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> From: "Udall, Jeffrey D" <JDUdall(--nospam--at)babcock.com>

> > Stability bracing. Lateral-torsional. See Yura/Hellwig and the SSRC
> > Guide to Stability Design Criteria for Metal Structures. Note that
> > their recommendations are conservative. It depends on how much time you
> > want to spend to optimize the design.
> 
> 
> Where is the conservatism in their recommendations? =20

If you have attended one of the Yura/Hellwig roadshows, they make an
issue of the fact that this is a very complex subject and that they have
boiled it all down to make the analysis recommendations commercially
practical.

Yura/Hellwig torsional restraint at mid-points on a flexural member
(case presented above): recommended to use the worst case at all points
on the member. Ostensibly, this is to make engineering time more
efficient and the extra cost in steel is usually minimal. This may be a
problem with architecturally sensitive details.

For convenience, an arbitrary rotation bewteen upper and lower flanges
was selected, with caveats, for the analysis recommendations.

If unbraced lengths are variable along the member, what KL do you use
for each segment in the bracing analysis? They acknowledge that their
research has not included this aspect and will verbally suggest that you
use engineering judgement (i.e. do what will allow you to sleep at
night).

If you look closely at the background and equations you will find the
other points of assumption or decision.

Of course, this approach is not really much different than former
conventional practice: The old rule-of-thumb of 2% brace force is an
"upper limit". It is made more conservative by using the ultimate
resistance of the member rather than the actual applied force.
Additionally, bracing is usually designed without consideration of other
attachments having strength and stiffness.

Don't let bracing be a black box analysis. Know the limits.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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