concrete piers first, slab latter?

[sungrove]

I must say, I have been checking out all your projects and I am very impressed with what you have built.

I am considering building about a 30 x 50 building 16-17 feet tall using either a Web Steel vaulted building ( http://www.websteel.com/BuildingSystems.htm ) or an Outback steel building.

To get directly to my question: has anyone started by simply pouring piers, then erecting the building onto those piers and then pouring the slab after the building is built?

My reasons for thinking about doing it this way:
1. my building site is not very level and my space is pretty constricted, so regular excavation could be a problem.
2. I know from previous work I have done that there is some beautiful hard packed stuff not very far down that I'd like to have the concrete sitting on. I believe it's called glacial till. (I'm in the northwest) This stuff is about 5-7 feet down. It is like compacted coarse sand. If you are trying to dig into it by hand , you have to use a pick ax.
3. I want to do as much of this project myself and feel that with some help, I could deal with making these piers and I would imagine there are tubes I can purchase to use as forms
4. Latter, when making the slab, I want to insulate underneath and around the sides of it and then install pex for heated flooring. I envision also insulating between the slab and the piers so that the slab is totally thermally isolated. I like that if I use this pier first method, I don't have to figure out how to insulate the perimeter foundation from the slab.
5. In either style of building I am considering, the basic design is that the building loads all go to the vertical steel posts supporting the trusses much like a pole building. so, this proposed method seems appropriate.
6. I like the idea that maybe this would simplify my permit application process since such pier design calcs would seem more straightforward and easy to apply. I'm thinking that I could get the building permit to build the building on the piers and then apply latter for the other things. ( Slab, heating system, electrical, blah blah)

Anyway, what do you think, have you known anyone to do it this way? If you like the idea, would you put rebar in to link the piers to the slab latter? What size piers did you use? ( I understand this is an engineered issue)

Thanks,
Neil

 

[larry4406]

The pre-engineered metal buildings I put up (60,000, 90,000, and 100,000 sf) all were done with the piers first followed by the slab later - this is normal with many of the benefits you state. An often overlooked benefit is that the slab can them be protected from wet weather for finishing (roof on, no sides)

The piers had the anchor bolts embeded in them to receive the steel (this is a critical step, done with a digital transit), the top of the piers have a finished surface and are level with the final concrete slab. The pier has hair-pin rebar embedded in it to tie with the slab later.

The slab is a turn down slab at the perimeter.

Example pictures attached for a building near Richmond VA.

 

[buening]

Regardless it will be likely a structural engineer would have to do the foundation design and detailing. It all depends on how he details it. Most pre-engineered buildings require a stub wall around the perimeter for the siding to rest against on the outside. We typically detail the piers to be poured first, and then the foundation wall and footings to be poured second, and finally the slab poured last. Regardless, you won't likely get foundation details from the manufacturer due to different requirements depending on your local code. They provide the engineer the design loadings and the anchorbolt layout, we determine the pier dimensions and the wall/footing sizes and rebar details.

 

[sungrove]

Thanks for your responses. I had thought I would probably have to have the foundation engineered and these responses have confirmed that. I appreciate the affirmation that I'm on the right track with the piers first.

It also becomes obvious to me that this ends up being pretty technical and that I need good engineering work done. Is this something I must have done locally? I have seen on line companies offering engineering. But it seems the engineer would actually have to visit my site and do tests ( or have someone local do that?)

I realize maybe this is overkill. But on the other hand, it seems in my situation, the potential is there to end up with a very strong, long lasting and useful foundation and slab given quality design.

I got a quote of $500 for a foundation design a while back. Does that sound appropriate? Or too low for all the detail I'm interested in? ( heated floors, insulation under and around, piers down to glacial till, drainage design )

I'm obviously green at this, but I appreciate you taking me seriously.

I do like that a good strategy seems to be to pour piers first, build the building to the roof stage, and then pour the slab and perimeter footing and then finish the building sides.

Neil

 

[buening]

Check a combination of local engineering firms and get multiple quotes. You'll find it will vary quite a bit, but around $1k is a fairly good price. I've done as small as a couple hand sketch details to keep costs to a minimum on small jobs like this, so be sure to explain to the engineer what you want.

 

[edjwl2020]

hello I just joined this group, hope to learn a lot thanks