home
Convention / Exhibit

Obstructions
We've Come a Long Way, But the Journey Isn't Over

by J. Scott Mitchell, P.E., AFSA Technical Services Department

Over the last ten or so years, obstruction criteria has evolved immensely. In the 1991 edition of NFPA 13, Standard for the Installation of Sprinkler Systems, there were only obstructions located at the ceiling and obstructions below the sprinkler. This is the same edition that told us to treat certain non-continuous obstructions at the ceiling as vertical obstructions (13:4-4.1.3.1). In the 1994 edition, things got a little better. The committee started warming up to the idea that literal interpretations of these criteria pertaining to open web construction were a little more restrictive than intended. Thus, exceptions were born that made these scenarios a little friendlier. Criteria for horizontal obstruction against walls were also introduced (13:4-4.1.3.1.3), which were less stringent than the "beam rule." Then the 1996 edition broke it wide open, presenting a clear separation point between two types of obstructions. This came in the form of the assigned plane located 18 in. below the sprinkler deflector. It also gave general obstruction criteria for all sprinklers, then updated the obstruction criteria for each sprinkler type, i.e. standard spray, extended coverage spray, large drop, ESFR. Since then, little has changed with the structure of the criteria's presentation, though indication of the committee's intent has improved. Some of this is made evident in the Annex (formerly Appendix) notes and handbook commentary. This article seeks to share AFSA's Technical Services Department's perspective on obstructions to standard pendent and upright spray sprinklers and our vision of the direction the criteria will be taking.

Types of Obstructions
Obstructions to standard pendent and upright spray sprinklers can be categorized into three classes. The most critical class consists of solid continuous obstructions that are generally tight to the ceiling or roof deck and the tops of which are above the elevation of the sprinkler deflector. (See Figure 1.) Solid continuous obstructions in such a locale will impede heat flow to the sprinkler, resulting in delayed activation, and will obstruct the sprinkler discharge from the upper air layers on the other side of the obstruction. Since these obstructions most significantly increase the risk of flashover, they are subject to the most severe restriction, which is commonly called the "beam rule" and can be found in 13:8-6.5.1.2 of the 2002 edition. Obstructions that are not solid or not continuous and located above the elevation of the deflector are still in the Class 1 zone, but they are not as detrimental to the overall system performance. In other words, when an obstruction only affects a single sprinkler, the other sprinklers in the area should be capable of controlling the fire. Since open or non-continuous obstructions are less detrimental, the applicable criteria are less stringent (see 13:8.6.5.2.1.1).

The second most critical class consists of obstructions located entirely below the elevation of the deflector but above the assigned horizontal plane located 18 inches down from the sprinkler deflector. (See Duct A in Figure 2.) Whether continuous (affecting more than one sprinkler) or non-continuous (affecting only one sprinkler), these obstructions are less critical because they do not significantly impede heat flow to the sprinkler and do not significantly impede water distribution in the upper air layers of the room or area. They are less detrimental to the overall system performance because overlapping discharge from sprinklers. Obstructions in this zone contribute far less to the flashover risk. However, since these obstructions occur in the zone where the sprinkler discharge pattern is developing, considerable restrictions are warranted. These obstructions are subject to the "three times" rule, requiring the sprinkler to be located from the obstruction a distance at least three times greater than the maximum obstruction dimension (see 13:8.6.5.2). Now, we must point out that applying this rule to all obstructions violates the literal wording of 8.6.5.2, which says, "Regardless of the rules of 8.6.5.2, solid continuous obstructions shall meet the applicable requirements of 8.6.5.1.2." However, we don't feel that focusing on the top of the obstruction being above or below the deflector violates the intent of this section. While solid continuous obstructions are more detrimental than open obstructions (like bar joists) and noncontinuous obstructions (like exit signs), activation time is not delayed and sprinkler discharge is able to reach both sides of the obstruction when the top of the obstruction is not above the sprinkler deflector elevation. Our interpretation is supported by A.8.6.5.2.1.3, which indicates that the rules were written to apply to obstructions where the sprinkler can be expected to get water to both sides of the obstruction without allowing significant dry shadow on the other side of the obstruction.
The third class consists of obstructions located entirely below the imaginary plane 18 inches down from the sprinkler deflector. (See Duct B in Figure 2.) This is the least critical class of the three because it does not significantly impede heat flow to the sprinkler and sprinkler discharge to the upper air layers, nor does it impede the sprinkler discharge pattern from fully developing. These obstructions, depending on size, can keep the sprinkler discharge from directly reaching large portions of its designated floor area. But, this can be tolerated since fire control is the objective, not fire extinguishment. If these obstructions exceed 4 feet in width, additional sprinklers must be installed beneath them (see 13:8.6.5.3).

