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RE: Minimum steel in slabs on ground

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Bill:

It is true.  Section 1.1.6 clearly states that ACI 318 does not apply to
SOGs unless they transmit vertical loads from other parts of structure to
the soil (i.e. if it were a mat foundation rather than just a SOG).

Now, this does not mean that you cannot use the provisions in it as a
guideline for a SOG.  Feel free to do so.  I do.  Just means that a code
official cannot force you to do so (unless they have locally modified the
code).

I know of no code provisions that directly apply to SOGs except some VERY
limited provisions in the UBC (sections 1806.7.2 and 1815) and the IBC
(section 1805.8.2 and 1806.2.1 and 1806.4 and 1911 in the 2000 IBC).  And
none of those provisions address the issue of cover.  So, to me it is
certainly reasonable to use the ACI 318 cover values for SOGs as well.

Regards,

Scott
Adrian, MI


On Fri, 15 Oct 2004, Bill Allen, S.E. wrote:

> 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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