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Re: Pretensioning Of Anchor Bolts

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Pretensioning of an anchorage into concrete is a tricky undertaking for 
several reasons. Assuming you can induce the desired pretension, you have 
to eliminate bond along the length to ensure that the pretension is 
actually induced over the full length of the rod. Otherwise, the bond may 
release over time and relieve any pretension that was induced. I suspect 
a headed or nutted end on the embedment would be required...a hook would 
probably just pull out.
	Compounding this problem, I'm not aware of a specification or 
code that provides a procedure or even a minimum installed pretension for 
anchorages that are of the length that is typical of an anchor rod. The 
minimums for A325 and A490 bolts are for steel-to-steel structural 
bolting applications only.
	Also, creep deformations in the concrete can relieve the 
pretension even if it was induced in the first place.

All this and stress corrosion considered, hence AISC's stated 
recommendation as quoted below.

Charlie

Rick.Drake(--nospam--at)fluordaniel.com wrote:
> 
>      The Commentary to the LRFD Specification Section A4.3 indicates that
>      "The designer should be aware that pretensioning anchor bolts is not
>      recommended due to relaxation and stress corrosion after
>      pretensioning."
> 
>      Does anybody have any background or references regarding this
>      recommendation.
> 
>      Thanx,
> 
>      Rick Drake, SE
>      Fluor Daniel, Irvine
>