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RE: Partial Rigidity (was ALGOR, EAGLE, ETABS, STAAD etc.)

• Subject: RE: Partial Rigidity (was ALGOR, EAGLE, ETABS, STAAD etc.)
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Tue, 9 Jun 98 09:32:53 -0500

```>Based on the analysis result, we can design member
>sections, and then design the connector. It is necessary to calculate
>the connector stiffness and check if the connector stiffness is
>satisfied with analysis output. If not, adjust the connector strength
>and re-start the process again.
Not to rain on the parade, but I have a couple of problems with 'partial
rigidity,' and I think they bear on the notion of an 'exact' analysis.
First, connections aren't linear. Bolted connections in particular
involve friction and slip into bearing; both welded and bolted
connections carry residual stress (preload in the case of bolts) and both
have a very complex stress distribution, despite the way we idealize the
design. Consequently I doubt that connections really have a definite
could take a guess, maybe even do a 3D FEA model of my idealized
connection, but I wouldn't bet the farm that it'd match the real world
response in any recognizable way.

Statical determinacy also has a role, particularly when you're talking
limit analysis. Indeterminate load components redistribute themselves;
determinate loads don't, so it's hard to imagine the effects of guessed
stiffnesses on the ultimate load assessment. Perhaps it's possible, but
the underlying question is whether it can be done accurately (in the
classical sense of matching performance in the field) and whether it
makes economic sense.

Forgetting the effect of the idealizations for a moment, the process
described by Luo seems open ended. At both ends. You need loads to assess
strength, although the loads depend on the stiffness. But you don't know
the stiffness until the connection is designed, which isn't possible
without loading. There doesn't seem to be any place to start the process
unless you've  started already. It's different for member selection,
because you can select sections based on statical determinacy and tighten
stiffness starts to affect load transfer from the beginning--make a
connection stiffer and it picks up more load, possibly requiring
redesign, which again affects the stiffness. It has the faint odor of a
diverging process.

It seems to me that we're adding an enormous amount of complexity to
include second order effects when we really don't have all that firm a
grip on all the first order approximations we use in connection design. I
make a fairly good living doing finite element analysis; it's a
marvellous tool for putting numbers on judgement calls. But it's real
easy to get dazzled by the ability to do difficult arithmetic and assume