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RE: Minimum steel in slabs on ground

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As has been cited before, SOG's are not technically structural concrete and are not REQUIRED to be in compliance with ACI 318. But the ACI is not true to form. The ACI 301 is specifically for "Structural Concrete", but it does have some provisions for SOG (RE: ACI 301, Sect. 3.3.2.5.). Basically it states that you can not hook or float "welded wire fabric" in a SOG.

In that most SOG's are not "structural", the issue for the cover on a SOG is serviceability. There is no other guidance for cover in literature other than that contained in ACI 301 or ACI 318 of which I am aware.

I choose to be on the conservative side, and use the cover contained in ACI 301 or ACI 318. They are the same. As such the bottom of a SOG is formed by the earth, requiring a 3 inch cover. Slabs exposed to weather require a 1 1/2" cover. This requires 4 1/2" just for cover. Interior slabs still have the 3" cover to the earth, and you can reduce down to a total of 3 3/4" total cover. One could argue that a "prepared sub grade" constitutes a formed surface of sorts and reduce the bottom cover. But I would rather go to a 5" slab as a minimum. I don't think that the savings in concrete warrants the potential serviceability risk. There are other advantages to the 5" slab as a minimum, but are not relevant to the current discussion.

I won't get into the Fibermesh issue again other than to say that some of the most expensive slab repairs I have worked on contained polypropylene fibers. On one project, I saw a poly SOG cast adjacent to a SOG with rebar. The poly slab was badly cracked, and curled but was newer than the SOG with rebar.

I do not use fibers with the exception of bran fiber in my cereal in the morning.

Regards,
Harold Sprague

From: "Bill Allen, S.E." <T.W.Allen(--nospam--at)cox.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Minimum steel in slabs on ground
Date: Fri, 15 Oct 2004 15:24:40 -0700

Harold -

The same moment when I started specifying #3s and #4s for slab reinforcement
instead of EWWF I stopped specifying 4" slabs for the reason you cite. Now,
when I look in the code and check cover, I'm not sure I can justify it.

1-1/2" bottom cover + 3/8" tolerance in thickness + 2 x (1/2+1/8) for bar
and rib + 3/4" top cover (which I don't think is enough for C.I.P. concrete)
comes to 3-7/8".

What am I forgetting? Or is that 1-1/2" bottom cover not enough to
compensate for an uneven pad? Dealing with a specific increment in chair
heights?

Thanks,

T. William (Bill) Allen, S.E. (CA #2607)
ALLEN DESIGNS
Consulting Structural Engineers
http://www.AllenDesigns.com
V (949) 248-8588	 .	 F (949) 209-2509


-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
Sent: Friday, October 15, 2004 10:26 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Minimum steel in slabs on ground


Check the rebar cover for a 4" slab.  That is where you can run into
problems.

Regards,
Harold Sprague


>From: "Ed Tornberg" <etornberg(--nospam--at)comcast.net>
>Reply-To: <seaint(--nospam--at)seaint.org>
>To: <seaint(--nospam--at)seaint.org>
>Subject: RE: Minimum steel in slabs on ground
>Date: Fri, 15 Oct 2004 08:48:04 -0700
>
>I too find the PCA vs ACI vs WRI vs etc. methods "difficult" to sort
>out.  The only conclusion I can make is that slab design is an "art".
>Well, no, it really is a science, but it borders on art because there
>are nearly infinite variables involved in determining whether the
>slab's cracking (cracking is when and how, not if) will be adverse
>enough to irritate the owner.
>
>Why do the design manuals shy away from addressing the classic 4" slab
>that seems to be poured quite often?  I know they're really interested
>in rack loading, wheel loading, etc. for industrial use, but for your
>standard office/light commercial slab that the contractor wants to do
>as 4", the manuals virtually ignore it.  Example:  ACI 360 Table 9.9
>for joints for plain slabs (which happens to be a PCA table) - the
>minimum slab thickness is 5".  If we as engineers allow a 4" slab, have
>we violated something?
>
>Ed Tornberg
>Tornberg Consulting, LLC
>503-551-4165
>-----Original Message-----
>From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
>Sent: Thursday, October 14, 2004 6:31 PM
>To: seaint(--nospam--at)seaint.org
>Subject: Re: Minimum steel in slabs on ground
>
>Thanks for remembering Gail.  I found this buried in the old sent items
>folder, so here it is.
>
>Seaint,
>
>I just received my copy of PCA's "Designing Floor Slabs on Grade"
>thinking it would resolve some floor slab design questions.  But it
>brought up even more questions about minimum steel requirements.
>
>They present three different methods for determining minimum steel:
>'subgrade drag' (which has been discussed here before) which nets
>around .4% - .5% minimum steel, 'temperature method' and 'equivalent
>strength method'.  When running numbers for the second two methods, I
>come up with anywhere from 4%-6% steel based on area (assuming a
>moderate temperature differential during exposure).  These numbers seem
>to be excessively high, especially when compared to the first method
>and even a 1% minimum by rule of thumb.
>
>The PCA notes for a Type B slab with shrinkage control reinforcement
>say these slabs usually take steel designed by the subgrade drag
>equation. Yet the notes under the 'temperature method' section say it
>is "more desirable" than the subgrade drag method.
>
>These comments don't exactly conflict with themselves, but they sure
>don't point the reader in a clear direction.
>
>Any better thoughts on these last two methods?  Are the units in the
>book off?  I am inclined to stick with the 1% minimum and be happy.
>From what I've seen, even 15" airport slabs should only require a
>moderate weight mesh...
>
>Thanks,
>Jim Wilson
>wilsonengineers(--nospam--at)yahoo.com
>Stroudsburg, PA
>   _____
>

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