How to Address Multiple Ducts Under 4-ft Wide?

4/9/2021

I have two large ducts 20-inches below sprinklers. One duct is 32-inches wide, the other is 42-inches wide. There is maybe 4- to 7-inches between them, only a couple inch gap on each side, running the length of a hallway.

The only rule for a continuous obstruction I can find says no coverage is needed below if the obstruction is under 4-feet.

Individually these ducts aren't over 4-feet, but how does NFPA 13 address multiple obstructions like this?

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9 Comments

Mike

4/9/2021 08:08:25 am

If there is only a few inches between them and in between them, they essentially obstruct discharge to the whole hallway.

I would consider it one duct, possibly 2, and provide sprinklers beneath.

The argument could be made that the separation between ducts will not allow actuation of the heads but that depends on the AHJ.

If you want to try with one line, add 1" outlets for an additional line.

Dan Wilder

4/9/2021 08:13:32 am

While not considered a single continuous obstruction over 48", it "obstructs" up a large percentage of the available area from the ceiling/roof level sprinklers to floor coverage, additional sprinklers below would be warranted in this case (IMO).

Additional rules like "Obstructions against walls" show intent of protection below obstructions less than 48" wide but "technically" if the obstruction (or multiple) are below the 18" and do not exceed 48", the standard does not require additional protection (see 2019-A.10.2.7.1.2 for an example of a soffit or obstruction against a wall, more than 18" below the deflector, and up to 48" wide with the intent of not requiring additional protection).

I have not been able to apply a rule like this with an AHJ when the obstruction is up to 30" (See 2019-10.2.7.1.2(b)), but I have used the "intent" of 2019-10.2.7.1.2 to remove sprinklers from similar instances of obstructions against walls or by using the 3x rule per 2019-10.2.7.2.1.3.

Ray Ramos

4/9/2021 08:15:12 am

I think the obstruction rules (that has to do with size and proximity) will apply in this case, not the 4 feet rule but the other ones. Therefore you will need to add sprinkler above and below.

Jesse

4/9/2021 08:24:11 am

While you don't have a single contiguous obstruction over 48", effectively the 2 ducts act as an obstruction and will negatively affect the spray pattern from ceiling level sprinklers. I would protect underneath it.

Glenn Berger

4/9/2021 08:36:52 am

Rule of thumb - If you can't see the sprinkler from the floor, than it is obstructed and then sprinkler protection is required below the obstruction.

CJ Bonczyk

4/9/2021 09:05:42 am

A general rule of thumb in these instances (specifically corridors) I try and treat them as your would grouped obstruction similar to those addressed from FM global data sheets.

Art

4/9/2021 09:22:42 am

Maybe the intent of the code requirement for Open Grid Ceilings can be applied here. It requires 70% open and the thickness of the grid to not be greater than the clear openings. If you treat the ducts as the open grid ceiling then in your instance the gap between the ducts and between the walls does not comply and hence sprinkler coverage is obstructed and needs additional heads below the ducts. Curious what others think about this. Please let me know if you think I am way off on this.

Franck

4/9/2021 10:46:26 am

When I was still working for an insurance company and was reviewing installations (acting like an AHJ), I often asked for additional protection below grouped piping (or grouped cable trays).
If you ask yourself “would I need sprinklers below”, you already have the answer : Yes.
Your gut feeling is already that the situation is not that good.

The point of view of Art (above) is a practical one and can be used to solve such dilemma

Do not forget to consider also the distance between the obstruction and the sprinkler deflector. A 4 in. pipe can be an obstruction if located too close to the sprinkler

ken thackery

4/9/2021 02:00:27 pm

I have seen this go both ways with small grouped ducts and cable trays. It's best to stay within the code. Also I have referenced
A8.1.1(3)

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How to Address Multiple Ducts Under 4-ft Wide?

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