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Bearing Wall on Slab
5

Bearing Wall on Slab

Bearing Wall on Slab

(OP)
A restaurant owner has asked me whether he can put a cold storage area on an existing floor in a 20 year old building, The floor is over a basement, and consists of 3 x 10 at 12 in on center, 14 feet long. The floor would overdeflect under the 150 PSF load, so a wall below is needed.

The builder suggested a 2x6 stud wall without an additional footing. The line load fron the midspan reaction of the floor joist is about 1300 PLF. At the bottom of the slab (which I assume to be 4 inches thick)the area bearing on soil is 8+5.5=13.5 wide. The soil capacity is at least 3000 PSF and the resulting load from the floor on the wall is
1300/(13.5/12)or 1155 PSF.

It looks to me like there should be no problem, but I'm always hesitant to do anything without a footing. Any thoughts or comments would be helpful

RE: Bearing Wall on Slab

Trussdoc:

ACI prohibits "footings" of less than 6 inches (ACI 318-02, Section 15.7) so your 4" floor slab technically cannot serve as a foundation for a floor.

Did you check punching shear through the floor from the wall?  

Do you really know that the floor is 4" thick.  Sometimes slabs on grade are placed on uneven subgrades that result in a variety of thicknesses.

Why not sawcut the slab 18" wide and dig out a thickened slab footing, perhaps 8" thick.

RE: Bearing Wall on Slab

2
Check punching shear through the floor.  Should be OK.  You'd be surprised what a 4" slab gets you.
If you need a footing by code, perhaps a course of solid CMU on top of the slab, then your 2x6 stud wall on top of that.  CMU can be your footing, plus it keeps the wood out of water if the basement has water problems.

RE: Bearing Wall on Slab

2
"Designing Floor Slabs on Grade" by Ringo and Anderson covers this exact topic very nicely.  Assuming there are no control joints nearby, a 4" slab is good for 936 plf of wall load (assuming f'c = 3000 psi and k = 100 pci).  This value is from Table 11, page 54, and it includes a factor of safety = 2.5.  I have reduced the factor of safety on occasion to verify an existing condition, but you have to decide what risk you are willing to take.

DaveAtkins

RE: Bearing Wall on Slab

Dave, do they use the slab as a foundation for true building floor loads or for simple wall weight?

Trussdoc's original question was based on taking actual floor loads into the slab, which technically violates ACI 318.

RE: Bearing Wall on Slab

JAE-
I think you are correct.  ACI 318 specifically excludes slabs on grade (i.e., the Code does not govern them) "UNLESS THEY TRANSMIT VERTICAL LOADS OR LATERAL FORCES FROM OTHER PARTS OF THE STRUCTURE TO THE SOIL."  The textbook I refer to allows slabs to support walls, rack storage posts, etc.  However, I have allowed for THICKENED slabs on grade to support structural loads (typically at interior bearing walls).

DaveAtkins

RE: Bearing Wall on Slab

(OP)
Thanks to all for the information. I am going to have the contractor drill a hole to confirm the slab thickness. If its not six inches, we add a haunch or some blocks.

RE: Bearing Wall on Slab

Hi JAE,
I have a very similar situation as Trussdoc.  Could you post section 318-02 section 15.7?  I have the 99 edition and for plain concrete footing, section 22.7.4 states 8 inches or less.  I am curious because the BOCA code and CABO both allow a 6" footing.  I am wondering if the 2002 has altered either section 15.7 or 22.7.4.

Thank you,
Mark    

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