×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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

bending coefficient "Cb" - W-beams with unbraced length > Lu

bending coefficient "Cb" - W-beams with unbraced length > Lu

bending coefficient "Cb" - W-beams with unbraced length > Lu

(OP)
does anyone understand this coefficient very well?

I have put lots of time into trying to figure out what the "AISC ASD Manual" means in its description of the coefficient.  

In a simply supported beam with a point load in the middle, I need to know if I should use Cb=1.75 or 1.32 or 1.0.  I checked three different sources and they all gave me different values.  These different values are linearly proportional to the allowable stress in the beam.

The explination in section F1-3 says that when the bending moment at any point within an unbraced length is larger than the moments at the ends of this length, Cb=1.0

this would lead one to believe that all simply supported beams should use Cb=1.0, however I found sources that say otherwise.

Please help me with this

RE: bending coefficient "Cb" - W-beams with unbraced length > Lu

The key phrase is "within an unbraced length".  If you have a simply supported beam that has a continuous lateral restraint on the compression flange (such as a metal deck or concrete slab) then you effectively have a zero unbraced length and the Cb factor doesn't apply because your Fb = 0.66Fy.

However, if you have SOME kind of intermittent lateral bracing, you have portions of the compression flange along the beam that is unbraced.  Each of these portions have a length, L that is used in the calculation of unbraced length.

So let's say you have a point load in the middle of the span and that point load has lateral restraint capability.  You then have two half-span lengths of beam that are unbraced.  Assuming a symmetrical arrangement, you would look at only half the span with zero moment at the end and you maximum moment at the other end (at the midspan).  You then design your beam for the maximum M with a Cb calculated based on M1 = 0 and M2 = M(max).  This leaves Cb = 1.75 in the ASD.

If you have other braces creating various and different lengths of beam unbraced between, you have to check each and every section of unbraced length for the maximum moment within that unbraced length.

Hope this helps.

RE: bending coefficient "Cb" - W-beams with unbraced length > Lu

(OP)
Thanks for the advice.  I forgot to mention that, yes, all the beams that I'm dealing with have the unbraced length greater than Lu.  

I also just talked to an expert structural engineer, and he informed me that when analizing a trolley beam used for lifting, Lu can be assumed almost double due to the bottom load not generating the beams tendancy to laterally buckle.

RE: bending coefficient "Cb" - W-beams with unbraced length > Lu

That would only be if the load was applied at the bottom flange.  If you had a trolley that was supported on the top flange, and hanging down on either side, the beam has a greater tendency to buckle laterally.

RE: bending coefficient "Cb" - W-beams with unbraced length > Lu

example
  Have a W12x16 beam, 50'-0" lg. The ends are supported by top plated columns. There is no load on the beam except the weight from the beam itself(assume no lateral force for example). Question is this: Will the beam fail in bending due to the unbraced length of 50'-0"?

bjm

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


Resources

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