From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
Date: Fri, 9 Feb 01 17:26:16 -0600
>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)