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Re: Pretensioning Of Anchor Bolts

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> So, where does the bolt get its capacity
>to resist external moment in addition to bearing the pre-tensile stress?
The tensile bolt preload induces compressive forces in the material it's 
bolted to. The preload exactly balances the compressive clamping force. 
When an external load is applied the compressive clamping force drops 
off; if the material being clamped is stiffer than the bolt the clamping 
force decrement is about the same as the external load increase, so the 
bolt tension stays about the same. If the resulting deformation is large 
enough to eliminate the clamping force, the bolt load starts to increase. 
You can prove this arithmetically or with a very simple finite element 
analysis if you have an hour or so to putz around.

When you get a fatigue failure in a bolt, the reason is usually 
insufficient preload. I've had a lot of people hand me broken wheel lug 
nuts bitching about their being overtightened, when in fact they weren't 
tightened enough, probably the tech forgot to tighten them past finger 

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)