×
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

Truss with inflection point

Truss with inflection point

Truss with inflection point

(OP)
We are analyzing a 100’ long truss that is part of a moment frame, and as such there is an inflection point towards the end of the truss under heavy snow loads.  The end fixity causes a fairly large compression force to be generated in the bottom chord over the last two panel points, and then it changes to tension.  If we consider the bottom chord a column from the end to the first bridging, the effective length is too long (40’ ) for us to support the compression force, whereas if we consider the buckling length to be only to the inflection point (8’) we are fine.  The latter seems to make more sense.   The question is this, does anyone with experience with this know of any resources that address the issue of column effective length with non-uniform axial load?  

RE: Truss with inflection point

I believe that the common understanding for continuous beams is that inflection points are not brace points - I would say this is true of your truss too.

RE: Truss with inflection point

Agree with JAE -

For backup see AISC 13th edition Commentary to Appendix 6 - 6.3 "For beams with double curvature, the inflection point can not be considered a brace piont because twist [can occur] at that point..."  

RE: Truss with inflection point

(OP)
Thanks for the response.  I agree indeed, but at least with beams we get the Cb factor for modifying the LTB equation in a very analogous situation.

RE: Truss with inflection point

(OP)
Upon further analysis it looks like we only need to brace from the wall to the first panel point 40" out and the compression load drops to acceptable levels for the 40' unbraced length.  So we'll probably do that but it still leaves me suspect...   

RE: Truss with inflection point

Steelmover,

I agree with you in theory. I cannot see why the bottom chord should be worse than an 8' cantilever column (which gives an effective length of approx 2.2x8' say 18'.

You will be hard pressed to find any reference for this though.

If you think about standard design for an unbraced compression chord, you design it for full length compression even though the full axial load is only on the middle section.

Best to stick to standard practice.

RE: Truss with inflection point

(OP)
We are bracing inflection point as it is fairly cheap, but when I have time I will run a buckling analysis of it and let you guys know what the upper bound safety factor was without it.  If anyone comes across some research though still let me know.

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