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Re: AISC LRFD Bearing Strength at Bolt Holes

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Heath,

It is fascinating to see how design philosophy differs somewhat from what
manufacturers of structural fasteners generally assume to be how our
fasteners function in shear.

For example, you wrote:

"I agree that the connection capacity would be the sum of the individual
bolt strengths. I think that this approach can be justified for normal
length connections with the assumption that as the material ahead of the
edge bolt starts to yield the load will be redistributed to the other
bolts in the connection, until they all yield or tear."

I would say fastener producers typically agree with that statement . . . in
aerospace applications or under circumstances where the bolts are installed
in an 'interference fit' application.  (i.e. no clearance between bolts and
holes.)  That's the only way we feel we 'know' they are working in unison,
meaning that the sum of their individual shear strengths relates to the
connection's shear capacity directly.  This would be similar to the way
rivets respond due to the lack of a hole around the fasteners.  (With
various clearances at different compass positions.)

In 'loosely toleranced' holes such as we have in structures, we tend to
think of connections operating like a zipper in shear, with one or a few
bolts in shear while others are not even bearing on the plates until much
more displacement has taken place and after the some significant shearing
has already taken place on some of the bolts.  A good graphic depicting what
I'm describing may be found on page M-59 in the Industrial Fasteners
Institute's 6th edition of the Fastener Standards Book.  Consider also how
permission to use 'over-sized' holes could make a big difference.

Right or wrong, I know we producers tend to think this way.  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 comparitively 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.)

I will go further to admit that the ductility typical to an A325 strength
bolt may very well mean total shear capacity is not compromised (or very
little) when such 'zipper action' is taking place.  On the other hand, I
would have a hard time accepting that A490 bolts would respond in the same
way.  Although we aren't necessarily including impact in our discussion,
observation of shear connections in the wake of the WTC attacks suggests
bolt shear can be a very complex subject.

It would be an interesting topic for me to see some research from the
structural market to compare with aerospace or automotive papers.

David Sharp

p.s. I've found the friendly disagreements here lately to be a great
opportunity from which to learn.




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