sprinklernotes Vertical Obstructions 18 inches or Less Below the Sprinkler Deflector
You asked the following question: “We have a system being designed as an ordinary hazard group 2. There are walls that do not go all the way to the ceiling. In some places, we have a clearance of 18 in. or more between the top of the wall and the deflectors, but in others, it is less than 18 in. For the latter, section 8.6.5.2.2.1 allows less clearance but only for light hazard occupancies. How should I address these partial walls?” In response to your question, we have reviewed NFPA 13, 2016 edition as the applicable standard. Our informal interpretation is below. This is really a two-part question. Let’s first talk about walls that stop more than 18 inches below the plane of the deflector. Walls are ignored when the top of it is more than 18 inches below the plane of the deflector. It is understandable that most AHJ’s would be concerned about the shadow created by the partial wall. As crazy as it may sound, we ignore these items. When reviewing this issue, it helps if one keeps in mind that the primary function of the sprinkler is to control the upper layer temperature so as to avoid flashover. The only driving requirement is in section 8.5.6.1 that states: “unless the requirements of 8.5.6.2, 8.5.6.3, 8.5.6.4, or 8.5.6.5 are met, the clearance between the deflector and the top of storage or contents of the room shall be 18 in. (450 mm) or greater.” The partial wall should be viewed the same as a bookshelf that is full but doesn’t go all the way to the ceiling. Just like a bookshelf, the wall is not a structural member and acts simply as something contained within the space. As such, it would need to be 18 in. or more below the deflector. As you identified, there is criteria for floor-mounted vertical obstructions (such as walls that don’t go to the ceiling) in section 8.6.5.2.2. This section is an allowance whereby the contents of the room can violate the 18-in. clearance required by section 8.5.6.1. It is limited to lower hazard fires as posed by light hazard occupancies. As shown in Table 8.6.5.2.2, allowed obstructions start at 3 in. below the deflector and stops at 18-in. below the deflector. Once you hit 18 in. from the deflector, there’s no required minimum separation distance per this section. Once past 18-in., there’s no minimum required separation for any applications when using spray sprinklers (other sprinklers such as CMSA and ESFR have a minimum clearance of 36 in.). This also supports the understanding that we ignore partial walls that are greater than 18-in. below the deflector. So, what do you do in an ordinary hazard occupancy when you don’t have an 18-inch clearance? One choice is to install an additional sprinkler (as discussed in section 8.6.5.1.1) on the other side of the partial wall. Another option is presented by section 8.6.5.2.1.7 that states: Sprinklers shall be permitted to be installed on the centerline of a truss or bar joist or directly above a beam, provided that the truss chord or beam dimension is not more than 8 in. (200 mm) and the sprinkler deflector is located at least 6 in. (150 mm) above the structural member and where the sprinkler is positioned at a distance three times greater than the maximum dimension of the web members away from the web members. From the sprinkler’s perspective, the top of a beam and the top of a partial wall looks the same (aka has the same impact on the discharge pattern). As such, it is entirely appropriate to apply this allowance to partial walls that meet the same defined dimensions. Since this is not explicitly identified by NFPA 13, the AHJ would have to concur and as I always say, discussing such things BEFORE the submittal is the best approach.
