×
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

JOIST UPLIFT BRIDGING LOAD TEST

JOIST UPLIFT BRIDGING LOAD TEST

JOIST UPLIFT BRIDGING LOAD TEST

(OP)
We have a roof where we are removing a ballasted roof and replacing it with a single ply membrane roof. We now have a small net uplift on the roof.

The SJI Specifications say that bottom chord bridging is required near the first B/C panel point.
We have done some limited testing on a mock-up and have determined that the tested joists have a moderate uplift capacity without this additional bridging.

What I am wondering is have there been any studies that establish general guidelines for this situation. There are several different joist sizes on this project and it is beyond our budget to do multiple load test to prove every joist type works.-                               

RE: JOIST UPLIFT BRIDGING LOAD TEST

Contact Vulcraft

RE: JOIST UPLIFT BRIDGING LOAD TEST

(OP)
I DID AND THEY NEVER REPLIED.

RE: JOIST UPLIFT BRIDGING LOAD TEST

For $25 you could buy the SJI Uplift Manual:
http://www.steeljoist.org/publications/default.asp#Print%20Publications
Or you could measure the bottom chords, figure out the section properties and provide bridging (bracing) to keep the kl/r under 150 or so.  I'm sure the uplift compression on the bottom chord is small, but you still need some bracing.
Third option is to call someone besides Vulcraft.  Quincy Joist or the SJI comes to mind.

RE: JOIST UPLIFT BRIDGING LOAD TEST

Without the bridging at the first bottom chord panel point, the bottom chord would actually cantilever from it's end to the first existing row of bridging, and the "K" factor would be 2 or greater. However, the bottom chord is not uniformly loaded in compression... load is zero at the end and increases towards the interior bridging, so it's not strictly a direct Euler buckling calculation... in other words, it's effective "L" is less than the actual "L". I've seen the use of 0.85 of actual length used to account for the "un-uniform" load. In the least, you should use "KL" = 2 x 0.85 x "L" = 1.7 x "L". Measure the angles, use the highest axial load at the interior bridging, do your calcs, and you're good!

RE: JOIST UPLIFT BRIDGING LOAD TEST

(OP)
Spats,
Do you have a reference for the 1.7L. I agree with your logic but i would feel more comfortable seeing it in print.

RE: JOIST UPLIFT BRIDGING LOAD TEST

I don't have a reference offhand. I feel the 0.85 is conservative. Why not use KL = 2L and be done with it, as long as it figures.

RE: JOIST UPLIFT BRIDGING LOAD TEST

(OP)
SJI e-mailed me with this info:

"Before uplift bridging was required the industry used a k-factor of 2 for checking buckling of the bottom chord and 1st web member." This is stated in older versions of Technical
Digest #6.

The B/C bridging requirement was added in late 1985 when the K series joists were introduced.

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