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Re: Pre-engineered Metal Building

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I think we are in an area of personal preference, or maybe engineering judgment.
 
PERSONALLY, I would not use WWF to transmit load.  If, in fact, the WWF is in the slab and not laying on the subgrade or vapor barrier, its purpose is to hold the drying shrinkage cracks together to facilitate aggregate interlock.  Loading the WWF in tension reduces its crack control effectiveness.
 
I didn't really mean to say that the coefficient of friction between slab and vapor barrier was zero.  I meant to say that's what I'd assume it to be.
 
For the young engineers out there stuck with having to specify WWF as tension steel, my recommendations: 
-  Provide enough As to carry load in addition to that required for temperature and shrinkage.
-  Require chairs or blocking to hold the WWF off subgrade during concrete placement, then inspect it.
-  Specify WWF lap splice requirements.
 
John P. Riley, PE, SE
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Doesn't the hairpin lap with the WWF in the slab, which then can act as the tension tie reinforcing between opposing columns.  I thought that that was why the hairpins always had a wide spread on them, to engage enough WWF (to give you enough As) for the tension.  With this assumption, it is possible to assume no slab to subgrade friction.
 
Of course, the above assumes that the WWF is placed properly in the slab.
 
On the other hand, a slab on a vapor barrier does not have a coefficient of friction of zero.  I would, however, agree that the coefficient is lower than if no vapor barrier is used.  I seem to recall that the PTI design manual for slab on grade design gave some rule of thumb values for the friction coefficient for a slab with a vapor barrier.  Unfortunately, I no longer have a copy of it, so I can not pass along the value(s).
 
Charles F. Espenlaub, III, P.E.
Martin-Espenlaub Engineering
Tel  215-665-8570
Fax 215-561-5064
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I have a problem with using hairpins, PERIOD.
 
The hairpin solution to horizontal thrust requires resistance to be developed through friction between the slab and subgrade.  If the slab is cast directly onto the subgrade, I would assume no more than about 0.30 for the coefficient of friction, and that's prior to applying a factor of safety (suggest 1.5 minimum).  And if a vapor barrier is used under the slab, I suggest the coefficient is zero.  Furthermore, you must determine the size of slab mobilized and reinforce accordingly.
 
A better solution, in my opinion, is to tie opposing piers together with deformed bars.
 
By the way, hairpins are used around these parts, without problems as far as I know.  And if I could get them to work on paper, I'd specify them because they are cheaper.  Probably the reason they work in real life is that passive resistance is achieved in the backfill soil.  But passive resistance offered by soil within frost depth (42" here) should be ignored.
 
Just an opinion.
 
John P. Riley, PE, SE
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I have a situation where the bearing elevation for the columns of a metal building is below the finished floor elevation by 4".  The slab is 6" thick.  The detail I usually use has the hairpin anchored to the anchor bolts though a mechanical connection.  This is not possible in this situation.  Has anyone seen a detail where the hairpin runs through a hole drilled in the column web?  Does anyone see any problems with this?  Any help would be appreciated.
 
Thank you,
Chris Towne, E.I.T. (Pending Test Results)
Chapman Technical Group