Metal Building Foundation.
Metal Building Foundation.
(OP)
I am trying to design a slab on grade for a metal warehouse building. This slab also needs to act as foundation for column vertical tension and compression loads. This is my first time designing a foundation like this. I would like to know, how the moments due to these vertical loads can be closely calculated?
Also, I shall appreciate any other design consideration.
Also, I shall appreciate any other design consideration.






RE: Metal Building Foundation.
RE: Metal Building Foundation.
RE: Metal Building Foundation.
I have a large uplift force in the column. So, I have to size the footing to handle this uplift. The thing is, the moment generated in the footing due to this uplift is nothing compared to the size of the footing. I was trying to find out a rationality in ACI-318 behind having the footing unreinforced.
I have a 8 kips uplift in the column, which requires a footing size to be 4'X4'X3.33'(deep). By the way, the down ward force on the footing is only 6 kips. Can I have this footing unreinforced?
RE: Metal Building Foundation.
RE: Metal Building Foundation.
Mike McCann
McCann Engineering
RE: Metal Building Foundation.
RE: Metal Building Foundation.
For footing, the minimum reinforcement is .0018Ag. I like to divide this total amount in two halves and provide half close to the bottom and other half close to the top of the footing. I do not want either top or the bottom of the footing unreinforced, nor I want to provide any more total reinforcing than minimum required.
Can you please comment?
RE: Metal Building Foundation.
We have had numerous discussions on the 0.0018 min. That should be provided on each face, not 0.0009Ag on each face.
RE: Metal Building Foundation.
I don't agree! (couldn't resist))
RE: Metal Building Foundation.
I don't see why you wouldn't use a portion of your slab to resist uplift. If you are using a hairpin for horizontal rx, it must be integrally tied around the column anchor bolts, so I see no reason that you wouldn't at least include the weight of the chunk of slab covered by the hairpin to add to your uplift resistance. Also, we typically use a pier on top of the spread footing connected with a couple of zee bars (reinf bent into a zee, or zed) to tie the footing and pier together. This way, you can keep the footing thickness to a minimum. As well, we direct the continuous turn down footing bars adjacent the column footing to run through the back of the pier, which also increases the uplift resistance. HTH.
Peggy
RE: Metal Building Foundation.
I didn't agree to begin with either. I am still not sure I do, but I just wanted to throw it out there.
RE: Metal Building Foundation.
DaveAtkins
RE: Metal Building Foundation.
StrlEIT: Even after the mammoth thread a while back, I think it's strictly for T&S and has nothing to do with flexure. Therefore, I don't see the problem with putting some fraction T&B. To avoid hijacking the thread, that's all I'll type about it. It's a small amount of steel in most cases, so if somebody wants to be conservative, then who cares... If anybody wants to fight about it ( LOL ;) ), he can re-open the other thread.
RE: Metal Building Foundation.
I re-read your post fouryth from the top of the string and you mentioned you were using a 4 X 4 X 3.33 footing to resist 8 kips of uplift. If that is net uplift, then your FS with this footing is only about 1.0 considering 140# concrete. By code, the FS needs to be 1.5. Seems like more concrete is needed, or using some of the slab too to get the 1.5 FS.
Am I missing something?
Mike McCann
McCann Engineering
RE: Metal Building Foundation.
Shin, what i would feel comfortable with is that you desing isolated footing for your vertical loads, if you want to reduce the size of your isolated footing (incase your uplift is the critical load governing the size of footing), give it a litle soil cover of say 2 to 4ft, this would cater for your vertical loads, and connect the slab on grade with your pedestals comming from the Pad, using hairpins as to resist the horizontal load, so Pad for Verticals and slab for horizontals !.
There are also other ways, but mostly this is what i use, its simple and probably most commonly used
RE: Metal Building Foundation.
RE: Metal Building Foundation.
"Mike, he might be using one of those new load combinations with 0.6D in it instead of 0.9D."
You are correct.
RE: Metal Building Foundation.
The uplift wind forces were very high. We tried to show 3-4 feet thick footings, which became a cost problem with the Owner. I asked the pre-engineered project manager how they dealt with this (they also did design-build) He responded that they take as much slab above the footing that can be engaged before it fails. That slab load was enough to offset uplift.
RE: Metal Building Foundation.
I am not sure if their approach was completely right. The amount of concrete that is needed to counter the uplift is quite large. Given that the slab on grade is usually quite thin, this should translate to a very large surface area of the slab. To mobilize all this area, they probably had to provide good amount of reinforcing to handle the induced moment. And reinforcement should not be that cheap.
Unless, the slab on grade is quite thick, the solution provided by the manager may not be that feasible.
RE: Metal Building Foundation.
Large footings for uplift are very common for PEMB. Often, downward soil presuures become very very small. Engaging the slab for uplift may result in slab cracking (and then what), and I would avoid it if possible. With use of hairpins or foundation tie-beams and centrally loaded foundations (if this is possible), you can drastically reduce the moments created by the horizontal and vertical forces.
Also, refer to ACI for embeddment of anchorage for the uplift forces. A thickened slab region may not be sufficient for very large loads.
RE: Metal Building Foundation.
Regarding the last paragrph of your last comment, this can be true, but with top steel and the anchor bolts welded to an embedded plate placed deep in the footing, it should not be an insurmountable problem. Or you can deepen the footing, at least locally in the area of the anchor bolts, to make sure it works in uplift without thickening the whole footing if the dead load is not needed.
Also, 271828, yes, that's right. I'm too fixed on WSD for the overall view of things I guess.
Mike McCann
McCann Engineering