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Unbraced Length

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Title: Unbraced Length

A new document entitled TR14:  Designing for Lateral-Torsional Stability in Wood Members http://www.awc.org/Publications/TR/index.html, which obviously does not cover steel, does provide some helpful background information on lateral torsional stability (including derivation of lateral torsional buckling equations in Appendix A). Here's the description:

Lateral-torsional buckling is a limit state where beam deformation includes in-plane deformation, out-of-plane deformation and twisting. The load causing lateral instability is called the elastic lateral-torsional buckling load and is influenced by many factors such as loading and support conditions, member cross-section, and unbraced length. In the 2001 and earlier versions of the National Design Specification(r) (NDS(r)) for Wood Construction the limit state of lateral torsional buckling is addressed using an effective length format whereby unbraced lengths are adjusted to account for load and support conditions that influence the lateral-torsional buckling load. Another common format uses an equivalent moment factor to account for these conditions. This report describes the basis of the current effective length approach used in the NDS and summarizes the equivalent uniform moment factor approach; provides a comparison between the two approaches; and proposes modification to NDS design provisions.

HTH

Buddy
John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791
http://www.awc.org
The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.

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The guidance provided herein is not a formal interpretation of any AF&PA standard.  Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.

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From: Udall, Jeffrey D [mailto:JDUdall(--nospam--at)babcock.com]
Sent: Wednesday, December 10, 2003 10:36 AM
To: seaint(--nospam--at)seaint.org
Subject: Unbraced Length
I'm presently doing a literature search as part of a university thesis
project. I'm hoping this list can give me some assistance.
I'm trying to determine the critical bending moment of a beam when it is
subjected to a uniform load. The beam is continuously supported laterally
and torsionally on the tension (bottom) flange. The top flange
(compression) is not restrained. Current practice is to consider the
unbraced length of the compression flange as the full span of the member.
In order to reduce this span, stiffeners are added at appropriate
intervals that essentially tie the compression flange back to the
laterally braced tension flange.
However, these stiffeners are costly in terms of labour, especially when
there are large numbers of them and the spans are significant. I am
trying to determine the effective span of the compression flange given
that it really is attached to the tension flange through the web. The
stiffener connects the two flanges together by treating the stiffener as a
column, and is therefore quite stiff. The web serves the same purpose but
acts only in bending in the weak axis. This is not nearly as effective,
but it does provide some degree of strength that is being ignored. I am
hypothesizing that the effective unbraced length of the compression flange
is less than the full span length, and determination of such will reduce
1. the need for stiffeners, and/or 2. the size of the beam.
I'm reviewing some of the texts by Nethercott, Bleich, Galambos, and a few
others, but I can't seem to find my specific situation. Galambos (in his
Stability book 5th ed.) makes mention of it but does not elaborate.
Has anyone looked at this arragement before? I'm looking for references
if you've got them.
Thanks.
Jeff Udall
Cambridge, Ontario (Canada)