The Correct Deflector Height for Pendents & Uprights in Obstructed Construction
Here we're continuing our discussion on layout for standard spray, pendent and upright sprinklers.
This segment is specific to standard spray type sprinklers, and specific to pendent and upright sprinklers. All of the criteria we're discussing here is from NFPA 13.
So, in our last segment, we talked about the correct height of this type of sprinkler for Unobstructed construction. That's relatively straightforward. We had a set range and a few caveats.
Today, we're talking about Obstructed construction.
So NFPA 13 Chapter 10 is specific to Standard Spray sprinklers.
Section 10.2 covers pendent and upright sprinklers. We’ve been hanging out in this area of the code a lot recently.
Subsection 10.2.6.1 is the Distance Below Ceilings (or height, in other words)
Finally, subsection 10.2.6.1.2 covers Obstructed Construction.
This is where we are left with five options which we can choose from as a designer. Let’s go through these visually.
OPTION #1: THE GENERAL RULE
Option #1: The General Rule.
Now that’s not a formal term, I’m using it here casually. But the gist is that the sprinkler deflector must be 1-6 inches (25-150 mm) below the bottom of the structural member, and a maximum distance of 22-inches (550 mm) below the ceiling or roof deck.
This is our traditional Obstructed Construction approach. You’ve probably seen this image and this rule already a few times in prior series that we’ve gone through. When we get into obstructed construction, this is the general guidance that we're gonna have about the height of the sprinkler.
Now the benefit of Obstructed Construction is that we have a little more tolerance, or leniency, in the height of what the sprinkler is allowed to be. We don’t have to be within 12 inches (300 mm) of a ceiling or roof deck where we are considered Obstructed Construction.
So, where does the deflector go if we choose this general rule option #1? Well, that could be anywhere 1-6 inches (25-150 mm) below the bottom of the structure and still within 22 inches (550 mm) of the ceiling or roof deck. That’s our Option #1 and that's pretty straightforward. That's going to be our most common approach for Obstructed Construction.
OPTION #2: OBSERVE OBSTRUCTIONS
Option #2: The deflector is allowed to be above the bottom of the structural member, up to 22-inches (550 mm) down from the ceiling or roof deck, but, we have to meet the obstruction rules of NFPA 13.
Under this specific set of rules, we’re basically saying that we’re going to be able to cover the floor area properly by throwing water underneath an obstruction, by putting sprinklers on both sides of the obstruction, by having sprinklers below an obstruction, or by the obstruction having a small enough impact that we’re just allowed to ignore it.
We will spend a lot of time on each of these in future obstruction segments.
For now, if we want to locate sprinklers above the bottom of structure and this is the path we want to go down, then those options are gonna be listed in NFPA 13 10.2.7.2. That’s where we’ll want to start and work through our different ways of acknowledging and designing around the obstructions. Now, I don't wanna go all through these rules today because that segment would be a whole lot longer than this one, and we will certainly go through each of these in our series of obstructions. But just know for today that if we go with this option number #2, that we are basically observing the obstruction rules so that we can effectively throw water below obstructions or place sprinklers on each side of the obstruction as if it's a wall and then we can design around the obstruction and comply with NFPA 13, allowing us to have sprinklers above the bottom of the structural member. So, the gist of this option is that we can be above the bottom of the structural member as long as we get water below it.
OPTION #3: SPRINKLERS IN EACH BAY
Back to #3 on the height of a sprinkler for obstructed construction, and that is Option #3, Provide sprinklers in each bay, 1-12 inches (25-300 mm) below the ceiling or roof deck.
Now, this option just has sprinklers in each bay. What is a bay? That’s the box created by structural members. If we have beams that frame into girders, then a bay would be the space between two beams with boundaries that end at each girder. That would be a structural bay.
Now that sounds easy enough, a simple enough approach, but what if each bay happens to be very narrow? Well, from a sprinkler design perspective, that could be problematic.
Let’s say we have beams that are only 2-ft (600 mm) apart. Do we really want sprinklers spaced at 2-ft (600 mm) on center?
No, that wouldn’t be good.
That wouldn't be good for a number of reasons. One is that it's costly to add that many sprinklers and fittings and outlets to a project, but also it's going to drive up our hydraulic demand. If we have that many sprinklers and we have to have minimum pressure at each sprinklers, now we're throwing a whole lot more water than we otherwise would need to. And as a designer, we want to be more efficient with our use of water.
But option #3 nonetheless says that we can position sprinklers in each bay. Again, that's more feasible when our bays are a little bit larger.
OPTION #4: COMPOSITE WOOD JOISTS
Option #4: If we have Composite Wood Joists, with channels that are no more than 300 square feet (or 28 square meters), and the channels are sufficiently firestopped, then we can have sprinklers 1-6 inches (25-150 mm) below the bottom of the joists and up to 22 inches (550 mm) below the ceiling or roof deck.
Now we covered composite wood joists in our structural series but just as a reminder, composite wood joists are built up “I” shape composed of three pieces, you have a top cord, a bottom cord, and a solid web. They have solid webs, usually made from plywood or OSB. They are not a single solid structural piece of lumber nor do they open-web joists. Composite wood joists are their own thing again in that “I’ shape, top cord, bottom cord, and the web.
Now, this option obviously only applies to composite wood joists, but if we have deep enough joists, this allowance would keep us from having a sprinkler within each bay.
