Interior footing size for column

[jimtheengineer10]

A modular home was installed on an existing concrete foundation wall and there are 12 steel columns installed down the center of the home. The columns are sitting on the existing concrete slab. There is a column with a 17,919 pound load on it. The code official called me and asked if the columns need to have a typical 24"x24"x12" interior footing under the columns. The modular company installed a 1/4" thick 6"x12" steel plate between the column and the slab. Is this sufficient or do they need to install an interior footing under the column? If so can it go on top of the slab or does the slab have to be cut and the column installed below the slab?

I know you take pounds divided by the soil pressure typically to determine the footing size but in this case the column load would be applied to the top of the concrete slab so the weight on the soil would be spread out through the slab. Can you take the load divided by 72" (6"x12" steel plate) and determine the psi load on the slab and if the load is less than the slab's psi no footing required?

 

[DaveAtkins]

The plate bearing on the slab is not a problem (249 psi), but the slab could be a problem. The load cannot spread throughout the entire slab area, because the slab would fail in upward bending between the columns.

You need to check bending in the slab, and punching shear through the slab, for the ultimate column load. If it does not work, you could pour a footing on top of the slab.

DaveAtkins

 

[JAE]

If you don't have a geotechnical report the code suggests 1,500 psf for soil bearing.
That means you'd need a 3'-6" square footing.

The slab-soil support combination capacity depends on the stiffness of the soil and the slab as well as the slab concrete strength.

With that much load on a 6x12 plate I'd think you'd have punching shear problems per DaveAtkins comments.

Is this sufficient or do they need to install an interior footing under the column?

If you don't know how to check this you should hire someone who can or who can teach you. This is basic foundation engineering.

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[jimtheengineer10]

Thank you.