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RE: Minimum steel in slabs on ground

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As has been said many times on this list, ACI 318 and its cover
requirements do not apply to slabs on grade. I routinely used 4" slabs
on grade usually with #4@18", unless the geotechnical engineer or
somebody tells me otherwise. The City Of Los Angeles allows you to use a
3.5" S.O.G.


-----Original Message-----
From: Bill Allen, S.E. [mailto:T.W.Allen(--nospam--at)cox.net] 
Sent: Friday, October 15, 2004 2:25 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Minimum steel in slabs on ground

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
>   _____
>
>Do you Yahoo!?
>Yahoo! 
><http://us.rd.yahoo.com/mail_us/taglines/aac/*http:/promotions.yahoo.co
>m
>/new_mail/static/ease.html>  Mail Address AutoComplete - You start. We
>finish.

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