BRACED FRAME BEAM UNBRACED LENGTH FO AXIAL 5

[F1955]

IF YOU HAVE A BRACE FRAME BEAM WITH AXIAL LOAD BRACED AT 2'-0" O/C AT THE TOP FLANGE, WOULD YOU CONSIDER IT BRACED ABOUT THE Y AXIS FOR AN LY=24" OR USE THE ENTIRE LENGTH CONSERVATIVELY IN THE Y AXIS?

 

[dozer]

If the braces don't provide torsional restraint then torsional buckling of the overall length is a possibility. Unfortunately, this is more complicated than I have time for right now.

I highly recommend attending the "Basic Design for Stability - Columns and Frames" seminar offered through AISC. I just went to the AISC web site and looks like you missed this round of seminars but keep your eyes peeled.

Maybe you can find someone who has notes from this seminar. Good luck.

 

[Taro]

AISC's steel solutions center will fax you the relevant portions of the seminar notes.

Another good reference for axial-torsional buckling with a more in-depth discussion is Guide to Stability Design Criteria for Metal Structures, edited by Galambos.

 

[dawn836]

well when i face this situation and i want to make this member brace the column, this member must be able to hold about 2% of the axial force the column have.

Following the path of this force you will find that it will make My on the last column of your building which is connected to this brace so you must put in mind this considerations.

 

[dozer]

The paper, "Fundamentals of Beam Bracing," by J.A.Yura, states: "...design rules based on strength considerations only, such as a 2% rule, can result in inadequate bracing systems.

Actually, if this is a discrete brace system you could actually use 1%, however, stiffness of the brace must be adequate. Again, I refer you to the seminar I mentioned above. Hopefully, we can wipe out the design of braces using strength only in our life time.

 

[SmithJ]

F1955, if I understand your question correctly, you are wondering if Top Flange Bracing ONLY is adequate to brace a beam that is taking axial (compression) load.

My answer would be NO. I would not use an Ly of 24" unless both flanges are braced or the brace extends at least 2/3 of the depth of the beam. As Dozer noted above, if the brace does not provide torsional restraint, then it is not adequate.

On the other issue raised, I have always used the strength method (2% rule) as a basis for my bracing design. Dozer, Am I to understand that this is no longer acceptable? I have read the "Bracing for Stability" ourse by Yura and Helwig but never did come away with an effective way to determine the stiffness or stiffness limits of a brace (discrete or othwerwise). Do you have any info or resource that could better clarify this question for me?

Thanks.

JS.

 

[F1955]

The member is a roof beam braced at 24" with wood roof trusses. Would you consider a composite beam braced?

 

[dozer]

SmithJ,

I have "Bracing for Stability" dated Feb 2001. This was the booklet that came with the seminar at this time. Within this booklet is a paper titled "Bracing for Stability - State of the Art", by Joseph Yura, rev'd 11/7/99. This gives stength and stiffness requirements for four different types of brace systems.

 

[haynewp]

I have read some articles on the ability of a composite beam to resist torsional buckling under axial load. The headed stud has some capacity to restrain the beam from twisting about the top flange.

For your other beam with the roof trusses at the top chord, I would also say no it is not adequately braced for axial loading. If you could also attach kickers from the truss to the bottom flange of the beam, and the trusses are adequate to take the brace load, then I would consider the beam braced.