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RE: Raised garage slab - bad idea

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I have done a few of these and have basically resigned myself to NOT doing them anymore for the following reasons:
1. 2x12 joist quality has been of very poor quality and select grade SPF which is what is normally used here is a special order item, difficult to get. It have seen 3/4" to 1" knots on the tension fiber of the joists (#1/2 SPF).  I don't know if this is poor grading or acceptable to that grade but by item 3 below, I have no tolerance for this.
2. You have to be very careful about the waterproofing detail at the garage door, this is where you you are going to have a problem. Ie. run the membrane outside and return down the fdn wall.
3. Try analyzing the joist using the wheel loads from say a Ford Expedition with the addtion of 4 passengers.  I could never get a single joist to carry the full wheel load and (i think) required lateral distribution of about 40% (i.e. one joist could take 60% of the wheel load). The biggest problem was shear, which of course is when the wheel is close to the entry door and here, the joists is very stiff so you probably won't get the lateral distribution you would get a mid span. What about the dork who (over) loads his one ton pickup with gravel and drives into his garage?
4. The venting issue is a nightmare, as I have always had living space (usually storage room) so you have cold space over warm space with nowhere to vent the floor unless you cross strap with 2x4. Even then, the venting is rather passive for my liking.  I have addressed this by requiring the garage to be heated and have not installed vapour barrier on the ceiling.
5. I believe this system was tried in Vancouver, B.C. using I joists (which do not have the required shear capacity). They had a garage floor collapse which I believer was caused by rotten joists.  I have heard through the grape vine that wood frame garage floors are banned outright.
6. I am typically do 20x20 double garage with an 8" suspended slab, pay particular attention to top steel at the corners ... done.  I could put a semi truck on one of these slabs and it would not fail, no venting problems and it's a permanent structure. I have also designed a shoring system with dimensional lumber that residential framers can assemble (alot a scared proper form tables) and alot of the product is reused in construction of the home.
 
I have not come up with an economical water proofing membrane but the last 3 slabs I have done have turned out really well, to the point that we haven't worried to much about water proofing (although I always say "Waterproofing membrane by others..." to C.Y.A.. I also like to stay under 25mpa on my concrete to help with cracking. 
 
Kevin Elbury, P.Eng.
Elbury Dolan Consulting Ltd.
Kelowna, B.C.


From: Mark Pemberton [mailto:markpemberton(--nospam--at)sbcglobal.net]
Sent: Monday, April 12, 2004 5:18 PM
To: seaint(--nospam--at)seaint.org
Subject: Raised garage slab

I am doing residential structural plans for a construction company that would like to raise the garage slab above a sloped grade to reduce the amount of fill required.  This sounds like a bad idea, but I know I've seen it in plans in the past.  Does anyone have any suggestions regarding this condition?  My knee jerk reaction is to talk the company into doing retaining walls with gravel fill as I'm used to typically seeing.  Any comments are appreciated.
 
Mark Pemberton, S.E.
Pemberton Engineering
Davis, CA