Obstructed Construction: Wood Joist Construction

MeyerFire University | FX108.51F

By Joe Meyer, PE

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RESOURCES

FX153 SERIES

RESOURCES

NOTES & SUMMARY

• One-Page Summary [PDF]
• Notes Page [PDF]

CODE & STANDARD REFERENCES

• NFPA 13 – 2022: A.3.3.41.1(6) Examples of Obstructed Construction: Wood Joist Construction

FX153 SERIES

1. Introduction to Obstructed Construction
2. Difference Between Obstructed & Unobstructed Construction?
3. Why Does Obstructed & Unobstructed Matter?
4. Examples of Obstructed Construction: Beam and Girder
5. Examples of Obstructed Construction: Concrete Tees
6. Examples of Obstructed Construction: Composite Wood Joists
7. Examples of Obstructed Construction: Panel Construction
8. Examples of Obstructed Construction: Semi-Mill Construction
9. Examples of Obstructed Construction: Wood Joist Construction
10. Examples of Obstructed Construction: Bar Joists with Fireproofing
11. Examples of Obstructed Construction: Steel Purlin Construction
12. Examples of Obstructed Construction: Truss Construction
13. Examples of Obstructed Construction: Bar Joists
14. A Quick Recap of Obstructed Construction

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TRANSCRIPT

Obstructed Construction: Wood Joist Construction 

Continuing on our series of Obstructed Construction, here in this video we’re gonna cover Wood Joist Construction. So far, we’ve covered Beam and Girder, Concrete Tees, Composite Wood Joist Construction, Panel Construction, and Semi-Mill Construction. Today, we’re talking specifically about Wood Joist Construction. 

Now, Wood Joist Construction in this context refers to solid wood members with some specific dimensions which support either a floor or roof deck. 

Now these dimensions include: 

A nominal width of 2 to 4 inches (50 mm to 100 mm) 

Spaced up to 3 feet on center (900 mm) 

Spans up to 40 feet (12 m) between supports 

And also, what would be considered wood joist construction are: 

solid wood members that are less than 4 inches nominal width (or less than a 100 mm) 

And spaced more than 3 feet on center (or 900 mm) 

Now depths here in the definition are listed as 14 inches, up to 14 inches nominal (350 mm), but this can also be exceeded. Not exactly sure why it's mentioned up to 14 inches in depth when it then says it can exceed 14 inches in depth and still meet the definition. 

Now, one thing to note here that I’ve missed before is that Wood Joists Construction is different than Composite Wood Joists Construction. 

So, in NFPA 13, we talked about wood joist and we talked about composite wood joist. Wood joists and composite wood joists are two different things. 

Composite Wood Joists have a thin web member and a wider chord on the top and the bottom. It’s like a beam with an “I” shape profile. 

Wood joists are solid members with a rectangular profile. So composite equals that I-shape. A wood joist or a solid wood joist is just a rectangular shape.  

Wood joist construction or solid wood joist have been used to support floors and roofs for a very long time. Rough-sawn lumber can be cut into dimensional lumber and used to support roofs or floors fairly easily. 

These are limited, however, by the length of the spans that they can support, and by the size of the tree which they are cut from. Just because a solid wood joist can span 36-feet structurally doesn’t mean it’s going to be readily available or inexpensive. 

From a fire perspective, these solid dimensional lumber pieces do help us in some respect. They are wider in their width so they have more time that we can allow for charring on the outside before that fire works their way into the beam and compromises the structure. 

With the way that we install these and the spacing that we install these at, we tend to create smaller volumes which can slow or limit fire growth inside.

In this segment, we’re talking about wood joist construction as one example of Obstructed Construction. This discussion also though has major ramifications for combustible concealed spaces, which we will spend a lot of time talking about in future segments.  

Now, in order to omit sprinklers for composite wood joist construction, as example, ceilings have to be attached directly to the composite wood joists, or onto metal channels that are no more than 1 inch thick, that’s 25 mm.  

and each channel cannot be more than 160 cubic feet (or 4.5 cubic meters) 

and the firestop that blocks these volumes off has to be at least as robust as the web of the composite wood joist 

and, if the metal channels are used, 3-1/2 inch thick (or 90 mm) batt insulation has to be at the bottom of the joist channel. 

So there a lot of specific requirements for composite wood joists. 

For Wood Joist Construction on the other hand, or aka solid members, a ceiling only has to installed within 6 inches (150 mm) of the bottom of the wood joists in order to omit sprinklers. 

Now that’s a big difference between these two requirements.  

First, we talked about Composite Wood Joists. Now we're talking about Solid Wood Joists construction. But back to our topic of Obstructed versus Unobstructed, why does it matter? 

In this case, it's a really big deal. 

Let’s say we have an older building that’s being retrofit with a fire sprinkler system. The building has solid wood joists which are 2x14 that support the main entry level. These wood joists are exposed in the basement. There's no ceiling installed on the other side in the basement. 

Now, if this situation is considered unobstructed construction, then our sprinklers in the basement need to be within 12 inches of that deck above. How can we get sprinkler deflectors to be within 12 inches of the deck if the bottom of the beams are 14 inches down from the deck? 

Well, the short answer is that we can't. We'll create an obstruction to that sprinkler discharge and so we would need sprinklers in every single one of these wood joist pockets in order to get the protection we need. If each of the solid wood joists are spaced two feet on center, we're talking about five to seven times the amount of sprinklers on the project just to cover this area. That's crazy land. 

Instead, because of this example of wood joist construction that we have in the annex of NFPA 13, these solid members are considered to be obstructed construction. Again with Obstructed Construction, we can be as far away as 22 inches down from the deck, or as low as 6 inches below the bottom of those wood beams. That means we can locate sprinklers lower down in elevation and not within every little pocket. 

So, all in all, in this segment we introduced another example of Obstructed Construction, and that’s wood joist construction. We briefly talked about the differences between wood joist construction and composite wood joist construction, concealed spaces, and why the distinction is important in terms of obstructed versus unobstructed construction. 

In our next segment, we’re gonna look at Bar Joist Construction with Fireproofing. 

I'm Joe Meyer, this is MeyerFire University.