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# Re: Unbraced Length

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Unbraced Length
• Date: Wed, 10 Dec 2003 10:14:01 -0800 (PST)

I am not aware of any published literature which addresses the situation you have described.

By definition, the twist at the ends of a laterally unbraced span is zero. While the addition of the vertical web stiffeners locally increases the torsional stiffness of the beam, the twist, at that location, does not become zero. Intuitively, however, the allowable compressive flexural stress (F sub b) of such a stiffened beam should be greater than the F sub b calculated on the basis of the full beam span.  By how much? Only an experimental research work can provide an answer. Alternatively, you may develop a finite element program to solve the governing differential equations and run parametric studies.

Finally, I am curious about the structural arrangement described by you. Are you referring to a beam where a load is hanging from the bottom flange, such as a monorail?

Rajendran

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

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