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.
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