Introduction to Obstructed Construction

MeyerFire University | FX108.50A

By Joe Meyer, PE

RESOURCES

• Notes Page

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TRANSCRIPT

An Introduction to Obstructed Construction 

INTRO

I’m Joe Meyer and I’m thrilled you’ve joined us for this series. This is one of the first series we start to get into is some good healthy technical content, and we have a lot to share on this topic. I hope you enjoy this one as much as I have putting it together and I’m hopeful that you’re gonna continue to enjoy this the more complex the topics get. 

So, this series is covering Obstructed Construction. I want to introduce a few concepts today that we’re going to be exploring in a lot more detail in this series. 

So what is the term Obstructed Construction, and what does it mean? 

Well, the term Obstructed Construction is one of two ways that we classify the construction of ceilings and roof decks. The other term is Unobstructed Construction. 

Obstructed Construction is a way we describe the construction type that (1) affects how hot gas layer moves along the ceiling of roof deck above a fire, and (2) affects how water distribution happens from a sprinkler down to a fire. Is the flow of heat affected by the construction type at the ceiling of roof deck? Is water discharge from a sprinkler affected by that same construction?

On a fundamental level, if the answer to either of these questions is yes, then we’re looking at Obstructed Construction. 

DEFINITION

So what are examples of Obstructed Construction?

Well, the actual enforceable language from NFPA 13 is limited. That’s in chapter 3 for the definition of Obstructed Construction. But the annex of NFPA 13 covers ten different examples of what the committee considers to be Obstructed Construction. Where is it in the Annex?  

The definitions chapter of NFPA 13 is Chapter 3. Within Chapter 3, we have the definition for Obstructed Construction, which is…

Panel construction and other construction where beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire. (AUT-SSI)

The annex material that gives us all these examples of Obstructed Construction is just that mirrored in the annex. 

OBSTRUCTED CONSTRUCTION EXAMPLES

There are ten examples that are given to us for Obstructed Construction in the Annex of NFPA 13, they are:

• Beam and Girder Construction 
• Concrete Tee Construction 
• Composite Wood Joist Construction 
• Panel Construction 
• Semi-Mill Construction 
• Wood Joist Construction 
• Bar Joist Construction with Fireproofing 
• Steel Purlin Construction 
• Truss Construction (wood or steel) 
• And Bar Joist Construction (wood or steel) 

As part of this series, we’re going to break out what each of these examples functionally mean and when they crop up in the built’s environment.

SPRINKLERS 

But why does Obstructed vs. Unobstructed even matter? 

It’s all about locating sprinklers. 

Where are sprinklers allowed to be? How far apart can they be? Should they be up high near the roof deck above structural beams, or down low so they can throw water beneath them?

We have a flowchart for sprinkler spacing that we’re showing here (a link is below maybe add arrow, pointing down to link for emphasis) which is the decision tree we use for sprinkler spacing out of NFPA 13. Some sprinkler types and some configurations rely on us to determine whether we have Obstructed or Unobstructed Construction just in order to figure out what our maximum sprinkler spacing is.  

That’s a consideration that’s in a two-dimensional plane, but this discussion also affects the height of the sprinkler as well. We have another chart for this and that is the deflector height for a sprinkler. 

If we have Obstructed Construction, NFPA 13 is gonna give us a little more lenience. We’re gonna have a little more tolerance so to speak on where we’re allowed to locate sprinklers. If we’re Unobstructed Construction, generally we’re gonna be allowed to locate sprinklers lower down from a ceiling or roof deck than we otherwise could with Unobstructed Construction. 

CHART EXAMPLE

Let’s look at just one example where the construction label we get for the ceiling affects our sprinkler spacing. 

Look at Standard Spray sprinklers in our flowchart here.  

Now let’s say we have combustible construction, and we have exposed structural members that are more than 3 feet on center. Well, if our construction type is considered Obstructed, then our maximum sprinkler spacing is 15 feet or 168 square feet. We can’t go 15 by 15 here ‘cause we’re maxed out at 168 square feet per sprinkler. But if our construction type is considered Unobstructed, then with a hydraulically calculated system, our maximum spacing is still 15 feet but we’re allowed a coverage area of 225 square feet per sprinkler. That’s substantially different just by this distinction between Obstructed and Unobstructed construction. 

That’s one example but determining Obstructed vs. Unobstructed construction affects the maximum allowable coverage area for a sprinkler. It will affect the maximum spacing for a sprinkler, and it will affect the allowable height that a sprinkler can be. 

WRAP-UP

That’s it for the introduction for this series. Let’s get going. I’m excited that you’re here and I hope you enjoy this series. Thanks for being a part of the university platform. 

I’m Joe Meyer, this is MeyerFire University.