Examples of Obstructed Construction: Bar Joists

MeyerFire University | FX108.51K

By Joe Meyer, PE

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• Notes Page

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TRANSCRIPT

Obstructed Construction: Bar Joist Construction 

INTRO

Now the last of our examples of Obstructed Construction in this series is Bar Joist Construction.  

What kinds of bar joists would meet this definition that would leave us with Obstructed Construction? 

Well, Joe, didn’t you mention earlier that Bar Joists are an example of Unobstructed Construction? 

CHORD DEPTH

Dagnabbit. You got me! Good question. Well in the Annex of NFPA 13 where we find all of these examples of Obstructed Construction, the definition of Bar Joist Construction (Wood or Steel) refers specifically to construction consisting of truss shaped members, wood or steel, that have still tube or bar webs. These can have noncombustible or combustible roof or floor decks, but must have top and bottom chord members greater than 4-inches (100 mm) in depth.

So, just like our last example of Truss Construction, the real kicker here or our real differentiator is that the top and bottom chords need to have a depth greater than 4 inches or 100 millimeters.  

SUBSTANTCE

If we have a wood bar joist, which in this context is a joist with wood top and bottom chords and a steel tube for the web members, this could still meet our definition, but only if that top and bottom chord is more than 4 inches (or 100 mm) deep.  

That’s a pretty deep chord for a bar joist like this. If you have a 2 x 4 that’s turned on its side, well, you’re only seeing an inch and a half from the side profile. 

If it's turned vertically, the 2 x 4 is still only 3 ½ inches deep. It's not more than 4 inches deep. So, it's gotta be something more substantial than 2 x 4 construction in order to qualify for bar joist obstructed construction by this definition. 

Now what's the difference between truss construction that we did in the last video and bar joist construction? Well, not a whole lot. Both are really defined by having a top and bottom chord of more than 4 inches or a 100 mm.  

In a way, it doesn’t really matter what the difference is, whether it’s a truss construction or a bar joist construction, only that the NFPA 13 committee is giving us two examples with deep chords and saying that they both would be considered Obstructed Construction. 

SPRINKLER DESIGN

In either case, whether we have truss construction from our last video or bar joist construction that we're introducing today, with Obstructed Construction, the sprinkler spacing is gonna be affected, the height that the sprinkler can be, which has one example 1 to 6 inches down from structure, or as low as 22 inches down from the roof deck. That's what's gonna be affected by this overall discussion. 

Obstructed Construction gives us some leniency that we can use with sprinkler spacing and sprinkler locations. 

Now that's a real quick rundown of our last example of obstructed construction in this series. But much like truss construction, we just don't have a lot to explore here other than that 4-inch or a 100mm threshold, that's setting the stage for us. 

In our next video covering Obstructed Construction, which is the last in the series, we're gonna wrap up and revisit what we've covered throughout the series about Obstructed Construction in what implications that term or that designation has for us. 

I’m Joe Meyer, this is MeyerFire University.