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RE: Steel beam unbraced length limit

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I can understand the concern, where you have a very low allowable stress (or allowable moment for LRFD) because you are way out on the design curve into the elastic range of buckling. But supposively the AISC equations have taken this into account, and if it meets Code requirements (even with a 50ft unbraced length) then I guess we should be covered.


Will Haynes



From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Steel beam unbraced length limit
Date: Fri, 25 Feb 2005 09:12:01 -0500

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.

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Will Haynes [mailto:gtg740p(--nospam--at)hotmail.com]
> Sent: Thursday, February 24, 2005 1:50 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Steel beam unbraced length limit
>
> I have designed steel beams with unbraced lengths up to 20 ft
> before (above "Lr" value).  I have had other engineers tell
> me they won't design for an unbraced length this high even if
> it works by AISC equations, I guess due to their fear of
> fabrication and erection influences on the beam and this
> being magnified by a larger unbraced length.  Isn't this
> already accounted for in the AISC equations?  Any thoughts?
>
> Will Haynes

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