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

Continuous Beam Unbraced Length

Continuous Beam Unbraced Length

Continuous Beam Unbraced Length

I would like to know if anyone has any information on calculating an unbraced length when a beam is continuous over a column, and only the top flange is braced by framing members.  It is my understanding that the inflection point of the beam should not be used for an unbraced length of the compression flange, but then what should be used?  It seems to me that the beam-to-column connection could be used as a brace point if web stiffeners are provided, and the cap plate/bolts/column are designed to prevent twisting of the section.  Also, any comments on the applicability of Cb in these conditions?

RE: Continuous Beam Unbraced Length

This has been a big bugaboo issue for AISC for some time and they have consistently refused to directly address it in their Specifications.  By intuition, or logic, the "correct" procedure is to use the full length (column to column) in your case, and calculate the Cb based on that full length (using the 1/4 point moments across that full length).  Cb will generally be greater than 1.0 and you can then calculate the phiMn value for your beam and compare it against the NEGATIVE moments at either end for your capacity check.

For the POSITIVE moments, use the unbraced length between joists (I'm assuming joists here because you said that the top chord was braced but not the bottom chord) and a Cb = 1.0 - or you could go to the trouble of calculating a Cb over the short top chord unbraced length for the section between joists at the max. positive moment...but that is work that is usually not required....just use Cb = 1.0.

In past years, many engineers didn't use the above method due to the fact that Cb was calculated differently and many used the inflection point to column distance as the unbraced length - perhaps adding 20% to be safe...and ALWAYS using Cb = 1.0.  Some papers have indicated that this is not always conservative but I have not seen anything published to prove it.  Many beams have been designed through the years by this method.

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


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

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