The built environment continues to increase in complexity. New design techniques, architectural motifs, building and finish materials, processes, not to mention redefined occupancies, are challenging the relevancy of our current codes and standards, making it difficult to keep up. Several NFPA codes and standards are changing their edition cycles from four- and five-year cycles to three-year cycles to stay on pace.
While the architectural and engineering community is usually the first place for these changes to surface, those involved with the engineering, design and installation of fire and life safety systems are the ones left to deal with the results of new and innovative techniques and materials. With the introduction of these new features comes new undefined hazards.
For years, the fire protection industry responded to ongoing changes with proportional requirements for fire sprinkler systems. After all, these systems have enjoyed overwhelming success protecting life and property due to the continually updated requirements for their design and installation.
It stands to reason that those involved must have a thorough and relevant understanding of system requirements. And nothing is more important when it comes to fire sprinkler systems than where they are installed. I call it “spot the dot.”
As I sat in a restaurant decades ago, I caught myself wondering about how far I was from the entrance I came in and where my second exit might be. If a fire started, what is my best chance of getting out of this place alive?
My quest for answers became focused on why we put that dot (fire sprinkler) where we do. Is it just because the “book” (NFPA 13) tells us to — or could there be a methodology for fire sprinkler design? NFPA 13 has the answer if you care enough to look closely.
NFPA 13-9.1.1 (3) 2019 edition notes, “Sprinklers shall be positioned and located so as to provide satisfactory performance with respect to activation time and distribution.” For me, the other eight descriptions listed in this section for sprinkler installation are just side notes. Earlier editions of this section simply say, “with respect to activation time and distribution.” A better way to say it: “with respect to sensitivity and discharge.”
You may not believe there is much difference between those two statements. When put into the context of sprinkler spacing and location, the difference is as big as half-inch residential sprinklers compared to one-inch early suppression, fast response sprinklers.
However, the important question is when you put weight to this performance level — sensitivity and discharge — is one more important than the other? To answer this, you must understand the rules governing those who spot the dot. And a good place to start is with sensitivity.
There is significant weight to these two principles. NFPA 13 puts them in the same sentence as if they are equally important; I would argue they are not. The scale of balance is tipped when you take a closer look. To find the imbalance, you need to understand the construction definitions and the rules that go with them.
While we have certainly progressed from the days of drafting boards and light tables, we are still looking at floor plans, even if they are digital. Certainly, the 3D world helps us better understand the space than years before, but the piping plan is still a plan view of a sprinkler system. No wonder floor coverage and spray pattern get all the attention.
But, what about the “Z” dimension? What about the elevation of that dot? It seems many look at elevation concerning the centerline of pipe in their quest to make it fit rather than to make it go off.
So, where do we find these “sensitivity rules”? Turn in your book to 3.3.41 — Construction Definitions; these dictate the vertical distances involved with sprinkler layout. Here you will find two definitions: obstructed (41.1) and unobstructed (41.2).
After reading the definitions, you will most likely be thankful for the pictures, especially those in the NFPA 13 Handbook, because the terms used can be confusing, especially to those not familiar with structural engineering or construction terms.
The difference between the two categories may not be strikingly obvious or significant to other disciplines and trades. For fire protection engineers, it is arguably the difference between life and death. I will go so far as to say that if you cannot recite these definitions by memory, you have no business designing, reviewing plans or inspecting fire sprinkler systems. It is that important
Deflector Location in Obstructed Construction
The first construction definition defined is obstructed. It is the harder one to remember. Do not confuse the use of the word obstructed as it refers to construction with that of obstruction to discharge. I know the annex and handbook supplements use the word spray pattern and obstruction in the same sentences and paragraphs, but this word means two different things regarding sensitivity and discharge.
When talking about construction, obstruction is a good thing. I would rather be in a building of obstructed construction than that of an unobstructed one during a fire, combustible or not. This is not obvious on the surface; many users tend to compartmentalize the standard and do not search for the connection between definitions and rules.
If you put this together with the rules referencing these construction definitions, you will see they define the vertical spot for the dot.
Keep your finger in 3.3.41 and flip over to chapter 10.2.6.1.2. Here you find that in obstructed construction, you are given five options for deflector location, depending on the type of obstructed construction you have
• Option 1: 1 inch to 22 inches maximum from the roof deck and no further than 6 inches down from the bottom “plane” of the beams/joists/trusses that are considered obstructed construction (see Figure 1)
• Option 2: 1 inch to 22 inches maximum from the roof deck with the deflector at or above the bottom plane of the beams/joists/trusses that are considered obstructed construction — but only in the cases where you can meet the beam rule (10.2.6.1.2). See Figure 2.
• Option 3: 1 inch to 12 inches maximum from the roof deck inside of every bay (or structural pocket created by the individual solid-webbed members themselves), regardless of depth (see Figure 3).
• Option 4: 1 inch to 6 inches maximum from the bottom of composite wood joists to a maximum of 22 inches below the roof deck. The only condition here is that the composite wood joists are fire-stopped into structural pockets of not more than 300 square feet (see Figure 4).
• Option 5: 1 inch maximum below the bottom plane of concrete tee stem legs spaced less than 7 feet, 6 inches on center. The nuance here is that the depth of the concrete tee leg is not considered. See Figure 5.
The deflector cannot be more than 1 inch below nor above the bottom of the stem legs unless, similar to Option 2, its horizontal location from the stem leg meets the beam rule (10.2.6.1.2). If you draw this, it will look something like Figure 6.
As you can see, the deflector is below the bottom of the structural member considered to be obstructed construction. And all are solid-webbed, except for the case of deep chord bar joists (top and bottom chords greater than 4 inches). Think about it. We are allowed to bring the sprinkler down as far as 22 inches — almost 2 feet! — from the roof deck. What is it about this type of construction where we can drop the sensitivity that low?
The answer is the solid webbing. If heat is what we need to activate a sprinkler, then what better way to capture the heat than to create structural pockets using the solid-webbed members to develop boundaries? This aids in heat collection, slowing it down and allowing the ambient temperature to rise, whereby the sprinkler feels the heat and activates.
Exceptions to this telling feature are fire-proofed bar joists and deep-chord bar joists; the rest are solid web. And realize that combustibility does not play a role here. We are talking about steel, concrete or wood.
When applying the obstructed construction rules for deflector distance, you can end up with the deflector above the bottom of the lower chord or solid member, which is OK. However, it is then that you need to consider spray pattern or discharge and move into the other use of the word obstruction. This will be discussed later in this series.
For now, let’s concentrate on the uncomplicated examples. Figure 2 illustrates obstructed construction and the rules governing and relying on it.
Exceptions to the Obstructed Rule
Now, for the exceptions that go along with this section. First is what I call the panel exception: If the structural pocket described earlier, created by obstructed construction members, constitutes an area of 300 square feet or less, then the spacing of the members can exceed the 7 feet, 6 inches maximum of the basic definition (see Figure 7).
Another unique construction type found in obstructed construction is concrete tees. Years ago, they were stand-alone but eventually found their way into the obstructed definition. Interestingly though, the rule governing this type of construction remained the same. However, the deflector should be located a maximum of 1 inch below the plane of the bottom of the tee legs.
It can be up into the tee legs' channel, but only as much as the beam rule will allow. Keep in mind that this is regardless of tee leg depth. So, save yourself some grief and install the deflectors at the 1-inch plane below the tee legs and move on.
To summarize, the definition for obstructed construction can be refined to these three points:
1. Solid-webbed member except for fire-proof bar joists and deep-chord open webbed joists.
2. Up to 7 feet, 6 inches spacing of these members.
3. Member spacing greater than 7 feet, 6 inches when the structural pocket or panel created by the solid-webbed members and associated girders or larger beams is no larger than 300 square feet
When I teach this concept, I include a little limerick to help commit it to memory: “One to twenty-two, maximum of six; solid web to seven-foot six."
Deflector Location in Unobstructed Construction
So, that is the hard one. Unobstructed construction is much easier. One way to look at this is: if it is not obstructed, then it is unobstructed. And the rule is very simple. The deflector distance is a 1-inch minimum to 12 inches maximum below the ceiling plane above (see Figure 8).
You would think that given the relative simplicity of these two rules, it would not be difficult to follow them. Unfortunately, I do not believe this is the case. I can walk into any big-box store or light-to-ordinary-hazard occupancy that does not include a ceiling and find at least three or more sprinklers that are not installed in the correct vertical distance from the ceiling plane.
Let me state the issue again just in case I was not clear enough earlier. We are not paying attention to deflector distance. Period. I am still confronted with installers, fitters and inspectors who think the only rules they need to pay attention to are for sprinkler spacing — that being 7 feet 6 inches off a wall and 15 feet between sprinklers. But those days were over more than 20 years ago.
I suspect this is because they view the centerline of pipe only for installation purposes rather than the centerline being set because of required deflector distances and sensitivity. All too often, I see typical grid branch lines running in bar joists off a cross main; inevitably, there will be a riser nipple that has been cut back because the centerline landed on a bar joist’s web.
It is simpler and faster to cut back and re-thread one end of a riser nipple than to cut a long one and prep and thread both ends with a new fitting. What happened when the riser nipple was cut back? Sure, the branch line runs straight through the bar joist now, but what happened to the deflector distance? Does the fitter understand what he has done to the sensitivity of the system?
If the designer sets the centerline at 12 inches using the sprinkler's size to get him within the 12-inch maximum, there is a good chance the deflector will be well past that distance since the riser nipple has been cut back. Now we have an entire grid branch line of sprinklers too far down from the deck.
A few weeks ago, I was in a conversation with a fitter/foreman who could not remember where and when the 9-foot rule is allowed. Why would we think anyone would have any idea what the rules are for deflector distances, let alone the importance they play in the success of a sprinkler system’s activation? My perception is that the only people who know and use the construction definitions and their associated rules are the “dot spotters” (designers).
After that, all bets are off. And since many sprinkler fitters are not taught these rules and the AHJ’s are not checking them, we all go home thinking we have done a good job. Everyone is safe and sound because every square inch of floor space will get some water on it. But the sprinkler has to activate for lives and buildings to be saved.
You will notice that in recent editions of NFPA 13, even more rules regarding allowed “shadows” or dry spaces are being defined. In my opinion, it is ridiculous to go to this level of detail all because people are hung up on water on the floor.
Light-and-ordinary-hazard occupancies, which make more than 90 percent of our built and occupied environment, are weighed much more toward life safety than property protection. Let us continue this discussion in the future, where we will discover that sensitivity and discharge are not as equal as they appear — and NFPA 13 and the building codes will prove it.
So, here is the challenge for you: Over the next few days, wherever you go, look up and determine the construction definition on the spot. Then look at the deflector locations and ask yourself, are the dots in the right spot? Or should you be looking for that second exit?