sprinklernotes
Obstructions Below Residential Sprinklers
You have asked the following question: "We are installing a NFPA 13D system. In the bedroom, there is a peaked ceiling with collar ties that are 8 inches X 8 inches. They are not beams (tight to the ceiling) but are located 51in. below the ceiling peak (similar to the bottom chord of a pitched truss). There are four collar ties with a separation of 4 ft. and 5 in. (measured to the side). There are four sprinklers in the room. They are located midway between the collar ties so 2 ft. 2-1/2 in. horizontally from their side and 1 ft. 2 in. vertical above the top of the collar ties. This creates a radial separation of 33 in. With this being a continuous obstruction, am I required to comply with the beam rule (13D:8.2.5.4.2)?" In response to your question, we have reviewed the 2010 edition of NFPA 13D as the applicable standard. Our informal interpretation is that the installation meets the intent of NFPA 13D. This is a very interesting problem for a couple of reasons. Let's start with the issue of continuous obstructions in section 8.2.5.4.2. A continuous obstruction is one that affects two or more adjacent sprinklers (this definition is not yet in NFPA 13D and was added to NFPA 13 in the 2013 edition). Such obstructions can be above or below the sprinkler. The location in relation to the sprinkler deflector is a critical parameter since different rules apply depending on said location. This is well addressed in NFPA 13 but poorly addressed in NFPA 13D (as discussed in the second paragraph). As for section 8.2.5.4.2 (also referred to as the beam rule), as Figure 8.2.5.4.2 portrays, this only applies to obstructions that are close to (and usually tight to) the ceiling. In other words, it is when the top of the obstruction is at, or above, the plane of the deflector. This is explicitly identified in NFPA 13, but you'll have to look at section 13:8.6.5.2.1.2 of a 2013 or later edition. As discussed in NFPA 13: A.8.6.5.2.1.3 in regards to the three-times rule, it states: "This works for small non-continuous obstructions and for continuous obstructions where the sprinkler can throw water over and under the obstruction,.." However, when a beam is above the deflector, you have to be able to adequately throw water beneath it. It's interesting that the discussion keys on the ability to throw water over the obstruction which really won't happen if the defector is only slightly above the top of the obstruction, but the physical relationship has been explicitly defined. Nonetheless, the beam rule is intended to be for obstructions close to, or tight to, the ceiling. This is the same criteria as provided in NFPA 13:8.5.6.1, which explicitly identifies the location of the obstruction. What makes this issue really interesting is that all of the rules in NFPA 13 D for pendent sprinklers, other than for fans, address obstructions tight to the ceiling. The only other rule for obstructions below the sprinkler is the cabinet rule for sidewall sprinklers. The standard is written around the normal situation where obstructions, like ducts, seldom exist. For all intents and purposes, the collar tie impacts the sprinkler the same as a duct. Effectively, there are no rules for obstructions below the sprinklers. As such, a literal application is that there are no requirements for such obstructions. Until the technical committee recognizes that obstructions below the deflector do occasionally occur and adds rules to address them, it’s reasonable to apply the criteria from NFPA 13. The four-times rule for residential sprinklers provides appropriate separation. Keeping in mind that NFPA 13 is a more demanding document than NFPA 13D, this meets or exceeds the intent of NFPA 13D. There is one small issue that must be resolved. Section 8.10.6.2.1.2 still states: “Regardless of the rules of this section, solid continuous obstructions shall meet the applicable requirements of 8.10.6.1.2.” This is the same text that section 8.6.5.2.1.2 used in the 2010 edition. The proposal for 2013 asked that a change be made to section 8.6.5 and no other sections were considered. The philosophy of how to treat the location of an obstruction is the same regardless of the type of sprinkler. From a sprinkler activation and discharge perspective, it is clear that the beam rule should be applied in the same manner for a residential sprinkler as it is for a spray sprinkler. The problem is that the committee has not yet been asked to look at it. This will be addressed in the next cycle.
