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RE: Pretensioning Of Anchor Bolts

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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