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RE: IBC Special inspection for Slab-on-Grade supporting HVAC unit.

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-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Friday, May 27, 2005 10:26 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: IBC Special inspection for Slab-on-Grade supporting HVAC
unit.

My point was that a SOG with wheel loads is still technically not a
structural slab (which you seem to agree with).  It _STILL_ needs to be
designed properly.

-----Original Message-----

'Sfunny this thread should crop up now. It so happens I've been
designing a SOG for parked chicken trailers. I have the weight data for
the typical trailer, and I note that the load on the front legs (when
the tractor is disengaged, props the trailer up in front) is really
pretty large.

I typically refer to Boyd Ringo's "Designing Floor Slabs on Grade" for
this kind of thing. Using the "modified" PCA Method that Ringo
describes, I come up with a required minimum slab thickness of a little
over nine inches.

Ringo goes further to describe a design method that turns the empirical
PCA data into design moments for which you can derive the required
reinforcement using the flexural design procedure of ACI 318.

In my case I came up with an "optimum" design of 8 inch slab thickness
with #5 bars @ 12" Top and Bottom. (Interesting also to note that Ringo
recommends a bottom bar clear cover of no more than an inch or so; his
rationale is that the 3" clear cover requirements of ACI 318 don't apply
because SOGs aren't covered by ACI 318).

When I mentioned this to my "boss," he objected. "We would only use a
single layer of rebar, say #4 @ 16" for an 8" slab!" I pointed out that
my design would not require expansion joints, because the reinforcement
would handle stresses due to movement, and he still objected. "Just put
in the joints, it's cheaper."

Well, he's right in one sense, because it IS cheaper. But by how much?

Using the current R. S. Means "Building Construction Cost Data", I
calculated that it costs about $3,000 more for this 2,500 S.F.
slab--$1.20/S.F. added cost--to do it my way. But I insist that the slab
will perform far better.

This area is expected to be "washed down" pretty frequently, and we are
even designing in trench drains going to a commercial septic tank to
settle out fecal solids, etc. To my mind, this is NOT an environment
where you want joints and "purposely cracked" concrete.

So we've agreed to disagree--he's the client and he'll get his way, but
to me this is being penny-wise and pound-foolish.

In the end, "structural" is in the eye of the beholder. To me, this is a
structural slab because of the service and the way you want (or rather,
"I want") the slab to perform. To him it's just a plain ol' SOG, no
different than a highway pavement. But they don't frequently wash down
highway pavements to get rid of fecal wastes into a trench drain system
('round here, buzzards and crows do all the waste disposal for highway
pavements).


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