Combined Footing Design, Types, Dimensions, Adv. & Disadvantages

What is a Combined Footing?

A combined footing is a concrete footing that supports two columns or load-bearing walls. It is usually rectangular in shape and is placed directly under the columns or walls it is supporting. The footing is usually wider than the column or wall it is supporting, and is typically made of concrete. Combined footings are usually used when the columns or walls are close together and when the loads on each are similar.

Combined Footing
Combined Footing

When two or more columns are near to one other and their foundations overlap, a combined footing is built.

Combined footing design

After careful study of the site conditions and the thickness of the ground cover, the architect or engineer formulates the basic plans of combined footing based on different types of footing.

There are many factors to consider when designing a combined footing, such as the soil conditions, the loads on the footing, and the size and shape of the footing. The soil conditions will affect the size and shape of the footing, as well as the type of foundation that is used. The loads on the footing will also affect the size and shape of the footing. The size and shape of the footing will also affect the type of foundation that is used.

Also read: Pile Foundations: Types of pile foundation, Design and Details

To design a combined footing, the first step is to determine the size and location of the footing. The footing must be large enough to support the load of the columns and must be located so that the load is evenly distributed. The next step is to determine the type of material to use for the footing. The most common type of material is concrete, but other materials such as stone or brick can also be used. The last step is to calculate the amount of material needed for the footing. This can be done by using the formulas for the area and volume of the footing.

Steps to Design a Combined Footing

There are many different ways to design a combined footing, and the most appropriate method will vary depending on the specific circumstances of the project. In general, however, the following steps are involved in the design process:

  1. Determine the loads that will be applied to the footing. This includes both dead loads and live loads, as well as any other external forces that must be considered.
  2. Calculate the soil bearing capacity at the proposed footing location. This will give you an idea of how much weight the footing can support without failing.
  3. Select an appropriate footing type. There are several types of combined footings, and the most appropriate one will depend on the loads and soil conditions.
  4. Design the footing dimensions. The size and shape of the footing must be carefully designed to support the loads while remaining within the soil bearing capacity.
  5. Prepare construction drawings and specifications. Once the footing has been designed, construction drawings and specifications must be prepared so that the footing can be built according to the plan.

Locate the point of application of the column loads on the footing

Width: The width of the footing should not be less than the gap between the columns, any narrower and the strength of the footing will be compromised.

Depth: The depth of footing should not be less than the gap between the columns.

A combined footing is to be used when:

  • This sort of footing is used to assist create a column when it closes the property line and the footing cannot be put ahead of the property line.
  • If the soil’s load-bearing capability is poor and the space required is greater beneath each individual footing.
  • When two or more columns must be constructed in a limited space, we may utilize combined footing.
  • To connect a perimeter or grade beam, the combined footing is necessary.
  • When an existing structure or sewer is near a building column, this method of footing is used.
  • The combination footing is utilised when an equal load distribution from the structure to the subsoil is required.
  • It draws the SFD and BMD after applying the shear force and bending moment calculated at the salient point.
  • It’s employed to make a construction that’s more cost-effective. Because the same footing supports two or more columns. As a consequence, the building expenses of the structure are reduced.

Advantages of a Combined Footing

  • The benefit of combined footing is that it may be used when two or more columns are required in a small area.
  • Another advantage of combined footing is that it may be used in areas where the soil’s load-bearing capacity is poor.
  • We built combined footings where the soil capacity of the construction area was uneven and we needed to transmit the weight uniformly to the sub-soil.
  • When a column is built to close the property line and the footing is not extended across the front of the property line, combined footing is used to help build the column.
  • The combination footing is frequently used to achieve homogeneous pressure distribution under the footing.
  • Assist in maintaining the column’s centre of gravity.
  • This type of footing is more advantageous than a two separate footing.
  • The combined footing equally distributes weight between the superstructure and the subsoil.
  • In tough building areas, the combination footing might be employed.
  • Because a common footing supports two or more columns. As a consequence, the structure’s construction expenses are lower.

Disadvantages of a Combined Footing

Sometimes the uneven pressure from the different loads in a combined footing may cause cracks or broken tiles in walls, especially if they are more than 20 inches in height.

As this is the basic column structure type, it is suitable only in vertical construction. Because the combined footing is wide and deep, the column needs to be placed deep into the earth. This type of construction works well for floors, but not for roofs.

