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.

Contents:

**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

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

**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.

**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.

**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**

is top reinforcement necessary for combined footing ??

the minimum depth required in all type of soil should not be less thsn 500mm.

this is so much helping.thanks for posting thins like this. awesome!

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

wow it is amaizing thnks

what is the minimum depth of foundation required ?

so much helpful ffor me

interesting design.

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 ?

very usefull information

great design

yaaa ofcorse

..

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

Unga kadamai unarchiku oru azhave illiya…..

useful information

thanks very use ful 4 me

thanks (Y)

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

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

haw to do concentrated moment load from column of combined footing.

kofreh tikelew

useful information.

Really useful for me as an engineer student!

thanks! those info's are very helpful!

nice info

hehey..

riz, age ko ug investigation..galisod ko taman

I am civil engineer and would love to make you as my friend.

good information thank you!

thank you (Y).

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

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

thank you

how to design fast by hand write?

I wud lyk t knw d design steps wit formulae.

I WANT TO HOW TO DESIGN THE PORTAL FOUNDATION IN DETAILS.

I want know designer combined footing in Acecoms Gear2003?

how to design isolated footing when not given column detail?

how can i download this article

## Gopal Mishra

go to the link at the bottom of the page, and from there, slideshow will open. You can Actions option on the left bottom of the slideshow, from there you can download it as pdf or ppt.

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.

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

how can design combined footing

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

Thank You

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,

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

how do design strip footing.

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