suitable for spread footings but may require additional stabilization measures, like compaction or geotextiles, in loose sand conditions.
     • Rocky Soil: Rocky soils provide excellent support for spread footings but may require specialized excavation techniques.
   • Soil Testing Procedures: Common soil testing methods such as Standard Penetration Test (SPT) and Cone Penetration Test (CPT) help determine the bearing capacity of the soil, which is crucial for designing the footing.
   • Adjustments Based on Soil: Based on the soil conditions, you may need to modify the footing design, such as increasing footing width or adding reinforcement for weak soils.
5. Determine Settlement and Concrete Strength: Next, we determine the total and differential settlement and the strength of concrete.
6. Select Steel Grade, Dimensions, and Thickness: Then, we select the steel grade, footing dimensions, and thickness.
7. Design Substructure and Superstructure Connection: After this process, we design the substructure and superstructure connection.
8. Check Uplift and Stability: Finally, we check the uplift and stability against sliding conditions.

Formula for Determining Area of Spread Footing:

Using the subsequent formula, the bottom area of the spread footing is calculated:

A = Qt/q

Where:

• The area of the spread footing (A) is determined by dividing the total load on the footing (Qt) by the allowable bearing capacity of the soil (q).

This formula helps ensure that the load is distributed over a large enough area to prevent soil failure.

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Cost Analysis of Spread Footing

1. Material Costs: Provide a breakdown of the costs associated with different materials used in spread footings, such as concrete, steel reinforcement, and formwork.
2. Labor Costs: Discuss how labor costs can vary depending on the complexity of the footing design and the site conditions (e.g., excavation in rocky terrain may increase labor costs).
3. Comparative Costs: Compare the costs of spread footings with alternative foundation methods like mat foundations or pile foundations. Mention factors such as the size of the building and the type of soil that can affect the overall cost.
4. Long-Term Costs: Discuss the potential for cost savings over the life of the building, such as lower maintenance costs if the spread footing is designed correctly.

Advantages of Spread Footing:

For providing the spread footing foundation, as a foundation within the building, there are the following advantages:

• For providing greater stability, the spread footing conveys and disseminates the load from the structure over a sizeable area of soil beneath it.
• Compared to other kinds of footing, spread footing has little or no risk of foundation failure.
• By controlling the damage within the structures, this type of footing increases the lifetime of the buildings.
• These footings provide continuous support to the structure.
• These footings are easy to implement within building structures and allow easy construction of basements.
• Compared to a load-bearing foundation, a spread footing has a wider bottom portion, which spreads the structure’s load over a larger area, providing greater stability.
• Concrete and reinforced steel are the primary materials used to construct this type of footing.
• In residential construction, these footings are commonly used.
• Reduces cracking caused by settlement.
• Stabilizes soil at the base of the structure.
• Cost savings in construction, design, and quality control.
• Differentiates in size and quantity.

Disadvantages of Spread Footing:

1. Not suitable for every soil type.
2. You have to fill all the lot in one go, but preparing such an amount of concrete on your own is quite problematic.
3. There is quite a decent amount of work with pouring the concrete, so you can’t do it without help.
4. Where not to use:
   • Add a section discussing the lessons learned from these projects, such as the importance of soil testing and the need for customization in footing design based on specific project requirements.

Where to Use Spread Footing :

• Example 1: A case study of a residential building constructed on weak soil where spread footings were used. The spread footings were designed to distribute the load evenly and prevent settlement. Details such as the soil type, challenges faced, and the adjustments made to the footing design are crucial.
• Example 2: A commercial building in an earthquake-prone area where spread footings were reinforced to withstand seismic forces. Modifications in design, such as additional steel reinforcement and increased footing depth, are highlighted.
• Lessons Learned: Discuss the importance of soil testing and the need for customization in footing design based on specific project requirements.

Where Not to Use  Spread Footing :

• Pile Foundations: Discuss when pile foundations are more appropriate than spread footings, particularly in areas with very soft or highly compressible soils.
• Caissons: Describe how caissons can be used in deep foundations where the soil near the surface is unsuitable for supporting a structure.
• Raft Foundations: Explain when raft foundations are preferable, such as when the loads from multiple columns need to be spread over a large area, or when the soil has low bearing capacity.
• Decision-Making Criteria: Provide a decision-making framework to help engineers choose between spread footings and alternative foundation methods based on site conditions, building loads, and cost considerations.

Long-Term Performance and Maintenance

1. Expected Lifespan: Provide information on the typical lifespan of spread footings, assuming proper design and construction practices are followed.
2. Common Maintenance Issues: Discuss common issues that may arise over time, such as settlement, cracking, or water infiltration, and how to address them.
3. Preventive Measures: Suggest preventive maintenance practices, such as regular inspections and monitoring for signs of distress in the footing or the surrounding soil.
4. Repair Techniques: Include a brief overview of repair techniques, like underpinning or adding additional reinforcement, to extend the lifespan of the footing.

Also Read: Clear Cover for Column

Shallow Spread Footings

If the depth of foundation, a factor in spread footing meaning, is less compared to the breadth of foundation then it’s referred to as Shallow or stepped Foundation.

Shallow foundation is basically a form of building a foundation that conveys building loads to the earth very close to the surface, in lieu of a subsurface layer or a range of depths as in a deep foundation.

It may be used where the bearing capacity of soil on which the structure is required to be constructed is maximum. Minimum depth of this Foundation is 800 mm and a maximum depth is no greater than 4 meters.

Spread Footing with Pier:

A pier foundation, an example of spread footing types, is an assembly of large diameter cylindrical columns to hold up the superstructure and convey sizeable super-imposed loads to the firm layer below. It stood several feet aloft the bottom. It is also referred to as “post foundation”.

The technology of Spread Footing with Pier

The most ordinary way is digging a square or rectangular borehole. Then formwork is made ready in the pit which settles the shape of the future foundation; then the reinforcement cage is settled, & concrete is slooshed. Then, the formwork is detached and canopied with a pillar.

Pier foundation is employed utilizing the below conditions:-

• When decomposed rocks are present in the top strata, & there are underlying strata of sound rock beneath them
• As stiff clays offer a plenty of resistance when driving a bearing pile
• If a structure required to be built on a slope
• The soil must have a little bearing capacity of water unless the pillars will sink under the ponderous load of the house

Spacing of Piers

Pier foundations are generally built 1-1.5 feet aloft the bottom to resist the moisture because the moisture impair the wooden structures.

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