Why does obstructed versus unobstructed matter?
OBSTRUCTED VS. UNOBSTRUCTED DETERMINATIONS
In short, it's all about sprinkler spacing.
Where are sprinklers allowed to be? How far apart can sprinklers be? Should they be up high within each beam pocket, or down low so that they can throw below each beam?
In Fire Protection design, the description of obstructed versus unobstructed construction matters because it qualifies how we layout, review and inspect sprinkler systems.
SPRINKLER SPACING FLOW CHART
Let's go into this in a little bit more detail.
We have two flowcharts that go along with this video, and links to them are below this video. Just as a side note, if you're ever looking for the most recent version of any of these, you can look up what that most recent version is and even get an updated version over at meyerfire.com/update.
Now the first flow chart is sprinkler spacing. Let's start with a standard spray sprinkler which is using a pendant or an upright orientation. Standard spray is our most common type of sprinkler which has a specific spray pattern to apply a relatively even distribution of water along the floor level.
In order to determine what the coverage area of a standard spray sprinkler is and what its maximum spacing is, we have to know a few critical items.
The first is what the occupancy classification is for the area. Each room can have its own occupancy classification, and even different areas within a single room can have a different occupancy classification.
Now let's be clear what we're talking about here. The term occupancy has different meanings. When we're talking about life safety or passive Fire Protection or egress or occupant loads, the term occupancy is a designation from the building code that identifies what uses are occurring within that portion of a building.
In the context of sprinkler requirements, we use the term occupancy to describe the level of Fire load within an area. It only is affecting sprinkler design and not those other areas from the building code. We will spend a lot of time on occupancies and hazard classifications down the road.
WHAT OCCUPANCY YOU’RE UNDER
The next step depends on which occupancy we fall under.
For light hazard, we then have to determine whether the construction is non combustible or combustible and then we are directed to the different obstructed and unobstructed categorizations.
LIGHT HAZARD OCCUPANCY EXAMPLE
Let's take for example a light hazard occupancy like an office, a residential area, or a movie theater.
Let's say we are using standard spray pendant sprinklers, and the construction is generally classified as non combustible. Well, if we are hydraulically calculating the fire sprinkler system, then our maximum area of coverage per sprinkler is 225 square feet or 20.9 square meters, and our maximum spacing between sprinklers is 15 feet 4.6 meters. This comes out of NFPA 13.
For many commercial spaces, this limitation will become quite common.
MORE COMPLEX LIGHT HAZARD EXAMPLE
But what if our situation is slightly more complex, where we have an art gallery with exposed wood beams. The art gallery could be considered a light hazard occupancy, but we then qualify as combustible construction, and let's say we have wood members that are exposed and are spaced more than three feet apart. Well now in this scenario, we have to determine whether we have obstructed or unobstructed construction. If we're obstructed, then we still have a 15 foot maximum or 4.6 meter spacing between sprinklers, but our largest possible area of protection is only 168 square feet or 15.6 square meters.
Now keep in mind this is still a light hazard space, because of the way that there is combustible construction with exposed structural members, our spacing is significantly less than what otherwise is allowed. If we had planned down 225 square feet per sprinkler and we later get this slap across the face as a surprise, then there could be some major changes to the layout.
OTHER SPRINKLERS AFFECTED
So we used the example of standard spray sprinklers, but the terms obstructed and unobstructed construction also have ramifications for spacing of extended coverage pendants and uprights, CMSA or control mode specific application sprinklers, and ESFR or early suppression fast response sprinklers.
Up until now, we've talked about the allowable coverage area and the spacing for sprinklers, but what about height? We have another flow chart to help determine what the required height of a sprinkler is. This logic also depends upon whether our area is considered an obstructed or obstructed construction. And this effects any sprinkler with a pendant or upright orientation.
In fact if we have a standard spray extended coverage center or ESFR sprinkler, the very first consideration we have to make when we're determining the sprinkler height is the obstructed versus unobstructed classification.
Let's work through a quick example. Let's say that we have concrete Tees in a parking garage where the centerline of the concrete Tees stem is 6 feet on center. Well, this is going to be considered obstructed construction under concrete Tee construction and as long as we observe the beam rule that's defined for obstructions in NFPA 13 that our sprinkler deflector can be at or above a line that's one inch below the bottom of the concrete stem.
So here, even if we have a very deep concrete T, we have an allowance that the sprinkler height can still be below thick concrete T.
So why does obstructive versus unobstructive matter? In three ways. The first is to determine the maximum allowable coverage area for sprinkler. The second is to determine the maximum spacing for a sprinkler. And the third is to determine the allowable height of the sprinkler.
As we continue on we'll get into some more examples of the different definitions and arrangements as we dig into all this in more detail.
I'm Joe Meyer, this is MeyerFire University.
Sentry Page Protection