Column load on existing slab near sawcut

[BadgerPE]

Good Friday afternoon to all!

Does anyone know of a procedure to evaluate the capacity of a slab on grade supporting a column load near a sawcut joint? I have an existing project where an interior (gravity only) column is required and I am trying to establish what the capacity of the slab (#4 bars @ 24" O.C.) is to determine if an independent footing is required. I have evaluated the slab for punching and it is acceptable but as far as subgrade checks go, the only method I have seen is from the Designing Floor Slabs on Grade book by Ringo and Anderson and that method requires the column located 1.5*radius of relative stiffness which this column obviously is not.

My gut instinct is to just require the contractor to put in the independent footing. However, there has been a little animosity on the project and I would like to help the contractor and owner alike in both time and materials. So I would appreciate any help that any can offer (if anyone is still working on "this, the day that the weekend begins")!

 

[TXStructural]

I think you will be loading a "footing" that is too thin, too close to the edge. If you really want to know if it will work, run the numbers, placing the column at the end of a slender footing. With the cut allowing a sharp bend to occur, the slab will rotate under the column, resulting in bending in the column. Even without the sawcut, the problem we see frequently with heavy point loads on SOG is the eventual depressing of the slab locally, and corresponding cracking. Additionally, s SOG is not structurally reinforced, and it will not act as a two-way beam, which is the function of a typical footing. SOG reinforcing is for T&S and to maintain continuity only, not to provide significant flexural strength.

Running a quick check for your example, if you use a symmetrical square footing on an allowable soil bearing of 3 ksf, the 47 k loading requires a footing approximately 4'-6" square, 9 inches thick.

I would not place this load on the SOG. And please do not let "a little animosity on the project" cloud your engineering judgement. There will always be a sad story behind every unreasonable request made by a contractor or owner. And a similar story behind every engineering failure.

 

[hokie66]

This is a no-brainer, and the contractor and owner probably know it. In no circumstances should a column with that much load be supported on a thin slab on grade. These questions are best answered quickly and authoritatively. No fancy calculations necessary.

 

[BadgerPE]

Impressive response TX! A lot of great info in that post! Just for the record I wasn't letting the animosity cloud my judgement, but rather I was just making sure I wasn't overlooking something.

 

[JAE]

Doesn't ACI require a minimum 8" for footing thickness? Or is it 6"? I can't recall. (see chapter 15).

 

[ajk1]

use a separate footing.