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Re: Lift Lug

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>The design, while technically correct, seemed odd because of a large hole to
> accommadate a smaller pin. Am I missing something?
I think so. I also think your lug is probably overstressed, although I 
haven't seen your numbers. You should be especially careful because 
lifting lugs are subject to fatigue failure since the loading tends to be 
cyclic and dynamic.

The AISC requirements obscure the fact that the load at the tension 
section comprises both a moment and a direct load. (Verify this for 
yourself by making a free body diagram of the segment of the lug between 
the tension section and the symmetry centerline. The reaction is offset 
from the applied load resulting in a bending moment equal to half the 
applied load times the average of the hole radius and the lug radius.) 
The maximum stress occurs at the hole ID and is the sum of the bending 
stress and the direct stress figured in the usual way. And don't forget 
that the curvature of the lug wants to increase the bending stress over 
and above the value calculated by Mc/I.

The bottom line is that the load is carried by an area fairly close to 
the hole. By increasing the hole diameter you increase the bending stress 
because you increase the offset. You don't want to do this. The apparent 
intent of the AISC rule is to keep the bending stress to a minimum and 
'simplifying' the calculation by specifying appropriate details to cover 
the moment loading so we can pretend it doesn't exist. The requirement 
for a 1/32 in diametral clearance has nothing to do with rattling, by the 
way--it's intended to minimize the bending moment.

This is a good example of basic engineering mechanics obscured by 
simplification gone wild, not unlike all the bugger factors and provisos 
and such applied to seismic design. It's also a good example of what 
happens when we violate the Einstein principle ("Things should be made as 
simple as possible, but not any simpler.")



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