WL Uplift on Bar Joist

[JackTrades]

I'm involved with a wind load analysis of a building built in 1987 and am trying to determine the uplift capacity of the roof support structure. The building has interior and exterior cmu bearing walls and a lightweight concrete deck on vent forms attached to bar joists of various spans at 4 ft. o.c.

The C&C pressures for the joists are 57 to 77.4 psf for zones 2 and 3 respectively and the DL is 37.5 psf (22.5 for 0.6DL).

My question is: without any reference concerning joist uplift loading on the as-builts, may I consider some percentage of the load capacity given in the bar joist standard load tables for uniformly distributed load as a capacity in uplift? The bridging shown on the plans seems to follow the requirments of the Steel Joist Institute Specifications. Also, there is a note on the drawings that states: "Bottom chords of joists shall match top chords."

Any advise would be greatly appreciated.

-Jack

 

[JAE]

The difference is in the lateral bracing of the chords.

For downward gravity conditions, the top chord is laterally braced throughout by the roof deck - Lb = 1 ft or so.

For the uplift condition, the bottom chord only has bridging to brace it laterally so the Lb is much larger and thus the capacity much smaller. You would have to field measure the bottom chords and do a more precise analysis using current applicable SJI specification requirements.

Alternatively, you could contact the manufacturer of the joists and see if they can do an analysis for you. One thing you could try is to suggest to them that they determing the maximum unbraced length of the bottom chord (i.e. the new bridging spacing) that would be required for the uplift loads and then add bridging accordingly.

 

[jike]

In addition to what JAE said, I would suggest that you make sure that you have bottom chord bridging at the both ends of the joist. This may not have been provided in 1987.

 

[JackTrades]

Thank you both for your helpful replies.

Also, prior to reading these, I had just gotten off the phone with Vulcraft tech support where it had been suggested that, if I could get a tag off one of the joists, it would have project I.D. and piecework numbers that could possibly be used by the factory to track it back to its design info.

I'll look at all these ideas.

Thanks again.

-Jack