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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 **square, **circular **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**

**Types of Spread Foundation**

Following are the** types of foundations** categorized under Spread Foundation,

### 1. **Wall Spread Footing**

**Wall Footing** is also known as **strip footing**. These types of wall footing strips of continuous concrete could spread the load of the structure to a **wide area of soil**. The is one shallow type of footing that can be constructed as plain cement concrete footing or reinforced concrete footing.

**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.

It is carefully designed considering soil bearing capacity and safe from any sliding and overturning effect. It also properly resist ground settlement under the earth.

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**

**Continuous footing **usually has more than **2 columns**, in which **loads of each column **are transferred to the **footing slab** directly, or through a **horizontal (longitudinal) **beam running **parallel to the footing**. In case when a **heavy load is spread **over a large area, a continuous** spread footing is used**.

**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.

**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 **

**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.

**Read More: Difference Between Footing and Foundation | What Is Footing Foundation **

## FAQs

### Spread Footing

A type of footing is used in which the width of the foundation is less compare to the width and it is spread over a large area of foundation to transfer the column and structure load to the soil below the foundation safely.

### What Is a 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.

### 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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