Also read: Isolated footing advantages and disadvantages- Types, Benefits & How to use

  • The downside of combined footing is that the ground-water level might occasionally rise.
  • As a result, the earth loses touch with the foundation.
  • The heat difference may be a problem if the foundation raft thickness is too substantial.
  • Because several columns share a single footing, the footing area is decreased.
  • It’s tough to keep the columns’ centre of gravity on combined footing.
  • The downside of combined footing is that its size must be adjusted according to the situation. It should be rectangular or trapezoidal depending on the situation.

Basic Principles of Combined Footings

There are three basic principles of combined footings:

  1. The first principle is that the footing must be large enough to support the loads that will be placed on it.
  2. The second principle is that the footing must be placed at the proper depth.
  3. The third principle is that the footing must be properly reinforced.

Other important considerations are:

  1. The footings must be designed to support the loads from both the structure and the soil.
  2. The footings must be designed to distribute the loads evenly between the two footing elements.

Types of Combined footing

Slab Type Combined Footing

Slab type combined footing supports two or more columns with bottom slab only.

Rectangular Combined Footing

Rectangular Combined Footing
Rectangular Combined Footing

When one of the footing’s projections is constrained or the width of the footing is restricted, the rectangular footing is used.

The footing is a vertically loaded beam that spans between columns and cantilevers beyond. Statics are used to create longitudinal shear force and bending moment diagrams. The faces of the column are examined for moment. The shear force is critical at a distance ‘d’ from the faces of columns or at the point of contra flexure. Two-way shear is measured under the heavier column.

In rectangular combined footing, each load is given an equal footing. This is also known as standard combined footing. The ground slabs carry weight evenly on each side of each column. It is a desirable footing for running water as well as soft soil. A rectangular combined footing supports two-column carrying an equal load.

Trapezoidal Combined Footing

Trapezoidal Combined Footing
Trapezoidal Combined Footing

When the load of one column is substantially greater than the load of another column, the trapezoidal footing is used. If one of the column is close to the boundary line, a trapezoidal column bearing the greater load becomes necessary to align the C.G. of the footing line with the C.G. of the overall loads of the column.

Strap Footing

The strap footing is required when the exterior column is extremely close to the property line. The strap footing is a combination of spread footing and two columns joined by a strap beam.

Strap Combined Footing
Strap Combined Footing

The load is not transferred by the connecting beam between two spread footings. The strap footing transfers weight from the highly loaded outer column to the inner column.

The strap combined footing is a form of combined footing in which two or more column footings are joined by a strap beam (concrete beam).

The strap beam’s objective is to keep the footings from overturning by connecting them with a concrete beam called a strap beam.

Slab-Beam Type Combined Footing

Two or more columns are supported by a slab-beam type combination footing with a bottom slab and beam.

Strap-Beam Type Combined Footing

Strap-beam type combined footing supports two or more columns with a bottom slab and beam.

Strap-Footings
Strap footings

A strap-beam type combination footing is used when one column is positioned on a property line, resulting in an eccentric load on a section of the footing. Install a beam in the footing that links to the neighbouring column footing to avoid the overturning effect. The rectangular footing is adopted when one of the footing’s projections is constricted or the width of the footing is limited.

Raft Combined Footing

raft foundation
Raft foundation
  • When the soil conditions under the foundation are insufficient to support the weight of the superstructure, raft footing is used.
  • When the building’s weight falls uniformly on the earth, a raft footing is highly effective.
  • By enhancing the bearing force of the ground, the raft combination footing protects the structure from collapsing.
  • The raft is built to ensure that the soil’s permissible bearing force is not exceeded.
  • It has a great hold on the soil structure and prevents the building from overturning. 
  • When the strip footing or spread footing is insufficient to support the weight from the superstructure, the raft footing is built.
  • A raft footing is built to transmit a balanced load from the superstructure to the subsoil.
  • The raft footing protects the building from suddenly settling.
  • A square raft footing and a rectangle raft footing are the two forms of raft footing.
  • Read more about the Raft foundation.

Conclusion

A combined footing is a type of foundation that is used when two or more columns are close together. This type of footing is used to distribute the load of the columns evenly across the entire footing. The combined footing is also used to provide stability to the columns. The dimensions of the combined footing are determined by the size and weight of the columns that it is supporting.

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