Sprinkler Locations for Deep Waffle-Ceiling?

4/30/2021

We are designing a project which has Ordinary Hazard with unique beam pockets.

We have ceiling pockets which are 988 cubic feet (28 m^3) and so sprinklers are provided within them. The beam depth is large at about 4-feet (1.2 m). Each beam width is about 1-foot wide (300 mm). The beam center-to-center distance is 6'-6" (2 m).

Are sprinklers needing to be provided below the bottom of the beam, or not?

If sprinklers need to be provided below the bottom of the beam, what is the coverage area if sprinklers are already provided in the ceiling pockets?

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7 Comments

andrew k

4/30/2021 08:24:30 am

If you can measure from each sprinkler to the CenterLine of the adjacent beams(in both directions) and maintain proper sprinkler coverage (probably 130sqft or less) then we can consider the "beams as walls" and coverage below beams is redundant and not required.
What do you all think?

Franck

4/30/2021 08:39:06 am

I don't know if we can consider the beams as walls, as there might be storage, equipment, etc. undernetah that would not be properly wetted by the sprinkler system. If it was a unique beam, it could maybe considered acceptable, but with multiple occurence, you may not control the fire underneath and finally overtax you ceiling sprinkler protection by opening too many heads, as the fire couled spread in the "unwetted alleys" below the beams.
But again, this is a gut feeling...

Maybe it could be interesting to make an elevation drawing with the beam depth, beam separation distance and water distribution pattern from the sprinkler (depending on its position below the ceiling), obstructed by the beams...

Luke M link

4/30/2021 09:29:38 am

Assuming you are using standard pendent or upright sprinklers, I believe NFPA 13-2019, 10.2.7.1.2(2) would apply. For spacing against a wall, refer to NFPA 13-2019, Figure 10.2.7.1.2(c).

Franck

4/30/2021 08:34:56 am

Personal interpration
If you apply table 10.2.7.1.2 (NFPA 13 - 2019), then your sprinkler is located 2 ft 9 in. (850 mm) from the beam if centered inside. This means that the maximum allowable distance above the bottom of the obstruction is 7-1/2 in. (190 mm). If you add this distance top the maximum allowable distance below the ceiling (22 in. or 550 mm), this means that if you beams is deeper than 29-1/2 in - or 2 ft and 5-1/2 in. (740 mm) you have an obstruction to the sprinkler discharge (as you have 4 ft - 1.2 m).

I am afraid that in you case you will have to provide sprinklers below one every 2 beams, centered below the beam.
For ordinary hazard, you need a maximum spacing of 15 ft (4.6 m) and a maximum protection area of 130 sq ft (12 m2).
The maximum distance between sprinklers below the beams should be 10 ft (3 m) as the distance in the other dimension is 13 ft (4 m).
Another solution is to provide sprinkler under each beam, with a maximum distance between sprinklers under the beam of 15 ft (4.6 m). But this would result on much more sprinklers and piping installation.

Interesting to see others point of views :)

Mike

4/30/2021 08:48:38 am

No you don't. Space within the pockets. Max Deflector distance is 22".

Dan Wilder

4/30/2021 10:03:50 am

I'm missing something I think....
Pocket is 988ft³ / 4 ft = 247ft²
Beam spacing is 6'-6" OC
247ft² / 6.5ft = 38' long bay?

How is this not falling under Concrete Tee construction and using 10.2.6.1.2 (5)? The width is less than the depth and it meets the criteria.

In addition, if these were steel I beams with 1' flanges, I would not be thinking about putting sprinklers under the flanges and I would utilize 10.2.7.1.2(2) and use the spacing to the centerline of the obstruction.

Casey Milhorn

4/30/2021 01:10:47 pm

If I am clear on the situation, you would just apply the "4' rule" as most designers call it. There isn't a diagram or drawing in NFPA 13 for this rule and you might be surprised how many designers don't know this rule, I believe mainly because it doesn't have a diagram or drawing in the book. It's in the first part of the obstruction rule section. I can't see how this wouldn't apply to this situation. As far as spacing, you are still covering floor space, so you will max out at 15' along the pockets (depending on the length of the pocket) and be 6'-6" between heads the other way. If the concrete tee rule can be applied, you might be able to skip every other bay. You could also technically use the obstructed construction rule to save a little pipe on the sprig length, but being 22" down doesn't really help anything other than that. The beam will still be an obstruction at 4' deep.

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Sprinkler Locations for Deep Waffle-Ceiling?

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