JOIST UPLIFT BRIDGING LOAD TEST

[NITTANYRAY]

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.-

 

[jike]

Contact Vulcraft

 

[NITTANYRAY]

I DID AND THEY NEVER REPLIED.

 

[JedClampett]

For $25 you could buy the SJI Uplift Manual:

http://www.steeljoist.org/publications/default.asp#Print Publications

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.

 

[spats]

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!

 

[NITTANYRAY]

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.

 

[spats]

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.

 

[NITTANYRAY]

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.