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Concealed Spaces and Special Sprinklers
You have asked the following question: “We have a four-story hotel with open web wood trusses spaced 1 ft. 6 in on center at the roof. The top chord of the truss is 2 by 6 (on edge) with 2 by 4 (on edge) web members and bottom chord. Overall truss height is 30 in. to 38 in. and they are 32 ft. 6¾ in. in length. When calculating the percentage of the opening to determine if the space is obstructed construction, is it necessary to take the dimension of the top chord into consideration if the deflector is located below the top chord? If the cross-sectional area is not 70 percent open, are sprinklers required in between each bay? Are special combustible concealed space sprinklers required for part or all of the combustible space?” In response to your question, we have reviewed NFPA 13, 2016 edition as the applicable standard. Our informal interpretations are explained below. Let’s take the questions in the order they were asked. The top chord must be included in the determination of how much of the cross-sectional area is open. The location of the sprinkler does not impact this issue since it is all about heat flow along the ceiling. The dimensions of the top cord of the truss in this application is critical when determining if the construction is considered obstructed or unobstructed. If the depth is greater than 4 in., NFPA 13 considers this to be obstructed construction. This is stated in Sections A.3.7.1(8) and (9). Section A.3.7.1 provides examples of what NFPA 13 considers to be obstructed construction. Then A.3.7.1(9) states: Truss Construction (Wood or Steel). The term truss construction refers to parallel or pitched chord members connected by open web members supporting a roof or floor deck with top and bottom members greater than 4 in. (100 mm) in depth. As for the second question, there is no direct requirement to place the sprinklers in every bay (also called a channel NFPA 13) for obstructed construction, as required for ESFR sprinklers. What does drive the issue is allowed distance below the ceiling and the ability to comply with the obstruction rules. If the truss assemblies were considered obstructed construction, then sprinklers have to be put into every channel since the beam rule can’t meet. If the truss is at least 70 percent open, measure the 6 in. below the structural member to the bottom of the top chord and then try to meet the three times rule required by the obstruction criteria. This is a challenge with closely spaced trusses. Due to the truss’ dimensions and the spacing between truss the options are limited within Section 8.6.5.2.1. The only real option that would be applicable is Section 8.6.5.2.1.7, which 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. This too is likely a challenge which could require installing sprinklers in every channel. Next, Section 8.15.1.6 needs to be reviewed due to the lack of space from deck to deck for the spray pattern of a standard sprinkler to fully develop. Section 8.15.1.6 requires sprinklers that are specifically listed for use in concealed spaces to be installed in concealed spaces that meet all of the following three conditions; a horizontal concealed space (slope no more than 2 in 12), a combustible upper surface, and a depth of no more than 36 in. Since this is Type V, it is assumed it has an upper wood surface and that it is exposed. If the upper surface is covered in insulation, the interpretation is the upper surface is not exposed so a sprinkler that is specifically listed concealed spaces would not be required. Since this concealed space has portions that have a depth of 30 to 38 in., concealed space sprinklers are required. Then Section 8.15.1.6.2 allows those special concealed space sprinklers to be installed throughout a concealed space that has portions which may exceed a depth of 36 in. Thus, the areas having a depth greater than 36 in. to 38 in. can use the concealed space sprinklers as well.
