Metal Building Foundation. 2

[shin25]

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.

 

[271828]

I think most folks would thicken the slab in the vicinity of the column, creating something that looks like an integral spread footing. Then design this footing like you normally would. This isn't the only way to approach the problem.

 

[zaes73]

i dont think you can use the slab for your vertical loads, for vertical loads you need to increase the size of the slab below the columns, and design this specific portion just like you would design a regular Pad, u can use the slab on grade for horizonzontal thrust. But be carefull that in case of wind load you will get uplift on your column footings, so maybe the increase of slab size under columns becomes un-practical and you may need soil cover on top to prevent uplift.

 

[shin25]

Thanks for the responses.

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?

 

[BigH]

zaes73 - I don't see why you think you can't use your slab for vertical loads. 271828 was correct in that you will thicken the slab at the columns so that it can take the vertical forces/reinforcement. It is done all the time. - a thickened raft slab. As for your uplift, why not install screw piles below your foundation to take the uplift? That way you don't have to go to strange lengths to handle the uplift load. You will have to put in the proper reinforcement to integrate the screw pile top into the foundation.

 

[msquared48]

4 X 4 X 3.33 is not unusual for a metal building footing - I have these all the time - just pour monolithically with the slab. As forbeingunreinforced, as I am in a seismically active area, I would always provide theminimum reinforcing.

Mike McCann
McCann Engineering

 

[DaveVikingPE]

I feel a lot more comfortable using a wall foundation that's independent of the slab-on-grade. This kind of foundation is sized for uplift, etc. Uplift is always a problem if the building has potential to be partially enclosed. Re: reinforcing - start with the minimum required, perhaps that will work just fine.

 

[shin25]

msquared48,

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?

 

[Lion06]

shin-
We have had numerous discussions on the 0.0018 min. That should be provided on each face, not 0.0009Ag on each face.

 

[271828]

"We have had numerous discussions on the 0.0018 min. That should be provided on each face, not 0.0009Ag on each face."

I don't agree! (couldn't resist))

 

[pldpe]

Shin,
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

 

[Lion06]

271828--
I didn't agree to begin with either. I am still not sure I do, but I just wanted to throw it out there.

 

[DaveAtkins]

Also don't forget that if your metal building column is not centered on the footing pad/thickened slab, you must account for the moment created by the eccentricity.

DaveAtkins

 

[271828]

Peggy: I agree with including a liberal chunk of slab also. For one thing, many people don't size these correctly for uplift and we don't exactly see buildings flying through the air during wind storms. Undoubtedly, there are additional things helping besides just the footing: slab, wedge of soil (if applicable), wall, turned-down slab, etc.

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.

 

[msquared48]

shin25:

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

 

[zaes73]

-BigH what i meant by saying " i dont think you can use the slab for your vertical loads" was that a regular depth of the slab on grade being used through out cannot be used, as i proceeded with my post i also mentioned that "for vertical loads you need to increase the size of the slab below the columns" so i too agree with increasing the depth.

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

 

[271828]

Mike, he might be using one of those new load combinations with 0.6D in it instead of 0.9D.

 

[shin25]

271828 has written-
"Mike, he might be using one of those new load combinations with 0.6D in it instead of 0.9D."

You are correct.

 

[BSE05]

Recently worked on a pre-engneered warehouse building that was competively bid. The sucessful bidder fixed the columns to the footings to reduce steel cost. The increased cost of concrete was more than the steel savings. Watch out for that trick, only allow pinned column base connections.

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.

 

[shin25]

steelylee,

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.