Branch Line Spacing in Obstructed Construction?

1/5/2023

I have a situation with obstructed construction; solid 18-inch steel beams with pockets being over 320 square feet. The beams are 10-ft on center.

The area is part of a warehouse, Ordinary Hazard Group 2.

Can I space my branch lines 14-ft apart if I'm protecting every pocket?

Sprinklers are 5-ft (on center) from beam and 14-ft to the next branch line.

Thanks in advance!

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8 Comments

Dan Wilder

1/5/2023 06:51:17 am

You can space sprinklers or branch lines up to 15' on center (standard sprinklers) per NFPA 13-22_10.2.4.2.1(b) provided the maximum coverage area of the sprinkler does not exceed 130 sq. ft.. Area of coverage is detailed in 13-22_9.5.2.

In the example (if I am reading this correctly), 1 sprinkler per bay centered (bay is 10' wide), the maximum branch line spacing cannot exceed 13' on center (10' x 13' =130 FT²). The only way to exceed the 13' branch line spacing is to provide sprinklers at a lesser spacing:

130 FT² / 15' (between BL) = 8'-8" maximum sprinkler spacing
130 FT² / 14' (between BL) = ~9'-3" maximum sprinkler spacing

I also am questioning the obstructed designation with beams 10' on center and the pockets over 320 FT² (bays are 10 x >32'). (1) Beam & Girder construction are for members spaced not more than 7'-6' on center. (4) Panel Construction allows for spacing greater than 7'-6" on center but cannot exceed 300 FT².

Both definitions above are detailed in 13-22_3.3.43.1 Annex for obstructed construction. While this does not change the area of coverage for this example, it does change where the deflectors can be located and may or may not prevent a layout that would allow branch lines being 14' on center (by use of lowering the deflector distance and/or the ability to spray beneath the beams based on spacing to the beams).

Anthony

1/5/2023 07:44:41 am

As Dan said you don't have obstructed construction. you have mildly annoying construction.

Maybe look to running the lines perpendicular to the 10'-0 18'' deep beans with 6''~10'' sprigs. This will allow you to have typical lines and hanger locations (every 10') so Jimmy sprinkler fitter is just installing the same line 100 times. Have a typical 20' line cut at 5-10-5 with 1'' welded outlets. Those typical pieces will be reversable and will drastically increase your productivity. There is nothing more annoying than getting up in the lift then having to lower the lift to swing a 20' pipe around.

Alex

1/5/2023 10:16:23 am

Another reason why I love this forum. Wouldn't have thought about that issue. Great advise

Casey Milhorn

1/5/2023 03:09:42 pm

Love it! Mildly annoying construction. They need to add that definition to NFPA 13!

But seriously, Dan is spot on. This is not obstructed construction. Remember, you do NOT treat beam pockets with the same rules as ceiling pockets (created from architectural features). You don't even have to protect every pocket if you can defeat the obstruction rules and if you are not exceeding spacing for whatever occupancy you have going on.

NEVER apply common sense to definitions when looking at any NFPA standard (go to a dictionary ONLY when the standard doesn't give a definition). Those definitions are at the front of the standard for a reason. Then chase down the rabbit trail and THEN follow the trail to any extra comments in the appendix. This is one of those topics that require reading the entire section, knowing your definitions, and referring to the appendix.

Casey Milhorn

1/5/2023 03:09:51 pm

Patrick Drumm

1/6/2023 02:54:09 pm

Casey, great addition regarding the dictionary. Rhode Island is working off NFPA #13 2016 Edition, so I always have "Merriam Webster's Collegiate Dictionary Eleventh Edition" handy for all definitions of terms not defined by NFPA. I have also found NFPA's "Glossary of Terms" helpful in hunting down definitions of terms.

Glenn Berger

1/5/2023 09:22:05 am

To qualify for obstructed construction, you need deep beams spaced 7-6 on center or less. The 130 sf max area of coverage per sprinkler applies.

Jesse

1/5/2023 02:19:55 pm

Yeah I don't think you have obstructed construction. But when you consider the beam spacing and the requiring sprinkler spacing its a pain!

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Branch Line Spacing in Obstructed Construction?

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