Combined Footing – Basic to Advanced Guide 2020 Updated

Combined Footing – As it is clear from the name combined footings it is the combination of two footings.  First of all, what is footing? and what is its function?

The function of a footing or a foundation is to transmit the load from the structure to the underlying soil. The choice of suitable type of footing depends on the depth at which the bearing strata lies, the soil condition and the type of superstructure.

A combined footing is one which supports two or more columns at the founding level. This situation arises because of

i) column spacing is close and the safe bearing capacity of the founding stratum is low. Hence individual footings will overlap.

ii) column spacing is wide but loads of them are so large that the footings if designed individually, will overlap.

In this article, we will discuss the Advantages and disadvantages of combine eccentric footing and How to design a combined footing with using axial load and biaxial moment? We will also Learn Under what circumstances is a trapezoidal preferred to a rectangular shape two column combined footing?

Why combined footings are used?

A combined footing is used based on three circumstances. It is used when the two-column are so close to each other that their individual footings would overlap. A combined footing is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when kept entirely within the property line. By combining it with that of an interior column, the load is evenly distributed. A combined footing may be rectangular or trapezoidal in plan. The trapezoidal footing is provided when the load on one of the column is larger than the other column. They are

(i) When the columns are very near to each other so that their footings overlap.

(ii) When the bearing capacity of the soil is less, requiring more area under the individual footing.

(iii) When the end column is near a property line so that its footing cannot spread in that direction.

Combined columns footing are the footing which connects two columns and if we connect more than two columns it’s called continuous footing.

Types of Combined Footings

  1. #1. Slab Type Combined Footing
  2. #2. Slab-Beam Type Combined Footing
  3. #3. Strap-Beam Type Combined Footing

#1. Slab Type Combined Footing

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

#2. Slab-Beam Type Combined Footing

Slab-beam type combined footing supports two or more column with bottom slab and beam.

#3. Strap-Beam Type Combined Footing

Strap-beam type combined footing is used when one column is located on a property line, resulting in is an eccentric load on a portion of the footing. Therefore provide a beam in the footing to the adjacent column footing to restrain the overturning effect.

  • Rectangular Combined Footing
  • Trapezoidal Combined Footing

This type of footing is provided under the following situations:

1. When the columns are located extremely close to each other and their individual footings are overlapping.

2. In case of soil having the low bearing capacity and a large area is required under the individual footing.

3. When the column end is situated near the property line and it is not possible to extend the footing area on the side of the property line.

The main purpose of using combined footing is to distribute uniform pressure under the footing. To accomplish this target, the centre of gravity of the footing area should be equal to the centre of gravity of the two columns.

If the outside columns close to the property line, conveys heavier load, it is necessary to provide trapezoidal footing to maintain the centre of gravity of the footing in line with the centre of gravity of the two-column loads. Otherwise only a rectangular footing may be provided. Following extra points should be kept in view to provide combined footings.

1. This type of footing is considered as an inverted floor, where footing load is carried by the columns and loaded from underneath by uniform earth reaction.

2. The area enclosed by combine footing should be equivalent to or more than that acquired by dividing the total combined load into the columns by safe bearing capacity of the soil.

3. The shape of the combined footings should be selected in such a manner that it concurs the center of gravity with the center of gravity of soil reaction.

Check this Post for Design of RCC combined footing with axial load and uniaxial moment as per Indian code.

  • What are the types of combined footing?
  • What is the difference between combined footing and strap footing?
  • Why combined footings are used?
  • What is regular footing?
  • What are the advantages of combined footing?
  • Why do we provide combine footing?
  • What’s meant by combined footing?
  • What is the difference between combined footing and continuous footing?
  • How do I detail combined footing?
  • Where do we use combined footing and isolated footing?
  • Why isolated footing is necessary?
  • Why do we use square footing?
  • What are the practical application of combined footing?
  • What is a wall footing?
  • In what circumstances the combined footing is preferred?
About the Author
Er. Mukesh Kumar
Er. Mukesh Kumar is Editor in Chief and Co-Fonder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.