Combined Footing:
If two or more columns are supported in a row and a common foundation is provided them then the footing is called combined footing. If the individual footing has to accommodate two or in exceptional cases more than two columns are known as combined footing.
Usually, the combined footing is made also of reinforcement concrete. In the combined footing the footing assumes to rigid and resing on homogeneous soil. When the load-bearing capacity of the soil is low and when under the individual footing required more area then combined footing is construed.
The combined footing should be rectangular combined footing, trapezoidal combined footing, strap combined footing, and raft combined footing. In this combined footing the maximum bending moment takes as the design value for the reinforced concrete footing.
The development length check for the longitudinal steel. The longitudinal bars curtail the economy. To get uniform pressure distribution under the footing the combined footing is widely used.
What Is Combined Footing?
When two or more columns are located near to each other then a common footing is provided for all the columns is known as combined footing. If the individual footing has to accommodate two or in exceptional cases more than two columns are known as combined footing.
If two or more columns are supported in a row and a common foundation is provided them then the footing is called combined footing. To get uniform pressure distribution under the footing the combined footing is widely used. Generally, the combined footing is made also of reinforcement concrete.
If two or more columns lie in a straight line and are carried by on a single spread footing, then it is called a combined footing. Isolated footings for each column are generally economical.
The combined footing is provided to equally distribute the load to the super-structure to the sub-soil. The selection of the ideal type of footing depends on the depth at which point the bearing strata lies, the soil condition, and the type of superstructure.
Advantage of Combined Footing
The advantage of Combined Footing is as follows.
- The advantage of combined is if there is required two or more columns construed in a small area then we can use combined footing.
- Another advantage of combined footing is where the load-bearing capacity of the soil is low there we construed combined footing.
- Where the soil capacity of the Construction area is unequal to distribute the load evenly to the sub-soil we construed combined footing. This is another advantage of combined footing.
- The common advantage of combined footing is when a column closes the property line and footing is not spread the front of the property line then combined footing help to construct the column.
- To get uniform pressure distribution under the footing the combined footing is widely used.
- And help to maintain the center of gravity of the column.
- This type of combined footing is more profitable than another footing.
- The combined footing is provided to equally distribute the load to the super-structure to the sub-soil. This is another advantage of combined footing.
- Because there is a common footing used for two or more columns. So the construction cost of the structure is reduced.
- The combined footing can be used in the difficult construction sides.
Disadvantage of Combined Footing
The disadvantage of Combined Footing is as follows.
- The common disadvantage of combined footing is, sometimes the ground-water level is rise.
- Which causes the loss of contact of soil with the foundation.
- If the foundation raft thickness is high, the problem may arise due to the heat difference.
- The footing area is reduced because a common footing is used for many columns.
- In combined footing, it is difficult to maintain the center of gravity of the columns.
- The general disadvantage of combined footing is that its size should be varied to the place. Sometimes it’s should be made rectangular or sometimes trapezoidal.
Application of Combined Footing
The Application of Combined Footing is as follows.
- There have been many practical applications of combined footing in our construction field.
- A combined footing is considered when a column closes the property line and footing is not spread the front of the property line then combined footing help to construct the column.
- The Dimensions of one side of the footing are determined to some lower value so that column footings may be combined.
- If the load-bearing capacity of the soil is low, and the area requirement is more under the individual footing.
- In combined footing, if there are required two or more columns construed in a small area then we can use combined footing.
- The combined footing is required to intersect a perimeter or grade beam.
- This type of footing is applied when the proximity of building a line or existing building or sewer, adjacent to a building column.
- The combined footing is applied when an equal load distribution is required from the structure to the sub-soil.
- It is applied to the shear force and bending moment calculated at the salient point and then draws the SFD and BMD.
- It is applied to make a more economical structure Because there is a common footing used for two or more columns. So the construction cost of the structure is reduced.
Types of Combined Footing:
There are four types of combined footing, that are-
- Rectangular combined footing.
- Trapezoidal combined footing.
- Strap combined footing.
- Raft combined footing.
1. Rectangular Combined Footing
Nowadays, the rectangular combined footing is mostly used in construction. The combined footing is usually a rectangular shape so it is called rectangular combined footing. This type of combined footing usually supporting two or more columns. The combined footings are used when there is a need for an equal distribution of load to the sub-soil.
2. Trapezoidal Combined Footing
This type of combined footing is used when we have to transfer an unequal load from the super-structure to the sub-soil. A trapezoidal combined footing is considered when a column closes the property line and footing is not spread the front of the property line then combined footing help to construct the column.
