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RE: Bolt Pre-Tensioning

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As I understand the analysis, the pretension force is constant until the
applied tension (due to seismic, wind, etc) causes a tension which
exceeds it.  Thus the initial tension CAN be exceeded, but the two are
NOT additive as Rokach suggests.  (It's the same principle as a
pretensioned beam)

> ----------
> From: 	Robert Rollo[SMTP:rrollo(--nospam--at)]
> Sent: 	Wednesday, July 22, 1998 9:01 AM
> To: 	'seaint(--nospam--at)'
> Subject: 	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)] 
> 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. 
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