Does a Mezzanine Change Ceiling-Level Density?

10/24/2023

Does sprinkler protection beneath an open grated mezzanine change the ceiling level sprinkler design density?

e have a building with a storage room with a grated mezzanine at 8' AFF. Beneath the mezzanine is Class I-IV storage on shelves to the bottom of the mezzanine and on top of the mezzanine is similar storage to 10' high.

NFPA 13 requires sprinkler protection for open grating over 4' in width but there is no indication on if this affects the ceiling sprinkler design as it might for in-rack sprinklers.

My feeling is the system would have to be designed as if it was ~18' high storage with heads under the mezzanine that match that density, but I am not sure if this is correct or if there would be a benefit to covering the grating entirely and possibly considering it storage under 12' below and above.

Thanks in advance for your take.

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6 Comments

Pete H

10/24/2023 06:56:03 am

Treat the mezzanine in this case as a separate floor.

That said, the protection scheme for the storage commodity must extend 15'-0" beyond the extents of the storage, which will often be to the border of the mezzanine. So some of your ceiling system is likely going to have to be able to protect for the storage commodity as is, in which case, it might make sense to just keep that protection uniform in case of storage above the mezzanine becoming likely.

10/24/2023 07:27:38 am

That said, due to the open grating, I agree that a permanent cover over the open grating would be required or at the VERY LEAST water shield heads to be used for the system under the mezzanine to establish heat collection.

Jesse

10/24/2023 08:08:04 am

The density requirement for the ceiling doesn't change. Storage become high-piled storage at different heights for different commodities. Class I-III at 12-ft and Class-IV at 10-ft.. It sounds like the governing hazard class sin this area would be OH2. If so, you could get by with an OH2 design in this area, but I would probably design the ceiling the same throughout presuming homogenous storage, and then OH beneath the Mezz.

Dan Wilder

10/24/2023 08:23:52 am

If it's just open metal grating, with no other horizontal barrier separation or sprinklers within a rack, no I would not apply a storage height reduction. The sprinklers are there to provide protection below the obstruction, not to protect the commodity (directly) or to be considered a horizonal barrier to lessen the storage height.

If the below grating layout complies with an in-rack storage design criteria (spacing between sprinklers, flue space location...) then yes for a design density reduction but not a storage height consideration. However, there is an annex section that walkways should not be considered flue spaces (A.3.3.7.1) so that layout would need to be done carefully.

If the mezzanine walkways are solid, including the rack shelf at the mezzanine level, then yes, storage height reduction w/out any in-rack consideration. The other rack levels can and should be open. Design density would be per level.

I'm looking at the "Horizontal Barriers" definition.
13-19'-25.7.1 * Where required by sections of this chapter, horizontal barriers used in combination with in-rack sprinklers to impede vertical fire development shall be constructed of sheet metal, wood, or similar material and shall extend the full length and depth of the rack.

Also looking at sections (13-19'-23.2.4.1 Bin Box & Shelf Storage) - Not sure your specific storage design but I'm thinking it doesn't involve pallets so this section may apply without any additional horizontal barrier requirements.

21.2.4.1.1 - Bin box and shelf storage that is over 12 ft (3.7 m) but not in excess of the height limits of 21.2.1 and that is provided with walkways at vertical intervals of not over 12 ft (3.7 m) shall be protected with automatic sprinklers under the walkway(s).
21.2.4.1.2 -Protection shall be as follows:
(1) Ceiling design density shall be based on the total height of storage within the building.
(2) Automatic sprinklers under walkways shall be designed to maintain a minimum discharge pressure of 15 psi (1.0 bar) for the most hydraulically demanding six sprinklers on each level. Walkway sprinkler demand shall not be required to be added to the ceiling sprinkler demand. Sprinklers under walkways shall not be spaced more than 8 ft (2.4 m) apart horizontally.

Glenn Berger

10/24/2023 08:32:41 am

Your approach is definitely acceptable. That is based upon storage for the entire height of the facility with matching design criteria used below the mezzanine (obstruction).

Anthony

10/24/2023 08:48:53 am

Open grating is considered a full obstruction

"Class I-IV storage on shelves to the bottom of the mezzanine" is there sprinkler coverage below this mezz? If not there should be and that coverage should be designed for the height from the floor to the grating under the mezz.

The storage on top of the mezzanine should be sized from the top of the mezzanine to the ceiling and extend that coverage 15' further out into the room if there is a lower hazard surrounding that area.

"to be designed as if it was ~18' high storage with heads under the mezzanine that match that density" it would be acceptable to provide the higher hazard level of protection.

Does a Mezzanine Change Ceiling-Level Density?

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