They can not make an equal rectangular footing because there is come an unequal load from the structure. This type of footing does not have any particular shape and size.
3. Strap Combined Footing
When the external column is very near to the property line then the strap footing is used. Two columns connected by a strap beam and the strap footing is a combination of spread footing. The connecting beam between two spread footing does not transfer any load. The strap footing is used to transfer load from the heavily loaded column to the inner column.
4. Raft Combined Footing
When the soil conditions under the foundation have not adequate strength to bear the load of the superstructure then the raft footing is provided. It provides a perfect grip on the structure of the soil and prevents the tendency of overturning the building. It is provided when a large number of the floor are exited.
The raft footing prevents the sudden settlement of the structure. There are two types of raft footing – one is a square shape and another is a rectangular shape. The square mat footing is constructed to provide stability to the building. When a column is too close to the building or structure where raft footing is required.
When two columns are so close to each other or they overlap each other where raft footing is necessary to provide. The raft footing is constructed where there the strip footing or spread footing is not enough to bear the load that’s coming from the superstructure. To distribute a uniform load from the superstructure to the subsoil raft footing is constructed.
When the Combined Footing Is Used?
A combined footing is used when a column closes the property line and footing is not spread to the front of the property line then combined footing help to construct the column. The combined footing is used when the dimensions of one side of the footing are determined to some lower value so that column footings may be combined.
The combined footing is used when an equal load distribution is required from the structure to the sub-soil. It is used to make a more economical structure Because there is a common footing used for two or more columns. So the construction cost of the structure is reduced.
It is used to the shear force and bending moment calculated at the salient point and then draws the SFD AND BMD. If the load-bearing capacity of the soil is low, and the area requirement is more under the individual footing then combined footing is used.
Combined Footing Design
- When two or more columns are supported in a row and a common foundation is called combined footing.
- The point of application locates the column loads. Such that the pressure of allowable soil does not exceed when computing the area of the footing.
- The footing such that proportion the resultant the loads pass through the center of the footing.
- The shear force and bending moment calculated at the salient point and then draw the SFD and BMD.
- To design the longitudinal steel check the longitudinal shear.
- The development length check for the longitudinal steel. The longitudinal bars curtail the economy.
- When draw and detailed the reinforcement and prepare the bar bending schedule.
- For the longitudinal movement, the reinforcement is placed in the appropriate positions.
- In the combined footing the footing assumes to rigid and resing on homogeneous soil.
- In this combined footing the maximum bending moment takes as the design value for the reinforced concrete footing.
Frequently Asked Questions (Faqs) That Could Be Included in Your Article on Combined Footings:
What Is a Combined Footing?
A combined footing is a type of foundation that supports two or more columns with a single footing, designed to distribute the load evenly to the subsoil.
When Is a Combined Footing Used?
Combined footings are used when columns are close together or near property lines, where individual footings would be impractical. They are also used to ensure uniform load distribution and reduce construction costs.
What Are the Types of Combined Footings?
Combined footings include rectangular, trapezoidal, strap, and raft footings. Each type is chosen based on specific structural and soil conditions.
What Are the Advantages of Combined Footings?
Combined footings help in reducing the overall construction cost, especially when multiple columns are close together. They also provide stability and ensure even load distribution to the subsoil.
What Are the Disadvantages of Combined Footings?
Challenges include potential issues with groundwater levels affecting soil contact, difficulties in maintaining the center of gravity for columns, and variability in footing size and shape.
How Are Combined Footings Designed?
Design considerations include calculating loads, determining the appropriate size and shape of the footing, and detailing reinforcement to handle bending moments and shear forces effectively.
Where Are Combined Footings Commonly Applied?
They are applied in scenarios where space constraints or soil conditions require distributing loads from multiple columns efficiently. Common applications include urban construction and areas with varying soil capacities.
What Are the Key Differences Between Types of Combined Footings?
Rectangular footings are straightforward and common, while trapezoidal footings accommodate uneven loads. Strap footings connect spread footings to distribute load, and raft footings are used when soil conditions demand a larger foundation area.
How Does Soil Condition Affect the Choice of Combined Footing?
Soil bearing capacity, depth to bearing strata, and soil type influence the type and design of combined footings. Engineers consider these factors to ensure stability and longevity of the structure.
What Are the Steps in Designing and Constructing a Combined Footing?
The process includes site analysis, load calculations, footing design (size and reinforcement), construction planning, and ensuring compliance with building codes and regulations.
