Contact US

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!

*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

Inflection Point Lateral Bracing

Inflection Point Lateral Bracing

Inflection Point Lateral Bracing

I've heard many times that a moment curve inflection point can be assumed to be a brace point when considering lateral torsional buckling.  I have recently come across a case that calls that assumption into question.

Have any of you done laboratory experiments on this phenomenon or know someone that has?  I understand that many of you may have successfully used this assumption for many years, but I'm looking for someone with definitive test experience to discuss this with.


RE: Inflection Point Lateral Bracing

The inflection point should NOT be considered a brace point.  The correct way to take into account the inflection point is by using the proper Cb factor for the moment diagram to increase the moment strength of the beam calculated using the full unbraced length.  

RE: Inflection Point Lateral Bracing

Note, the previous post is AISC's official stance on the subject (as published in their "AISC Steel Design After College" short course notes).  Note also that the Cb values provided in the LRFD manual are based on a totally unsupported beam between brace points.  AISC does however in the same short course notes provide per Yura revised (and less conservative) Cb values for the typical case where the top flange of the beam is braced by a composite deck.  This is not the same thing as assuming Lb as the distance between the inflection points.  Lb is still assumed to be the full length of the beam and Cb is calibrated to take into account the top flange bracing.  

RE: Inflection Point Lateral Bracing

I agree with WillisV that this is "AISC's official stance on the subject".  

I do know that many engineers over the years did take the distance from the column support to the inflection point as an unbraced length lu in ASD and use that for the adjoining negative moment...along with Cb = 1.  

I was at a seminar in the early 80's when Yura was asked this question and I believe he answered "yes, as long as Cb = 1 you are generally conservative doing this".  However, it subsequently came out that there are some cases, with particular parameters, where it wasn't conservative.  

We used to take the IP distance x 1.2 and use that to be extra conservative.  But this was based on engineer's response to silence in the AISC specification about this situation.

Today, I use the full length with the proper Cb.

Also - you can add bottom chord braces near the IP and definitely use the shorter length.

RE: Inflection Point Lateral Bracing

For lapped Z's over supports, AISI uses the distance from the end of lap to the I.P. as the unbraced length of the bottom flange.  

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! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close