The note on the fire stopping NFPA 13 is specific that the fire stopping method has to be at least as fire resistant as the web of the composite wood joist. So, I wanna acknowledge that there's some further rules to get into there, and there's some caveats that I would say most architects are not familiar with if we're going this route. So just be aware that if we go this method, we're gonna need to coordinate those fire stopping requirements with an architect.
OPTION #5: CONCRETE TEES
Option #5: If we have concrete tee construction, and the stems are less than 7.5 feet (2.3 m) on center, and we position the sprinklers to throw beneath the stems, then we can have sprinklers at or above a plane that’s 1-inch (25 mm) below the bottom of the stems. So, we can have sprinklers below the stems or they can be located above the stems when we are throwing water underneath the stems. But this is going to allow us to exceed 22 inches (550 millimeters) from the duck above.
What is a concrete tee? Well, we know tt’s obviously concrete, but these generally have stems (or the vertical part of the tee), that extend deeper than they are thick.
If this is our scenario, then the code is gonna tells us we have to use the “beam rule” table to make sure we’re throwing water underneath the stem of the concrete tees.
Now, if you haven’t worked on a parking garage before – sometimes concrete tees can be very deep. If we had to have sprinklers within 22-inches (or 550 mm) of the deck, that could mean sprinklers in every single structural bay. Those bays can be very narrow, so that would again mean a lot of sprinklers, and a whole lot of water coming through our hydraulically remote area. That would be bad.
This allowance, this Option #5, gives us further grace in the case of having deep concrete tees that we can locate sprinklers further down our 22 inches (550 mm) in that situation.
Also remember that in this whole picture today we're talking about obstructed construction – if those stems are more than 7.5 feet (or 2.3 meters) away, center to center, then our area should be considererd unobstructed. And all of these will apply.
Those are the five options that we have in regards to setting the height of a sprinkler deflector for a standard spray, upright or pendent sprinkler.
Now, if that feels like a lot to take in, well, it can be the first time we go through.
There's a couple key things to remember that hopefully will simplify this whole process for us. Remember that when we're following a code or standard as a guiding principle, we want a code path for each decision we make.
Now, 99.9% of the decisions that we make in design won't be written down. Over time, we learn tendencies and we learn the rules, and we just innately memorize some portions of those, and we really only need to create a code path for novel, unclear, or unique situations. But we do, we do always follow a line of logic when we're following code.
So, if we're in design or plan review or installation or inspection and we don't feel confident in how we're getting to a certain approach, then go back to the basics and write down the code path.
You can just open your notebook manually write it down, or open Microsoft Word, just a blank sheet and start at the top.
This project has the International Building Code 2024 Edition applied. This project uses the IBC 2024 edition, which references the 2022 edition of NFPA 13. Great. Now go to the next line, and indent.
NFPA 13-2022 Edition. Great. Go to the next line, and indent.
Chapter 10 covers requirements for Standard Spray Sprinklers. Next line, indent.
Section 10.2 covers Pendents and Uprights. Next line, indent.
Section 10.2.6 covers Deflector Position. Next line, indent.
Section 10.2.6.1 covers Distance Below Ceilings. We’re getting closer. Next line, indent.
Section 10.2.6.1.2 covers Obstructed Construction. This is where we find ourselves today. This is where we have five options, and we only have to meet one of those five options.
So, what is our applicable deflector height? Well, if we follow the first option that we went through today (our general rule), then we’re taking about Section 10.2.6.1.2(1).
That’s it. That’s our documented code path from all the way on the adopted building code, all the way down to this nuanced detail requirement. Now if we need to justify our design or justify an install, or say we're doing plan review or inspection, and we need to question those things, well, here's the code path in how we get about going there. We've done the code path from the very top to this very detail.
The nice thing about doing a code path for unfamiliar territory is that we can easily save that and go back to it when we need to question it or deviate from that path in the future.
So again, if this whole process feels more complex than it really needs to be, just remember you always have that code path. You can always start at the top work section by section until you get the detail that you need to either justify or question something that's in the code's logic. That in a lot of ways can simplify a lot of things for us. Not to mention, improve our understanding and familiarity in working with the codes and standards. So that's one tip.
The other way of simplifying this whole topic is something we've already put together, and that's our flow chart that covers sprinkler deflector height. You'll see this scenario, what we talked about today on the first page in that flow chart where we've outlined standard spray sprinklers and obstructed construction, and then the five options.
Now, this flow chart is good as a starting point, and it'll get you moving in the right direction, whether you're a design plan review, install or inspection. But as always, be sure to actually open up the codebook and chart your own code path so that you feel comfortable and familiar with the code and the decisions that you're making when you do that.
So; for Standard Spray Pendent and Upright Sprinklers, we have five options in NFPA 13 for the allowable height of a sprinkler.
The first is our General Rule, which is 1-6 inches (25-150 mm) below the bottom of structure and up to 22 inches (550 mm) below the ceiling or roof deck.
The second is observing the Obstruction rules, and making sure water will effectively throw beneath obstructions or by covering the obstuction adequately with the obstruction rules.
The third is simply installing a sprinkler in each bay 1-12 inches (25-300 mm) down below the ceiling or roof deck.
The fourth option is specific to Composite Wood Joists, which allow us to put sprinklers below the bottom of those composite wood joists.
And last, our fifth option, is specific to Concrete Tee Construction where we can have the deflector below the bottom of the stem of concrete tees or above them as long as we're adequately throwing water below the bottom of the stem.
In our next segment, we’re going to cover sprinkler heights for peaked roofs and ceilings.
I’m Joe Meyer, this is MeyerFire University.
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