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Re: Lift Lug

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--- Christopher Wright <chrisw(--nospam--at)skypoint.com> wrote:

> You should be especially
> careful because 
> lifting lugs are subject to fatigue failure since
> the loading tends to be 
> cyclic and dynamic.

The lifting lugs will be used just twice. Once in the
fabrication shop to set the skid on a trailer and once
at the site to offload it at the skid's location.
Therefore, cyclic loading or fatigue is not an issue.

> The AISC requirements obscure the fact that the load
> at the tension 
> section comprises both a moment and a direct load.

I calculated the bending moment, at the location
indicated by you, and determined the corresponding
flexural stress. It is low. In fact, I assumed that
the load is applied as a concentrated load, although
in reality it will be a distributed load across the
diamter of the pin. Thus I did check the tensile
stress and the flexural stress.

> 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.

The empirical rule to keep the stresses in check is
provided through the conservative allowable stress of
0.45Fy instead of the normal 0.6Fy. So as long as the
combined stress due to tension and flexure is limited
to 0.6Fy, the design should be okay. I wonder if the
framers of the Specifiaction can enlighten us on the
restriction of hole dia to 1/32 of the pin dia.

In practical situations, it will not be economically
prudent to design different lugs for different skids
within the same project. I have some 50 skids, which I
have grouped into 3 categories. In each category, I
have designed a skid with the maximum weight. Assuming
that the rigger will use an appropriate shackle to
lift each skid, it is more than likely that in amny
skids, the pin dia. will be smaller that the hole dia.
by more than "1/32...".

> 
> 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.")

All the more the need for explanation to such
"empirical" design restrictions, within the
specifications. 

P. Rajendran


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