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Re: Minimum Slab on Grade Steel

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In a message dated 5/20/2004 5:33:10 PM Eastern Daylight Time, keith(--nospam--at)kdlengineering.com writes:

> It has always been my opinion that ACI 7.12.2.1 (0.0018 minimum temperature and shrinkage reinforcing) applies to elevated slabs and not slabs on grade.  I have always designed my sog reinforcing to ACI 7.12.2.1.
> 

Maybe I am missing something,  but if you don't think 7.12.2.1 applies to slabs on ground, than why do you design sog reinforcing to 7.12.2.1?

In point of fact,  ACI 318 specifically notes that it does not apply to soil supported slabs except blah, blah, blah.

I have actually never read of any recommendations that 7.12.2.1 be used as a guide for sog reinforcement - the intent of the reinforcement is completely different.  7.12.1.2.1 is structural integrity reinforcement - if your shrinkage cracking in an elevated slab is significant, you can potentially lose the integrity of your slab, you can also end up with corrosion damage to your primary reinforcement.

There are no code requirements for steel in slabs on ground because it is not a structural issue, unless the slab is design to support part of the building.  The steel in virtually all slabs on ground is not structural - it is for serviceability and/or aesthetic issues.  The steel keeps the cracks from opening.  It does not prevent cracking. 

There are lots of industry recommendations.  The recommendations are typically a function of the joint spacing.  The subgrade drag equation is often used - that often comes up with about 0.06% (.0006),  however, it is generally acknowledged that this amount of steel does nothing but get in the way.  A minimum of 0.1% (0.001) is generally recommended. There is really no logical justification for the subgrade drag equation, but designers latched onto it because it was something to calculate.  Researchers at University of Texas came up with a "modified subgrade drag formula" that about doubles the steel, but that never caught on. 

ACI 360 has some good information about shrinkage, etc., but it should be recognized for what it is,  i.e. the opinions of a group of people,  many of whom are not engineers.  Its the fiber salesmen, the dowel salesmen, the admixture salesmen, construction management majors, etc. Some of these people are very good in their particular field of expertise, but some are mostly interested in promoting their widgets and/or advertising their "expertise."  Construction management majors in particular tend not to have a good understanding of concepts like stress and strain.

The existing 360 is one set of opinions.  There will be a new 360 published within a couple of years, with a new set of opinions.  The new 360 will say something to the effect that if you have 0.1% or less steel,  it should be designed as if there were no steel, and joints should be at 24t, with a maximum of 12 ft.  Or maybe 15 ft - these are all empirical numbers, so it is hard to come up with a consensus for one number versus another number.  The committee discussion on a particular number can last several hours.

At greater than 0.1% steel, joint spacing can be increased.  At 0.5% steel,  contraction joints may be unnecessary - the so-called "joint-less" slab.

The publication by Packard on Thickness of Industrial Slabs is a very good write up on how to use the PCA method to select slab thickness,  but it says nothing about recommended steel.  The PCA method assumes that joints will be so closely spaced that no steel is necessary for shrinkage stresses.

Interestingly,  the 2005 ACI 318 will explicitly exclude post-tensioned residential slabs on ground.  It is not clear why just post-tensioned slabs are being excluded.

Gail Kelley


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