Is W6x9 laterally braced

[Plainsboro]

I have a 20 feet long W6x9 supporting 1.5 inch (rib) 20 gage metal deck. Deck is screwed with top flange of W6 with #12 x 1 1/4" Tek Screws @ 12" O.C.
Now is compressive flange of W6 is braced laterally or not for designing purposes.

 

[jike]

The top flange would normally be considered braced by the metal deck diaphragm and it's fasteners. However, if you have uplift (wind) on the beam, the bottom flange becomes the compression flange and it is NOT braced. You need to check both conditions.

 

[SlideRuleEra]

Assuming the applied load is "down" (top flange is the compression flange), in my opinion it is laterally braced.

 

[jike]

All the above comments, assume that the beam is simply supported and the normal loads are downward. If there is end fixity or member continuity, then the problem becomes more complicated and the guidelines for lateral bracing will be different.

 

[apetr26542]

As a rule of thumb does the #12 screw provide 2% shear capacity of the applied load. If you have 500 plf, and screws at 24" o.c. They would have to be capable of 2% of the 500 plf or 20 lbs shear since they are at 24" o.c. The limiting factor is the gage of the deck in bearing on the screw. On a 20 gage deck a #12 screw has about 140 lbs shear capacity.

 

[KarlT]

apetr26542

Isnt the 2% force supposed to be based on 2% of the compressive force at the top of the beam under full downward load?

I don't think its right to take 2% of the vertically applied load??

KarlT

 

[trainguy]

Your beam sounds quite shallow for the span.

20 ft / 0.5 ft = 40

You could have a deflection problem.

tg

 

[JAE]

Yes, I was going to say the same thing - 6" beam going 20 feet is not a good idea. I also agree with KarlT's point.

 

[UcfSE]

You could probably check the detail you have for compliance with the bracing stiffness/strength requirements in the AISC 3rd LRFD Edition. I say if the deck is part of a shear diaphragm then the flange is most likely braced unless the deck is spanning some abnormally large distance. #12 screws are kinda small in capacity compared to the usual plug welds but then again it's only a 6x9.
When I've wondered about this myself I followed the procedure outlined in the steel book by Salmon and Johnson, and checked the KL/r of the deck < 200 (spanning to the next beam or joist), and found that the deck met those criteria for bracing.