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Combined footings are constructed for two or more columns when they are close to each other and their foundations overlap. Design of combined footings with example is discussed.

The function of a footing or a foundation is to transmit the load form 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.

Combined Footings

Whenever two or more columns in a straight line are carried on a single spread footing, it is called a combined footing. Isolated footings for each column are generally the economical.

Combined footings are provided only when it is absolutely necessary, as

  1. When two columns are close together, causing overlap of adjacent isolated footings
  2. Where soil bearing capacity is low, causing overlap of adjacent isolated footings
  3. Proximity of building line or existing building or sewer, adjacent to a building column.

Combined Footings Design with Loads

Types of Combined Footing

Types of Combined Footing

  • The combined footing may be rectangular, trapezoidal or Tee-shaped in plan.
  • The geometric proportions and shape are so fixed that the centeroid of the footing area coincides with the resultant of the column loads. This results in uniform pressure below the entire area of footing.
  • Trapezoidal footing is provided when one column load is much more than the other. As a result, the both projections of footing beyond the faces of the columns will be restricted.
  • Rectangular footing is provided when one of the projections of the footing is restricted or the width of the footing is restricted.

Types of Combined Footing

Rectangular combined footing

  • Longitudinally, the footing acts as an upward loaded beam spanning between columns and cantilevering beyond. Using statics, the shear force and bending moment diagrams in the longitudinal direction are drawn. Moment is checked at the faces of the column. Shear force is critical at distance ‘d’ from the faces of columns or at the point of contra flexure. Two-way shear is checked under the heavier column.
  • The footing is also subjected to transverse bending and this bending is spread over a transverse strip near the column.

Rectangular combined footing

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Critical Sections for Combined Footing

Steps for Design of Combined Footing

  • Locate the point of application of the column loads on the footing.
  • Proportion the footing such that the resultant of loads passes through the center of footing.
  • Compute the area of footing such that the allowable soil pressure is not exceeded.
  • Calculate the shear forces and bending moments at the salient points and hence draw SFD and BMD.
  • Fix the depth of footing from the maximum bending moment.
  • Calculate the transverse bending moment and design the transverse section for depth and reinforcement. Check for anchorage and shear.
  • Check the footing for longitudinal shear and hence design the longitudinal steel
  • Design the reinforcement for the longitudinal moment and place them in the appropriate positions.
  • Check the development length for longitudinal steel
  • Curtail the longitudinal bars for economy
  • Draw and detail the reinforcement
  • Prepare the bar bending schedule

Detailing of Combined Footing

Detailing of steel (both longitudinal and transverse) in a combined footing is similar to that of conventional beam-SP-34. Detailing requirements of beams and slabs should be followed as appropriate-SP-34

See the Complete Design with an Example

Read More:

What is Punching Shear? Punching Shear in Slabs and Foundations

Isolated Footing Design Example and Excel Sheet

Gopal Mishra

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50 Comments

  1. thanks , can you give us the soil bearing capacity of types of soil . and required depth of foundation from the natural ground of 3 floor building

  2. I have to design 3 columns as a combined footing in which 2 columns on property lines and 1 column at centre … how to design this type of strap footing ?

  3. I want to know the modification that Br inch Hansen has brought to the analysis of Terzaghi's general shear failure of bearing capacity?

  4. Sir following this which is very valuable and refreshing course just like getting all the things for my professional growths

  5. Am a gud designer in Changing ladies curve.. bt 9 months in result,. lol

  6. how these affect the bearing capacity of the subsoil;nature of the soil, type of soil, load of building and theunderground streams.

  7. Is there any example of trapazoidal combine footing and strap footing? which solved by IS 456…

  8. I really appreciate the work done by you sir, being your old student of SJCE i have great respect to your and mode of teaching.

  9. It is very nice example. but for the trapezoidal footing the calculations are really complicated

  10. I appreciate the examples you forward about combined footing. I am 4th year civil engineering student at Wollo university.
    Thank You

  11. The examples you give for combined footing is very interesting. So I want to appreciate your presentation.
    I am 4th year civil engineering student at Wollo university.
    Thank you,

  12. How can i design strip and grid footing can u give us an example ,ur reply is highly appreciated thank you

  13. Dear Gopal,
    I would request to developm Excel sheets for variuos civil engineering project estimation or calculation bcs Excel is very easy and every person know it. I will also share and contribute.

    Thanks

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