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Re: Pretensioning Of Anchor Bolts

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Thanks for your comments Harold. I frequently forget that there are steel 
structures other than the buildings for which AISC's coverage is 
primarily intended.
	I'm not sure about the stress corrosion issue either. I suspect 
you are correct that some grades are more sensitive than others. I'll 
look into it and let you know what I find out.

Charlie

Harold Sprague wrote:
> 
> Charlie,
> 
> I agree with you in general.  The process is trickier and requires
> specialized inspection.  Not all special inspectors can do this.
> 
> The designer should specify the process and inspection required.  Bond break
> is essential along the shaft.  A headed anchorage is also required.  And the
> A325 A490 specification does not apply.
> 
> I generally design for lower service loads and pretension forces.  I still
> like to have about a 20 to 30% larger pretension load than the service load,
> but my pretension load will be a function of the applied service load and
> will be within the allowable stress range.  This assures me that I will not
> have a failure while applying the pretension load, nor will I exceed the
> clamping load in a service load condition.  This will also reduce the
> relaxation due to creep.
> 
> One thing that is still a bit of a puzzle is the tendency of stress
> corrosion.  I generally use ASTM A193, B7.  Stress corrosion is unique to
> specific alloys, and I have never seen anything to indicate that A193, B7
> has a tendency for stress corrosion.  Stress corrosion is characterized
> local corrosion and hydrogen production in a crack or pit in a stressed
> element resulting in crack propagation.  I have never seen this documented.
> Stress corrosion can be mitigated by coatings.  I would discourage
> galvanizing as this can lead to hydrogen embrittlement with some alloys.
> Stress corrosion is not the same as hydrogen embrittlement.
> 
> I would not discourage pretensioning anchor bolts.  There are several
> thousand elevated water tanks with properly pretensioned anchor bolts.  But
> I would advise a good design, specification and inspection.
> 
> Regards,
> Harold Sprague, P.E.
> The Neenan Company
> harold.sprague(--nospam--at)neenan.com
> 
> -----Original Message-----
> From: Charlie Carter [mailto:carter(--nospam--at)aiscmail.com]
> Sent: Tuesday, October 27, 1998 3:14 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Pretensioning Of Anchor Bolts
> 
> 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
>