×
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

Unbraced length of tube
2

Unbraced length of tube

Unbraced length of tube

(OP)
Does anyone know where to find the Lp and Lr values in the LRFD manual for square or rectangular tubes? I frequently use the tables for the wide flange sections. There are equations to calculate these values but it states that these equations are only valid for wideflanges and channels.

I realize that tube sections are more resistant to torsion and thought that it may not be an issue. Is it required to check for lateral torsional buckling and consider the unbraced length of tubes? If not, why not?

RE: Unbraced length of tube

Motor City,

Commentary Section 5.1 of AISC "Load and Resistance Factor Design Specification for Steel Hollow Structural Sections" (Free download I believe)states that lateral torsional buckling is not a significant issues and gives an example showing this assumption. Based on this, I usually don't worry about if for HSS.

Shepherd
 

RE: Unbraced length of tube

(OP)
EUREKA!

RE: Unbraced length of tube

Charlie Carter of AISC sent this response to a similar question on the SEAINT listserve on 5/6/04:

Lateral-torsional buckling (LTB) is a phenomenon of importance for W-shapes and similar members of open cross-section because there is comparatively low torsional resistance. In an HSS, however, there is comparatively high torsional resistance and the warping component is eliminated almost entirely.

In truth, there is a small reduction for LTB in a rectangular HSS bent about the strong axis. However, it is noted in the Commentary to the AISC HSS Specification that the reduction in bending strength is less than 7 percent for a typical span up to 40 ft on an HSS 20x4, which is about as bad a strong-to-weak ratio as you could get for an HSS. In view of this and the likelihood that serviceability limit states will control beam design anyway, it was decided that LTB need not be a design consideration as a matter of practicality.

So yes, I think it is acceptable practice to not reduce for LTB in HSS beams. Local buckling effects may get you below Mp, though.

Charlie

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