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Bolt Pre-Tensioning

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Dan (and others),

Maybe this explanation will help:

Put the palms of your hands together and press so that you are pressing them 
together with, say, 10 pounds of force.  Now, have co-workers (or your 
children) try to pull your hands apart.  Until they pull with at least 10 
pounds of force, there will still be some pressure between your palms.  The 
force from your co-workers (or children) is not additive to the initial 10 
pounds of compression, but merely reducing the amount of compression.

Alternate explanation:

Take a spring scale and attach it to a wall and apply, say, 10 pounds of 
force.  Have a co-worker or a child take a second spring scale and attach it 
to your hand.  Until the second spring scale reaches 10 pounds, there will be 
no increase in the force in the first spring scale.

Hope this helped.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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.