How to Sprinkler Protect an Exposed Waffle Slab?

10/25/2022

I'm working on the renovation of an existing building with waffle slab construction.

As part of our scope, the client would like to sprinkler the space, and the architect was asking if we would be required to provide sprinkler coverage in each of the waffle slab pockets if they left the structure exposed.

Originally I thought we could treat the space as ceiling pockets, but since it's not an "architectural feature" I'm thinking the exceptions in NFPA 13 11.2.7.2 would not apply. The pockets created are roughly 3-ft x 3-ft wide, and the structure is 2-ft deep.

If we're using upright sprinklers, and they need to be installed 1"-12" below the "ceiling" (in this case, the slab?), taking a look through the obstruction/positioning tables, it seems like the structure would create obstructions and a head would be needed in every compartment?

Agree? Disagree?

Any insight would be appreciated.

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7 Comments

Casey Milhorn

10/25/2022 08:09:23 am

I would recommend getting an official ruling from NFPA, and while you wait for that to come back (it can take awhile), I would get an informal interp from AFSA. I would argue this could be treated as concrete tee construction with no minimum deflector distance but with deflectors at a 1" minimum down from bottom of concrete "tee". If that doesn't fly. you can also classify this as obstructed construction, place a head at the center every 3rd to 5th pocket (depending on hazard and spacing), and keep deflector at 22" down from deck and no more than 2.5" above bottom of "tee". Sounds like you have a couple options here to avoid every pocket.

Yves Poulin

10/25/2022 08:14:09 am

Maybe if they drywall every pocket you will not need to sprinkler every waffle ?
If it's cost saving not sure

Eric R

10/25/2022 08:44:26 am

Depending on your occupancy class I think you could get away with a 9' x 12' or 12' x 12' sprinkler spacing pattern utilizing the obstructed construction rules.

From the 2016 edition of NFPA 13

8.6.4.1.2 Obstructed Construction
(2) Installed with the deflectors at or above the bottom of the structural member to a maximum of 22 in. (550 mm) below the ceiling/roof deck where the sprinkler is installed in conformance with 8.6.5.1.2

If you set your deflectors right at 22" down for the slab that would put them 2" above the bottom of the beams. Per 8.6.5.1.2 you are allowed to be 2.5" above the bottom of the beam as long as you are 1' away from it's edge. If the sprinkler is located in the center of each pocket then they should be at least 1' away from the edge of the beam if your pockets are nominally 3'x3'.

I would double check the pocket dimensions, and make sure you get the center-of-pocket to center-of-pocket distances to make sure your overall spacing conforms with your coverage area limitations based on the occupancy hazard.

Failing this, I also think you could likely consider the structure to be concrete tee construction, but if so, you'd want to get that approved by the AHJs involved.

Randy Rathert

10/25/2022 10:29:31 am

Nicely done. that makes for a complete analysis.

Jessica Lutz

10/25/2022 12:00:13 pm

...Precisely how I've seen it handled on a few projects.

Alex

10/25/2022 01:40:22 pm

Exactly how I've protected waffle ceilings before in the past! Well said.

Bobby V

10/27/2022 08:53:19 am

Sections taken from 2016 NFPA 13.

This type of construction falls into definition of obstructed construction, section 3.7.1, since the members impede heat flow.

Next, use one of the methods given in 8.6.4.1.2 for deflector placement in obstructed construction. You can use arrangement (2), but as Eric R mentions, since the deflector is 22" down from the deck, you then have to apply Table 8.6.5.1.2 to position the deflector to avoid water spray on the waffle. It then becomes a challenge to properly space sprinklers throughout the space while keeping the required distances away from the members.

I would recommend asking the AHJ for approval to use arrangement (5), which is used for concrete tee construction, because the depth of the tee does not matter.

There are other options...The architect could install cloud ceilings conforming to 8.15.24 to avoid sprinklers at the upper roof deck. Or he could just install a ceiling to cover the whole area, and then you would just have pendent sprinklers in the lower ceiling.

How to Sprinkler Protect an Exposed Waffle Slab?

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