Obstructed Construction: Composite Wood Joists

MeyerFire University | FX108.51C

By Joe Meyer, PE

RESOURCES

• Notes Page

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TRANSCRIPT

What are Examples of Obstructed Construction: Composite Wood Joists? 

INTRODUCTION

We’re continuing on our series of the “Obstructed Construction” designation in NFPA 13, today covering the example of Composite Wood Joists. 

To start, Obstructed Construction is a designation we give for construction that impedes heat flow or water distribution that affects the ability of sprinklers to control or suppress a fire. It’s defined in NFPA 13 Chapter 3.  

In the Annex portion of the standard, again, a non-enforceable but helpful part of the standard that clarifies and supports the rest of the book, we have examples of Obstructed Construction. Ours today is Composite Wood Joists, which is the third listed example of Obstructed Construction that is provided.  

COMPOSITE WOOD JOIST CONSTRUCTION

If you remember from our structural videos, composite wood joists are the engineered beams in the shape of the letter “I”. They have a solid wood web, usually OSB nowadays, with wood flanges on the top and bottom.  

Composite Wood Joists were created in the 1970s because of the desire for long floor spans without columns in “open-concept” homes. Because solid lumber was expensive and difficult to source in lengths longer than 20-feet, engineers developed the Composite Wood Joist which could handle the longer spans. They were notably more expensive up until a timber crisis in the 1990s when the prices were more stable than solid wood, and they then became more popular in mainstream residential construction. 

NFPA 13 DESCRIPTION

As described in the annex of NFPA 13, Composite Wood Joists can be up to 48-inches deep (1.2 meters), can be spaced up to 48-inches (1.2 meters) on center, and can span up to 60-feet (18 meters) between supports. 

Joist channels (or the open volume that runs parallel to the joists), should be fire stopped to the full depth of the joists with material equivalent to the web construction. The annex material goes further to say that individual channel areas cannot be over 300 square feet (28 square meters).  

FIRESTOP

One curious note on this section is that the annex portion of NFPA 13 here is telling us we need to firestop the space in-between the joist channels. Is that NFPA 13’s territory? What if that doesn’t happen? 

Aside from firestopping requirements elsewhere in the building code or other applicable standards, if we don’t qualify for obstructed construction, then sprinklers generally need to be located closer to the ceiling or roof deck above. With a reasonably deep composite wood joist, let’s say a 14-inch (355 mm) deep joist, that would mean sprinklers still need to be within 12-inches of the deck above. So, if we don’t qualify for Obstructed Construction, that means a sprinkler in every single joist pocket. Yikes! That could be a lot of sprinklers. 

Why is that? It’s because we don’t want a fire to send heat along a linear path far away from the heat source. If it does, we could be activating sprinklers far from the fire that could overtax our system.  

Also, we don’t want to intentionally give heat a pathway to continue burning and growing. We want to limit that spread as much as possible, and get water only where it needs to go, close to the fire. 

The firestopping here helps contain the movement of heat. 

COMPOSITE WOOD JOISTS VS. SOLID WOOD

Are composite wood joists different than solid wood members? Yes, they are.  

NFPA 13 differentiates between the two, because they are different. Composite wood joists are often spaced at greater distances and are typically deeper as well. They burn differently. Solid wood tends to char, and form a layer on its outside before it can slowly burn towards the center. Composite wood joists are manufactured with wood and glues, have less thickness in the web members, and therefore burn differently. 

Also, solid wood members tend to be spaced close together, are more shallow, and create much smaller volumes in-between them. It quickly becomes impractical to install so many sprinklers within each of these volumes. 

Because of this, NFPA 13 differentiates between Composite Wood Joists and solid wood joists. 

FIRESTOP MATERIAL

Last, on this topic, what does that firestop material need to be? It depends on the building code. NFSA addressed this topic in January 2013 with an informal interpretation in their TechNotes. If the applicable building code has specific firestopping requirements for limiting these joist channels, then that is going to stipulate the minimum requirement. In the absence of specific guidance, NFPA 13 is only telling us that we need to firestop using a material at least equivalent to the web of our Composite Wood Joist. If that’s 3/8-inch OSB, then that’s all we’d need to use to firestop the channel. 

When we think of firestopping, we usually don’t think of particle board as a method of stopping fire. Under the guidance from NFPA 13, that’s not really the overall goal here. The purpose here is to stop the movement of hot gases early in a fire from traveling along that joist channel, and going far away from the fire. We want to limit the spread of those hot gases so sprinklers nearby have a chance to activate and fight the fire, early in the stages of fire growth. 

SUMMARY

In terms of obstructed construction, Composite Wood Joists are listed as the third example of Obstructed Construction. The key takeaway in the annex example is the need for a firestop method on the full depth of the joist so that individual channels between joists are not more than 300 square feet (or 28 square meters).  

In our next segment we’ll cover Panel Construction as the next example in our series covering Obstructed Construction in NFPA 13. 

I'm Joe Meyer, this is MeyerFire University.