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RE: Minimum steel in slabs on ground

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You are right but:
a) in this case it is true
b) the building code is only a required minimum, nothing in the building
code prevents a designer from being more conservative e.g. applying ACI
318 cover requirements for slabs on grade

There is probably a number of documents that establish their own
criteria for cover. ACI 360R has a couple of suggestions for example.
 
LA City requires #4@16" with a 3.5" slab but this is only for slabs on
expansive soils and compacted fill.

-----Original Message-----
From: Bill Allen, S.E. [mailto:T.W.Allen(--nospam--at)cox.net] 
Sent: Friday, October 15, 2004 3:06 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Minimum steel in slabs on ground

Michael -

While I'm not sure about how many times something has been said on this
list, I do know that just because something is said several times that
a.)
doesn't make it true and/or b.) doesn't mean that's good enough for me.
For
example, the live load deflection for a floor joist doesn't have to be
less
than L/360, but that doesn't mean I'm going to use that criteria when
designing a bonus room on the 2nd floor with a span of 20 feet.

Still, I would be interested in what document establishes the criteria
for
cover for S.O.G.s if it's not ACI-318. WAGs don't count (even if they're
generated by LADBS).

Just curious, what is the LADBS 3.5" slab reinforced with? If nothing,
then
that's probably O.K. (until it cracks).

Good Friday afternoon stuff.

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: Michael Bryson [mailto:mbryson(--nospam--at)NYASE.com] 
Sent: Friday, October 15, 2004 3:37 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Minimum steel in slabs on ground


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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