Post bearing on concrete floor slab
Post bearing on concrete floor slab
(OP)
Hi - I'm working on a retrofit. The owner is knocking out a bearing wall and replacing it with a beam and posts to support the overhead floor joists.
The floor below is a concrete slab, 4" to 6" thick. The load to each post is about 2400#. If the slab is 6" thick, I can show it good to support the load, and I was thinking of just attaching the post to the slab with a Simpson post base. Would that be an acceptable solution? Or should I have the slab cut up at that location and have a footing poured for the column? If it's 4" I can't show it good and would have a footing poured in that case.
Thanks for your input!
The floor below is a concrete slab, 4" to 6" thick. The load to each post is about 2400#. If the slab is 6" thick, I can show it good to support the load, and I was thinking of just attaching the post to the slab with a Simpson post base. Would that be an acceptable solution? Or should I have the slab cut up at that location and have a footing poured for the column? If it's 4" I can't show it good and would have a footing poured in that case.
Thanks for your input!






RE: Post bearing on concrete floor slab
According to an old CRSI table I have, for 6" slabs, for one layer of 6X6 6/6 or 6X6 4/4, the slab is good for 4 to 500 psf, and with two layers of the same, from 6 to 800 psf, but I have found that these are conservative values.
If the slab can take it, a Simpson post base would be fine if there is no uplift, and no forklifts.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Post bearing on concrete floor slab
RE: Post bearing on concrete floor slab
The slab might be 6" - I'm having someone drill some holes to check that. The building is actually an old retail store building, so the slab could be thick enough. I don't think it's reinforced though. But I did some calculations assuming a plain 6" "footing," (2000 psi concrete) and I can show it good for that.
My spreadsheet is attached. In my next post I will attach a PDF file from an article I found on the web about strength design of plain concrete footings - that's basically the methodology I used to create my spreadsheet.
RE: Post bearing on concrete floor slab
RE: Post bearing on concrete floor slab
Personally, I almost always make them place a new footing, even if it is only 2400lbs and you can calc it out. Concrete slab-on-grade behavior is a highly unpredictable thing, and if it cracks down the road (no matter what from), rest assured you are getting a phone call! I hate giving my guarantee to existing structures unless I know just about everything about them.
Andrew Kester, PE
Florida
RE: Post bearing on concrete floor slab
I did have a couple of test holes drilled and it looks like what we're actually working with is two slabs... an original that was approximately 6" thick, and another that was poured on top of that one which is about 4" thick. So I basically have a 10" slab that was poured in two lifts (it's an older building, so the second pour was done a number of years after the first).
My gut feeling is that it doesn't make a lot of sense to cut out 10" of concrete to pour a new footing, when what is existing will probably function just as well.
Any thoughts?
RE: Post bearing on concrete floor slab
Explain to the client your ideas and feelings - in writing - so in case something should ever happen - they will know how to remedy.
Also tell them how much you just saved them.
RE: Post bearing on concrete floor slab
Review and use at your own risk.
www.PeirceEngineering.com
RE: Post bearing on concrete floor slab
PEinc - I'll check out the MathCAD file, thanks.
RE: Post bearing on concrete floor slab
www.PeirceEngineering.com
RE: Post bearing on concrete floor slab
Like Mike said, not really a safety concern, most likely you'd get some hairline cracks or settlement as the worse case scenario. Your 2400lb probably has a lot of live and dead load that will be largely NOT present for the life of the structure.
A large SUV parked on a 4" slab probably is way worse (average residential driveway).