Combined footing: Structural support for heavy loads

A footing or foundation is used to transmit a building’s weight onto the earth underneath. The soil type, the depth at which the carrying strata rest and the kind of superstructure all play a role in deciding what kind of footing is best. When there isn’t much room between the columns and the outer footing’s stability is at risk, using a combined footing can make sense. In the following paragraphs, we will go over all there is to know about combined footing, including its several types and all its characteristics.

 

What is combined footing?

When building a structure with many columns whose bases overlap, a technique known as “combined footing” is utilised.  The primary objective of employing combined footing is to ensure that the pressure exerted under the footing is distributed evenly. The combined footing evenly transfers the weight of the structure to the foundation underneath. 

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When is combined footing used?

The construction industry has a broad variety of uses for combined footings; however, these are some of the situations in which combination footings are often employed in the building industry.

• When a column is being built that is next to the property line but the footing is not being extended ahead of the property line.
• For columns to be able to share a footing, the width of only one side must be reduced, and this is when the combined footing is employed.
• When it is necessary to distribute the weight of the building uniformly to the ground below. 
• A combined footing is employed when the load-bearing capability of the soil is low and the area needed is more than that under the individual footing.

 

See also about: Design considerations

 

Types of combined footing

Rectangular combined footing

 

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These combined footings have rectangular foundation pads. They are formed in a rectangular manner and are able to sustain two or maybe more columns simultaneously. A rectangular combined footing is used wherever there is a need for a rectangular form and the ability to handle equal weights.

Trapezoidal combined footing

 

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A trapezoidal combined footing is the name given to the kind of footing that results when a combined footing is built in the form of a trapezoid. This type of combined footing is used where the load that is supported by one column is larger than that supported by the other column. The most important function of trapezoidal combined footing is to transmit the non-uniform weight of the structure to the surrounding soil.

Raft combined footing

 

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When the load-bearing capacity of the ground must be increased, a raft combined footing is employed. When the building’s weight is distributed evenly throughout the earth, raft combined footing is an excellent choice. By boosting the load-bearing capacity of the ground underneath the building, this combined footing mitigates the risk of the structure collapsing.

Strap combined footing 

A strap footing is a form of combined footing where a strap beam connects two or more column footings. By attaching the footing to one another with the use of a concrete beam known as a strap beam, the strap beam’s task here is to stop the footing from rolling over and causing damage to the structure.

 

Combined footing: Considerations to make

The following are some of the things that should be taken into account while building the combined footing:

• Area of the footing

The size of the footing is determined by the axial load of the column and the maximum bearing capacity of the soil. When two columns are joined together, more footing area may be created than is strictly required, relieving some of the stress on the foundation. Therefore, whenever the soil seems to have a limited load-carrying capacity and is more prone to collapse, more caution must be exercised when selecting a foundation location.

• Ground improvements

It is recommended in the geotechnical investigation that the soil be amended to increase its bearing capability. Under the watchful eye of a structural engineer, low-rise buildings might profit from ground stabilisation or renovation approaches. A building’s collapse, foundation failure, settlement, etc., are all possible outcomes of improperly treated weak ground. 

 

Combined footing: Design

The design of a combined footing that may be used for a variety of building applications is outlined in the following steps in detail.

• Find the point at which the loads from the columns are applied to the footing.
• It is important to balance the footing in such a way that the resulting load goes through the middle of the footing.
• Determine the size of the footing section in such a way that the maximum permitted soil tension is not exceeded.
• Perform the necessary calculations to determine the bending moments and shear forces at the salient spots, and then draw the SFD and BMD.
• Based on the amount of force exerted, you may determine how deep your footing should be.
• Perform the calculations necessary to get the transverse bending moment, then develop the transverse section to account for depth and reinforcement. Examine the anchoring and look for shear.
• Conduct a check for transverse shear on the footing, and as a result, establish the longitudinal steel
• Comprehend the idea of the reinforcements in order to accommodate the longitudinal moment, and then position them in suitable areas.
• Verify the length of the development for the longitudinal steel.
• Reduce the length of the longitudinal bars to save money.
• Render the reinforcement, paying close attention to detail.
• Prepare the timetable for the bar bending.

 

Combined footing: Advantages

The following are some of the benefits that come from using combined footing:

• One of the benefits of combined footing is that it may be used in places where there are requirements for the construction of two or perhaps more columns within a relatively compact space.
• Another benefit of combined footing is that it may be used in places where the load-bearing capabilities of the soil are limited. 
• Combined footings are used in situations when the soil capacity of the proposed site is inadequate. 
• When a column is being built up against a property line and the footing cannot be stretched out ahead of the property line, the combined footing is often used to aid in the construction of the column.
• The combined footing is extensively utilised because of its ability to distribute loads evenly across the pavement structure as well as aid in keeping the column’s centre of gravity where it should be.
• The combined footing is there to make sure the weight of the over- and under-structures is distributed fairly over the ground.
• Several columns share a single footing. This allows them to cut down on the total cost of building the project.
• Constructing on uneven ground is easier with the combined footing.

 

Combined footing: Disadvantages 

The following are some of the drawbacks of using combined footing:

• One of the most prevalent drawbacks of combined footing is that it may sometimes cause the groundwater level to increase.
• The temperature disparity may cause issues in the event that the thickness of the foundation raft is significant.
• Due to the use of a shared footing for several columns, the space devoted to the footing is minimised.
• When using combined footing, it might be challenging to keep the column’s centre of gravity in its proper places.
• One major drawback of combined footing is that the size of each component must be adjusted to suit the specific application. The ideal shape might be either a rectangle or a trapezoid, depending on the circumstances.

 

FAQs

In what situations is combined footing used?

When building a structure with many columns whose bases overlap, a technique known as the combined footing is utilised to ensure that there is equal pressure beneath the footing.

What is crucial to the combined footing of two columns?

When designing combined footings, the centre of gravity of both the column loads must essentially coincide with the centre of the area of the base. This is an important consideration in the design process. It is recommended that the base have a rectangular outline in the plan and be arranged symmetrically around the direction of loads.

What function does footing serve?

The building of the footings is an essential component of the foundation. They are normally fabricated from concrete that has been reinforced with rebar and poured into a trench that has been previously dug. Footings are there to provide support for the foundation and to stop the ground from sinking in.

Got any questions or point of view on our article? We would love to hear from you. Write to our Editor-in-Chief Jhumur Ghosh at jhumur.ghosh1@housing.com