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Re: Steel beam unbraced length limit

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> > From: Will Haynes [mailto:gtg740p(--nospam--at)hotmail.com]=20

> > I have designed steel beams with unbraced lengths up to 20 ft=20
> > before (above "Lr" value).  I have had other engineers tell=20
> > me they won't design for an unbraced length this high even if=20
> > it works by AISC equations, I guess due to their fear of=20
> > fabrication and erection influences on the beam and this=20
> > being magnified by a larger unbraced length.  Isn't this=20
> > already accounted for in the AISC equations?  Any thoughts?

> From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>

> In general, I would not place a limit on unbraced length as long as the
> beam meets the code requirements. However, I sometimes get nervous when
> the allowable stress based on the code is very low - although this
> forces the applied loads to also be very low, it implies that the beam
> does not have much lateral stiffness.=20

I recall a circumstance with a long span beam in a building designed by
an engineer under my suoervision. The design was good. The top flange
was properly braced; P-delta and lateral-torsional issues had been
considered. In drafting, a line was left off the drawing and it was
missed in review. An HSS strut was missing in the field (simple line,
big issue). The strut was fabricated and installed.

The erector noted to me that they could lightly push on the bottom
(tension) flange and it would flap like a flag. They took some joy in
showing this to everyone on site. This tightened up considerably with
the added strut but it highlighted that good design was not just about
meeting the stress limits in the design standard but designing to the
real life of possible conditions. A good interior air gust, or a
careless maintenance crew with a man-lift, combined with full roof load,
could make the design standard limits pointless.

The Yura/Hellwig work for the SSRC pointed out that section rotation
stiffness at torsional brace points can have an influence on beam
capacity. Also related, one of the issues that is hidden in the design
standards, is the affect of load application position (e.g. top flange,
mid-web, bottom flange) to section stability and capacity. A braced top
flange (e.g. floor slab) does not mean that your total member is
rotationally restrained but the design standards do not include an
unbraced torsional length (e.g. K*Lt). 
 
I think that 20'-25' unbraced for the flange of a mill W beam is not an
unreasonable limit. More is possible but probably not realistic or
economic with a simple W section or plate flange.

Regards
Paul Ransom

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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