Basis for Intent
For years we have been focused on putting water on the floor; making sure the sprinklers are spaced from each other and from obstructions so that the maximum floor area is receiving water. This is still a valid concern, but we are beginning to grasp the benefits of sprinkler discharge in the upper air layers of the room or area. Water droplets ejected from the sprinkler become heat sponges in the air. Each droplet will absorb a certain amount of heat and in some cases can absorb enough heat to form steam. This conversion not only consumes immense amounts of heat energy but also involves rapid expansion displacing oxygen. The result is a significant abatement in the combustion cycle. The combination of heat energy absorption and oxygen displacement usually stops a fire dead in its tracks. A critical, though not emphasized function of sprinkler discharge is that it prevents flashover, which is the most destructive phase of a fire's life. It only stands to reason that we see the most restrictive requirements applying to those obstructions contributing to a sharp increase in risk of flashover. Then we see the least restrictive criteria applying to those obstructions that only result in shadowing of floor area under the sprinkler. It is easy to slip into the mode of thinking that the sprinklers are supposed to extinguish the fire and that they are supposed to blanket every square inch of the protection area with water. This is simply not true. It is important to remember that (except for ESFR) sprinkler systems are designed to control the fire, allowing the responding fire service time to finish the job. Since the system is in a mode of fire control, it can tolerate obstructions. In fact, when an occupancy hazard fire control approach uses spray sprinklers, it can tolerate a 4 ft wide obstruction located more than 18 in. below the deflector. This causes a shadow possibly extending the full length of a room or area. This can be tolerated because the sprinkler discharge patterns are overlapping and adjacent sprinklers on the opposite side of the obstruction are able to control the fire. Is the sprinkler system discharging an even coat of water on the floor? No. Are there dry areas on the floor? Probably. Will the sprinkler system control the fire? NFPA 13 says it will, and we agree.

Why are the Criteria so Confusing?
If you've tried to apply the criteria of sections 8.6.5.1 through 8.6.5.3 and became confused, don't feel bad. The confusion may be attributed to failure of the designer and NFPA 13 to follow logical sequence. When a series of criteria are to be applied, one must start at the first and proceed through to the last. For obstructions to SSUs and SSPs, first apply the criteria of 8.6.5.1. If the arrangement satisfies this or is not applicable, proceed to 8.6.5.2 or 8.6.5.3 depending on the obstruction's location in relation to the 18-inch plane. This sequence is often ignored by skipping 8.6.5.1 and going directly to 8.6.5.2. For instance, with surface-mounted lights and recessed pendent sprinklers, designers often start by incorrectly applying the allowance for non-structural members in LH and OH occupancies to be ignored (see 8.6.5.2.1.4). In this case, 8.6.5.1 is the applicable section. Furthermore, expecting to find all applicable criteria in the applicable section would not be unreasonable. For example, if you are using SSUs and SSPs and are looking for position, location, spacing, and use requirements; you should be able to find all the applicable requirements in section 8.6. You should not have to research requirements in 8.8 because these are for extended coverage upright and pendent spray sprinklers. The same logic applies within the individual sections. Unfortunately, NFPA 13 does not always present criteria in this logical sequence. One such portion is the obstruction criteria for SSUs and SSPs. If I have a 4 ft 6 in. wide duct above the 18 in. plane, where do I find the applicable obstruction criteria? Following the sequence and looking at 8.6.5.1.2(2), I find that obstructions up to 4 ft in width are addressed, but not beyond. So I move on to 8.6.5.2, which specifically addresses obstructions above the 18 in. plane. Unfortunately this section tells me to apply the "3 times" rule with no limits on width, which is obviously is not the intent. I can only locate the applicable criteria if I read the requirements of 8.6.5.3.3 which says, "The requirements of 8.6.5.3 shall also apply to obstructions 18 inches or less below the sprinkler for light and ordinary hazard occupancies." This means that sprinklers must be installed under obstructions exceeding 4 ft in width regardless of where they are located. A more logical presentation would have the criteria located in the section dealing with obstructions above the 18 in. plane. Each time NFPA 13 is updated, these issues must be addressed by the appropriate committees. We intend to bring these to the table for the 2006 edition. In the meantime, if you notice inconsistencies or inaccuracies in the standard, please submit proposals to NFPA.

<top of page>



AFSA
Home | Copyright © AmericanFire Sprinkler Association. All rights reserved.
Search