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Re: Bolt Pre-Tensioning

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By the time the actual load on the bolt gets to the design strength (if 
it ever even gets close), all of the pretension has been lost due to the 
shear deformations. Thus, the design strength is independent of the 
installed preload in the bolt.

This issue is covered clearly in both the Commentary on the RCSC 
Specification for Structural Joints (in the AISC Manual) and in the Guide 
to Design Criteria for Bolted and Riveted Joints.

Hope this helps.

Charlie

Dan Huntington wrote:
> 
> 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.
>