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Re: QUERY: Fatigue

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>Sharpshooter! (in jest!)
I'm sure there's another term for someone who stands off and potshots 
other people's opinions, but I doubt it's as graphic. And again, I'm 
reluctant to give even the appearace of such, since I've been on the 
receiving end of a few. Your comments are well taken. 

>E. "You have taken care of fatigue" was intended to mean that the fatigue
>requirements have been met. The stress limitation of 16 ksi for the type of
>weld as per AISC for the number of cycles, and 0.5 Fy (endurance limit) in 
>the base metal at other points.
My reply was meant to caution that there's nothing magic about 16 ksi. 
There are plenty of welding details which might be considered for a 
fabbed T-section where the 16 ksi limit would be iffy at best. I don't 
have any particular problem with using 50% of the yield strength as a 
fatigue limit, since it's a conservative estimate for situations without 
great stress concentration. Given the wide variety and quality of welded 
details in building construction, using1/2 Fy as the fatigue limit would 
range from optimistic to foolhardy as a rule of thumb. As an example the 
base metal at stitch welded connections would have a stress category of E 
or F in many instances and have an allowable well below 16 ksi.

>There is a lot more to
>Fatigue than what has been discussed, but that may not be of immediate
>interest to the forum.
It probably should be. I suspect building design is hell bent on learning 
fatigue and fracture in weldments the hard way. Pressure vessels (where I 
learned about fatigue) and offshore structures and ship building before 
us all seemed to have their own separate learning curves. Building design 
has been living with structures where dead load pretty much dominates 
things, and fatigue is still pretty much an afterthought. I don't think 
the emphasis on limit design and 'prequalified' weld joints is helping 
much. The moment connection failures in the Northridge earthquake were a 
wake-up call in the area of weld quality--so we'll see if it takes. 

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