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RE: Raised garage slab - bad idea
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Raised garage slab - bad idea
- From: "Kevin Elbury" <kelbury(--nospam--at)elburydolan.com>
- Date: Wed, 14 Apr 2004 19:53:10 -0700
- Cc: "Mark Pemberton" <markpemberton(--nospam--at)sbcglobal.net>
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
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
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
Kevin Elbury, P.Eng.
Elbury Dolan Consulting Ltd.
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.