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Build to Include Existing Slab

Piccioni | Posted in Construction Techniques on June 8, 2005 08:22am

Hello everybody, and thanks in advance for your help.

About 6 years ago, while building my house, I made the mistake of buying a Quonset hut in order to shelter my tractor, tools, etc.. Because I considered this a permenant solution, and because the plan inspector insisted on it, I used an engineered slab, which is about 16″ thick around the perimeter.

Well, Quonset huts are a disaster: they leak, are very hard to insulate, or keep warm and dark as a tomb during winter. I want to build a full workshop on the same site, using traditional construction. It will be about twice the size of the existing Quonset hut.  Because my topsoil is so deep (16″ +) I will have some of the new workshop with a full basement, with the excavation providing a source of back fill (I spent about $4,000 on granular A bringing the floor of a small barn up level with the topsoil).

Ideally, I would like to remove the Quonset hut and reuse the slab. I could build two adjacent buildings, but this would be a pain in the butt going from one to the other. Ideall, this new structure would enclose the existing slab, which would save me having to pour that slab again at least if I can reuse it.

As I understand it, frost heave means I can’t join build up to and over the slab. I was thinking about having a frost wall installed around 3 of the 4 walls of the slab to stop heave, and a full basement wall on the fourth side, though I’m not sure how I’d backfill the inside of the new frost wall.

Has anybody encountered such a situation before? Ar there other alternatives I haven’t considered?

 

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  1. User avater
    CapnMac | Jun 09, 2005 12:59am | #1

    How locked into a shop basement are you?

    Since the existing slab seems to work (you did not mention any heaving, cracking, or other disreputable behavior), it would seem like the simplest thing would be to dowel the existing slab into a new one.

    The original engineering on the orignal slab will partially dictate the size & spacing of the dowels into the new slab.

    But, that gets you a good slab, of the size you want, and without any tricky manipulation of an existing slab or supporting that same slab with little or no backfill.

    Now, if a basement is a needful part of the design (ground slopes away to make a walk-in, etc.), then maybe what is needed is a transition from the basement to the existing slab; the better to not undercut the existing, etc..  Those answers really need an engineer.

    Occupational hazard of my occupation not being around (sorry Bubba)
    1. piccioni1 | Jun 09, 2005 04:09am | #2

      Setting aside the issue of the basement (I don't really need one) my question is really about tying an existing slab into a new building. Frost heave would probably reck havoc on the building if I tried to extend the slab, unless I create a frost wall to stop the frost heave.Or am I wrong? Could I just pour a much larger slab around the existing one assuming I doweled, etc.?

      1. VaTom | Jun 09, 2005 04:17am | #3

        Here you go:  http://www.cs.arizona.edu/people/jcropper/desguide.html  Design Guide for Frost-protected Shallow Foundations.PAHS Designer/Builder- Bury it!

        1. Piccioni | Jun 09, 2005 10:33pm | #4

          Thanks!

          I think this is exactly what I was looking for!

          1. VaTom | Jun 10, 2005 12:41am | #5

            You're welcome.  I live with a variation on that concept.  If you want, you can use the dirt under the insulation for both winter heating and summer cooling.  Opens up lots of opportunities.PAHS Designer/Builder- Bury it!

      2. User avater
        CapnMac | Jun 10, 2005 01:41am | #6

        Could I just pour a much larger slab around the existing one

        VaTom gives a better reference for your climate, but the short answer is yes.

        I worked on an USAF project that was to be built in four phases, each one a slab of 200 x 200' each of which was to tie to the previous, until they had a foundation for a 400 x 400' building slab.  There were many (oh, so many) enginering slab details to review on that one . . . )  Inside clear height was 45', too--not a small little thing.

        The key part in your situation is getting the old engineering tied to the new.  That means running bar and tying it to the dowels, and proper expansion joints and all.  All relatively 'standard' detailing for a structural engineer.

        Specifically, the dowels will provide two functions for you.  One is that they become and extension of the reinforcing of the existing slab.  The second is that they help limit any effects of movement between the two slabs.Occupational hazard of my occupation not being around (sorry Bubba)

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