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Re: Bolt Pre-Tensioning

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If you do a 'free-body' analysis you'll see that the applied force will not increase the tension
in the bolt *until* the pre-tension force is exceeded.


Robert Rollo wrote:

> this one always baffled me as well.  last time i did this, i recall finding a reference that
> finally satisfied my "intuition". McGuire maybe? If I remember right, it had to do with where
> you cut your free-body of the system and it proved that you would never exceed the initial
> pretension force?  Someone throw in here with me. am i hallucinating a false memory? if so
> i've got some sleep to lose.
>      -----Original Message-----
>      From:   Dan Huntington [SMTP:huntingtondj(--nospam--at)]
>      Sent:   Wednesday, July 22, 1998 11:01 AM
>      To:     seaint(--nospam--at)
>      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.