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RE: Bolt Pre-Tensioning

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The pretension force on an A325 bolt is much higher than the allowable applied force.  The clamping force will therefor not be exceeded by the applied force.  That is why machinery and dynamic loads require "torqued" bolts.  By tensioning the bolts the bolts will be under a constant stress and will not experience any cycles in stress and will not fatigue.

I have seen strain gauges on bolts in assemblies that were subject to fatigue.  The bolt stress does not cycle.  It remains constant.  The only exception is when the assemblage goes nonlinear and the prestress force (clamping force) is exceeded by the applied force.  

The pretension force and the applied forces are not additive.  If that were the case you are overstressed when you install the bolt.

The only way to exceed the pretension force in a bolt is to have an applied force greater than pretension force, which greatly exceeds the allowable capacity of the bolt.  

It is a very similar analogy to post tensioned concrete.  The PT cables do not see any increase in stress until the PT force is exceeded.

Harold Sprague, PE
Krawinkler, Luth & Assoc.


-----Original Message-----
From:	Dan Huntington [SMTP:huntingtondj(--nospam--at)kjww.com]
Sent:	Wednesday, July 22, 1998 10:01 AM
To:	seaint(--nospam--at)seaint.org
Subject:	Bolt Pre-Tensioning

I'm hoping someone can help me understand allowable bolt design
strengths more clearly.  Per Chapter J of AISC, when there is a bolt
subjected to direct tension, we are required to pre-tension the bolt to
70% (for A325 bolts) of it's design tensile tensile strength (0.70 x Ab
x Ft).  Yet we do not take this into account for the allowable design
value (0.75 x Ab x Ft).  Thus, we seem to be designing the bolt to 0.70
+ 0.75 = 1.45 x Ab x Ft.

I spoke with Abe Rokach, Dir. of Buidling Design for AISC, about this.
He indicated that even though the pre-tensioning force and design force
were additive, the bolt would still be well within it's ultimate
strength (1.00 x Ab x Fu).

I don't believe calculations support his explanation.  Nor do I find it
plausible the code is purposefully allowing the designer to double-dip
on capacity without tclearly conveying this to the designer.  Any
insight would be appreciated.

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