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# RE: Bolt Pre-Tensioning

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Bolt Pre-Tensioning
• From: Robert Rollo <rrollo(--nospam--at)TEAM-PSC.com>
• Date: Wed, 22 Jul 1998 11:01:09 -0500

Title: RE: Bolt Pre-Tensioning

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)kjww.com]
Sent:   Wednesday, July 22, 1998 11:01 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.

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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