×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Lateral Torsional Buckling Bracing...

Lateral Torsional Buckling Bracing...

Lateral Torsional Buckling Bracing...

(OP)
This is something that has bothered me for a long time, but couldn't ask because I thought it was something that was really trivial.

How can we determine that said beam is sufficiently braced somewhere along its length so that we can take L smaller than the beam length? For example, secondary beams spanning into the primary beam (without any bracing.) How can we determine that the secondary beams will brace the primary beam enough to cause an inflection of the buckled profile? Also, what criteria do we use to determine that a plate floor or a concrete slab braces the compression flange of the supporting steel beam sufficiently enough to allow us to only use Mr = SFy without using the lateral torsional buckling equation. I have a feeling that we neglect any beneficial restraint from floors, but I don't know why. Seems like floors can offer excellent restraint for lateral torsional buckling of beams.

Thanks! Would also appreciate links to reference for further reading,

Clansman

RE: Lateral Torsional Buckling Bracing...

Check out the new AISC Manual Appendix 6.  It has a section on beam bracing that covers this question.

As for a slab on the compression flange, that will most always provide LTB restraint.  It's hard to think of a time that it wouldn't.

RE: Lateral Torsional Buckling Bracing...

Oh yeah, other reading.

Most offices have at least one copy of the Yura/Helwig AISC bracing seminar notes.  That's the best source of guidance.  

The SSRC Guide to Stability Design Criteria has a chapter also.  

Finally, there are several papers, probably the best place to start is the AISC EJ paper by Yura called "Fundamentals of Beam Bracing."

RE: Lateral Torsional Buckling Bracing...

Please check out the discussion on a contemporary thread "Web Stiffeners for Flange Bracing?".  I found that Appendix 6 is quite helpful, but does not allow any calculation for continuous lateral compression flange bracing.
 
Your question is appropriate in the example of a roof beam with metal roof decking.  Do puddle welds constitute adequate lateral bracing for the top flange?  2% of the flange force (for nodal lateral bracing) or even 0.8% (for relative bracing) is usually a pretty substantial number.  Another example is a non-composite beam (no headed anchor studs) with metal decking and concrete floor slab.

In your example of secondary beams framing into a girder, I believe they will provide TORSIONAL bracing support per Appendix 6, especially if full fitted stiffeners are used, the connections will support the required torsional bracing moment, and the secondary beams have adequate stiffness.

RE: Lateral Torsional Buckling Bracing...

The old BS449 Clause 26 states that the restraining beams or slab must be capable of resisting 2.5% of the maximum flange force to provide effective lateral restraint.

RE: Lateral Torsional Buckling Bracing...

I have heard about the 2.5%, although in the past I have used 5% instead as a more 'conservative' approach. The lateral restraint required was surprisingly low

RE: Lateral Torsional Buckling Bracing...

code in UK specifies 2.5% of factored forces in compression flange as stated earlier. It is also worth noting that it is not satisfactory to simply brace a primary beam by connecting the braced beam to another unrestained beam so that they are mutually dependent. I agree with earlier statment that a concrete floor would certainly provide lateral restraint in pretty much all cases.  

RE: Lateral Torsional Buckling Bracing...

There is a free webinar sponsored by the AISC called "Steel Design after College" which discusses the latest research on what constitutes bracing and how it affects LTB, see www.aisc.org.  It costs nothing to view.

There are twelve sections in the webinar but I believe the pertinent discussion occurs in the first and second one.

RE: Lateral Torsional Buckling Bracing...

Do the newer BS standards include a stiffness requirement?  AISC recently added an explicit one and I was curious if the British standards had done so as well...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources