CODE & STANDARD REFERENCES
Unobstructed Construction: Truss Construction
So, we’re down to our last example of Unobstructed Construction that’s in the Annex of NFPA 13, and that is Truss Construction.
Now if you took part in our series on Obstructed Construction, you might remember that Trusses are also listed as an example of Obstructed Construction. What’s the deal? Why is it listed in both areas?
Let’s hop in.
So in order to qualify as Unobstructed Construction, trusses have to have top and bottom chords that do not exceed 4 inches (100 mm) in depth. That’s pretty much it.
The Annex states parallel or pitched chord members, open webs, and supporting a roof or floor deck – but all of those things don’t really limit what we see in trusses, at least not in most commercial applications.
The real kicker here to be unobstructed is that the top and bottom chords need to be 4 inches or less, that a 100 mm or less.
Now if we want to pick hairs, both definitions say top and bottom chord members need to be more than 4 inches for Obstructed, 4 inches or less for Unobstructed. Well, what if the top chord is different depth than the bottom chord? Well, technically that would fall outside of both of these examples, but for argument's sake, what's probably most important is that top chord where the hot gases are collecting and near where the sprinkler is getting ready to discharge. That's kind of where all the action's at.
So, when we’re 4 inches (100 mm) or less on the chords, we set up nicely with unobstructed construction.
When we’re more than 4-inches, obstructed construction. Fairly straightforward. Also, that top and bottom chord are pretty much always gonna be the same depth. That's just the way the trusses are made.
What are the ramifications?
Well, when we’re Obstructed Construction, it’s a little bit more lenient in that we can be as far as 22 inches (or 560 mm) down from the deck or 1 to 6 inches (25 to 152 mm) below the bottom of the structural member, whichever is higher. So, if we have deeper chords, we're allowed to be a little bit lower in elevation with that sprinkler deflector further down from the deck. If we are unobstructed construction, spray sprinklers are going to need a deflector height within 12 inches or 300 millimeters of that deck above, that's much less flexible situation.
Bottom line today, quick summary on a very quick video.
If we have trusses, check the cord depth, that'll guide us into either unobstructed construction, where we have four inches or less on that core depth, which is probably gonna be most cases, unless you have a really big truss or obstructed territory.
Obstructed construction, where we have cords that are greater than four inches or that a hundred millimeters in depth. From there, we follow NFPA 13 and get our guidance on deflector height and spacing.
This last example on Truss Construction wraps up our series on Unobstructed Construction. If you’ve joined us for this entire run, while we’ve introduced the concept of unobstructed construction, defined the term out of NFPA 13, and ran through the examples that the annex gives us.
These examples included:
So, with a better understanding of how we define unobstructed construction, we’re now set to move onto how these arrangements affect sprinkler position and that will help us out when we do design, review, and inspection.
Thanks for joining us for this. I’m Joe Meyer, this is MeyerFire University.
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