I need a slab poured inside an existing steel building. The building will be used as a shop and for farm equipment. It'll have tractors, gooseneck trailers, and big pickups on the floor, but no D5 caterpillars or things that heavy on it. Here are the parameters:

Location: central Texas, rural
40 x 60 feet
5" thick
1/2" rebar on 18" center
One 18" x 24" footing down the middle, running in the long direction
Two 18" x 24" footings across the short dimension, 1/3 and 2/3 of the way up the long dimension
Each footing reinforced with qty 4 5/8" rebar, tied around every three feet with 3/8" rebar
Formed on the outside with treated 1" lumber to avoid pouring the concrete against the building sides; forming to be left in place after the pour
Poured in three sections: center 20x40 foot bay first, then each end.

The soil is blackland clay, but the previous owner put up the metal building on a nice level pad of road base-like material that is 30" to 36" thick. It has been well compacted from about 7 years of use as a horse barn.

I have a bid from a gentleman whose work I trust for $14,600. This is an "out the door" price, including the dirt work. The dirt work includes sloping the slab appropriately to make the existing rollup doors at each end close against it.

Questions:

1) The quote knocked me on my rear -- I was expecting about $10000 at most. Am I naive, or is he gouging me? Maybe concrete work really IS that expensive these days? It works out to well over $6/sq foot.

2) Is it over engineered? I don't want any less than 5" thick, but are the footings really necessary? He'll pour it without the footings for about $5/sq foot. I want a solid foundation and it will be fairly heavily loaded at times but I don't want to waste money either.

3) What do y'all think of the idea of the form boards on the perimeter? I understand the engineering concern of corrosive concrete touching the metal sides, but I was hoping for a tight seal between foundation and sides.

4) What about the three-section pour? I think I know why these particular guys do it this way: they're a small crew and 20 x 40 is all they can handle in one day. I know the slab will need joints anyway, so it seems OK to me -- what do you think?

Any other comments are welcome -- thanks in advance for your learned insights!

If I left out any info just ask and I'll provide.

Jeff

 

Get a couple more local bids from reputable contractors. Compare them to see if they're about the same as the one you've got. Then go with whoever you trust most.
It's hard for anyone on a forum to even give a ballpark estimate if they're not from your area. Labor and material prices vary wildly by locale.

 

For the work that they're proposing to do, the quote isn't that outrageous. BUT...

Assuming this is a clear span building (at 40x60' it should be), there are no interior loads that merit the two transverse and the one longitudinal beams which total some 17CY of concrete plus the rebar and associated labor. Put 1/2 that amount of concrete into making the slab 6" thick and you'll be be better off in my opinion. And they can do it with a grid on 2 foot centers.

If the building was properly erected with correctly sized, reinforced piers under the main and corner columns (roughly 3'x3'x3' and 2'x2'x2', respectively), then you really only need to form and pour the slab. A modest perimeter footing would benefit the slab by keeping the edges from collapsing, but won't do anything for the building. A continuous perimeter run of rebar will probably do as much.

As for the "forming" boards around the perimeter, what is the purpose? ACQ pressure treated lumber is horrendously corrosive and leaving it in contact with the building won't be a benefit.

 

Thanks, Doug.

The building is clear span. There *may* be significant loading in the interior in the form of a beam and trolley lift, and there may also be significant amounts of heavy equipment inside at one time, so I'm inclined to go with the footings as insurance, but I'll ask the contractor about the 6" slab -- thanks for the suggestion.

LOL at the "correctly sized, reinforced piers". There ARE piers, but I'm sure they're nowhere near the size you suggest. Although serviceable, this is not a cadillac metal building with I-beam posts and joists -- it has steel pipe uprights and bowstring joists. I like the idea of the perimeter rebar and I'll ensure it goes in.

The purpose of the form boards are corrosion prevention. I researched the treated lumber corrosion issue, and you're right, it is rather corrosive, especially to fasteners (treated galvanized metal in surface contact, not so much). But the concrete is corrosive too -- I have a friend who also had a slab poured inside an existing building with no isolation between wood and sidewalls and he is already getting corrosion after just a couple years. I'm now inclined to go with the form boards but put a barrier membrane between metal and wood.

Thanks again for your help.
Jeff

 

"Three main factors distinguish pre-engineered building foundations from the rest: substantial horizontal column reactions, large column uplift, and a common need to design the foundations before the column reactions are determined. ...the first two are often overlooked by the uninitiated." (ref: "Metal Building Systems", 2nd ed., A. Newman, p.336)

Fact is, Newman is focussed more on commercial structures where design deficiencies rather than budgets are the principal causes of problems. An experienced structural engineer would recognize that the purpose of the foundation for a pre-engineered metal building is to hold it down, not to prevent it from sinking into the ground.

Your contractor can take simple remedial action by drilling into the (hopefully reinforced) piers and extending rebar (#6 minimum) into the slab and/or grade beams. I suggest 4' into the slab and as far as they can drill into the piers. Tie the portion extending into the slab to the reinforcing grid. It isn't a perfect solution, but it is probably better than what you have now.

Good luck with your project.

 

heh... I have that book (although I haven't read it cover to cover). Thanks for pointing that section out.

I'll discuss your suggestion with the contractor. Thanks.

Jeff