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RE: Minimum steel in slabs on ground

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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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