Suspended obstructions within 18" of the Deck

[Oremus]

I have a situation (light hazard, unobstructed construction) where each office contains both ductwork and 2X4 flourescent lights suspended from the ceiling all within the 18" envelope from the roof deck. There are no ceilings, everything is open to structure. The roof is sloped (.5:12) so there are some areas where the ductwork is tight to the roof deck so it's easy enough to apply the beam rule in that case but the other areas are becoming murky to me. I have some spaces where there is a 5" to 10" variable gap between the roof deck and the top of the ductwork. My deflectors are going to be within 1-12" from the deck. Seeing how there is this gap here it would seem to me the beam rule would not be used which then puts me into the 3X rule which exempts non-structural elements. So perhaps the suspended or floor mounted obstruction rules of 8.6.5.2.2? But that appears to be limited to 'privacy curtains, freestanding partitions, room dividers and SIMILAR OBSTRUCTIONS" - this whole section talks about privacy curtains and the likes... would this apply to ductwork below my deflector?

 

[cdafd]

Any diagrams

Roughly how many sq ft are the offices

 

[Oremus]

No diagrams but just picture your deflector 5" above a lot of 12" ductwork trunks. Would the suspended obstruction rule be the applicable code at that point?

 

[TravisMack]

If your deflector is above the duct work, I would do my best to comply with the 3x rule. Try to position the sprinkler so that I am 24" away from the duct as measured diagonally. However, I wouldn't go crazy with trying to comply, because for light & ordinary hazard, only structural members shall apply to the 3x rule. The annex goes on to state that light and ordinary hazard areas can handle more obstructions than storage or extra hazard type areas.

Now if you are using Ex Cov sprinklers, then you need to be exact with the 3x rule. If you can't comply, then put heads under the duct or on both sides as needed to comply.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Oremus]

Thanks Travis!

 

[cdafd]

So how many sq ft is the average office

 

[Oremus]

in the 320ish sq ft range.

 

[cdafd]

I am wondering if you can cover the room with 2-3 heads?

Located so you do not have obstructions