Post bearing on concrete floor slab

[CEmonkee]

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!

 

[msquared48]

The ability of the slab to support the load depends on the reinforcing used and thickness, obviously, plus a 45 degree line to the subgrade from the post/baseplate perimeter to establish the area.

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

 

[dhengr]

The problem with 4" residential slabs on grade is that they are all to often only 3 or 3.5" thick, in all the wrong places and they rarely have any mesh reinf. in them. Furthermore, you don’t know the strength or preparation of the subgrade material, or the conc. strength. I believe the values that MikeMcC gives (4 to 500psf) are industrial slab, distributed live loads, not to be confused with point loads on the same slab, thus his comment on forklifts. Check punching shear around the column. What you want to do usually works, but you could also get some localized slab cracking. Advice the client accordingly, you don’t have a crystal ball, do you?

 

[CEmonkee]

Hi Guys - thanks for your comments. Unfortunately I don't have a crystal ball - just one of those black plastic 8 balls

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.

 

[CEmonkee]

Here's the article I referred to.

 

[a2mfk]

I'm with dhengr, even if you know the exact properties you don't know the subgrade props. But you could have this verified by drilling a couple of holes in the slab where you want to put the post and do in-situ hand cone penetrometer tests. Cheap and easy, and gives you an idea of your in-situ compaction.

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

 

[CEmonkee]

Hi Andrew - thanks for your reply.

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?

 

[MiketheEngineer]

IMHO - I would think you should be OK. What if does fail?? Nothing is going to come falling down unless you punch a hole to the center of the earth. I have only seen one case where this was a problem - a highly loaded column on what turned out to be about 2.5'' concrete where the soil had also washed out!!

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.

 

[PEinc]

Attached is a MathCAD 11 file I found on the internet for a column on an unreinforced concrete slab.

Review and use at your own risk.

www.PeirceEngineering.com

 

[CEmonkee]

Hi MiketheEngineer - thanks for your input and advice, it's much appreciated.

PEinc - I'll check out the MathCAD file, thanks.

 

[PEinc]

Make sure you use an appropriate k value.

http://www.PeirceEngineering.com

 

[a2mfk]

Talk with the owner about the risk, its not life safety is performance of the finish materials. If they are putting ceramic tile around this area, any cracks in the slab will eventually telegraph through.

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).