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RE: AISC LRFD Bearing Strength at Bolt Holes

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David-

I agree with the points you made, but I think that we are talking about
2 different, yet related issues. But, since I haven't been exposed to
the automotive/aerospace perspective, I may be missing something. Let me
know if I am!

If I correctly understand what you've said (including the IFI figure),
it seems that you are looking at this from the perspective of bolt
performance. I look at the bearing limit state as a function of
connected material (plate) performance. The typical connection materials
used in buildings have yield and tensile strengths well below those of
high strength bolts. So, for typical plate thicknesses, the connection
plate stiffness (and strength) is below that of the bolts and bearing
load redistribution could be expected to take place through
deformation/yielding of the connection material. With the bolts in
standard holes, I would expect the post-yield deformation capacity of
the plate (at the end bolt) to be sufficient to allow the plate at the
interior bolts to attain its design bearing strength. However, as
connected ply thicknesses increase and/or higher strength connection
material is used, I would be concerned that load redistribution would be
forced through bolt deformation. This approaches the conditions you
discussed and is not desirable performance in my opinion either. I would
be interested to hear of research that investigated the effects of thick
connection plies and/or high strength plate material.

I think the issue you brought up concerning bolt shear strength as a
function of hole size has been addressed, although it is not apparent by
looking at the specification equations. The design shear strengths
specified by AISC and RCSC incorporate a 20% reduction in bolt capacity
due to joint length effects. This covers joints up to 50 in. long. IIRC,
the tests involved bolts in standard holes, so the reduction seems to
account for 'loosely toleranced' holes and well as load distribution to
bolts in large joints.   

You said:
"It is not uncommon to see large connections in which some bolts are
against one edge, and others against another edge, and so on due to
comparatively imprecise hole placement.  (And, if we've had any doubts
over the years, we remember those times we've seen the 'beater' or
sledge used to drive those bolts in. . . up against one edge or
another.)"

Good point. I doubt that the research accounted for this. From my
limited experience in a research environment, misalignment of holes was
typically not a problem and the tests I did probably started out with
near uniform bearing on the bolts.

"Consider also how permission to use 'over-sized' holes could make a big
difference."

I agree. Technically, once oversized holes are permitted, the connection
has to be designed as slip-critical. But, if it is not, I would expect
the joint length effects to be beyond what is assumed in the bolting
specs.

I get the impression that fastener design in other industries is more
involved in this one. It seems that assumptions used to arrive at the
structural design equations incorporate issues that are explicit
considerations in the design of fasteners for other applications. Is
this the case?

I'd be interested to read the aerospace/automotive research (well, at
least some of it). Any suggestions?

Back to you...

-Heath

p.s. Friendly disagreements help to give me a different perspective on
things. It's nice to get a different view than the one from my cube! 

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