Unobstructed Construction: Bar Joist Construction

MeyerFire University | FX108.52B

By Joe Meyer, PE

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RESOURCES

FX154 SERIES

RESOURCES

NOTES & SUMMARY

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

CODE & STANDARD REFERENCES

• NFPA 13 – 2022: A.3.3.43.2(1) Examples of Unobstructed Construction
• NFPA 13 – 2022: 3.3.43.2 Definition of Unobstructed Construction

FX154 SERIES

1. Introduction to Unobstructed Construction
2. Difference between Obstructed & Unobstructed?
3. ​Unobstructed Construction: Bar Joists
4. Unobstructed Construction: Open-Grid Ceilings
5. Unobstructed Construction: Smooth Ceilings
6. Unobstructed Construction: Standard Mill
7. Unobstructed Construction: Trusses

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TRANSCRIPT

Examples of Unobstructed Construction: Bar Joist Construction 
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INTRO

Now, we’re starting our series on unobstructed construction with the first example that NFPA 13 gives us – and that is bar joist construction. 

Now, quick recap, unobstructed construction is where heat flow or water distribution are generally not impacted by structural members. This means that heat and sprinkler flow are generally uninterrupted by the ceiling or roof construction.

DEFINITION

The definition of unobstructed construction, just to revisit that quickly, states such and adds that unobstructed has non-solid members where (1) openings are at least 70% of the cross-section area, (2) the depth of the member is not over the least dimension of the openings (we’ll come back to this), or, all construction except panel construction where the spacing of structural members is more than 7-1/2 ft or 2.3 meters on center.

What we’re looking for here is mostly open structure or structure that is widely spaced apart. Again, the central theme or the key here being that we’re concerned about heat flow and sprinkler distribution. If those two things are generally not impacted by structure, then we're looking at unobstructed construction. 

BAR JOIST CONSTRUCTION & FIREPROOFING

So, let’s hop into bar joist construction. 

Now during our series on obstructed construction, we talked about Bar Joist Construction with fireproofing and we talked about Bar Joist Construction. Well, what’s the deal here? What’s the difference? Why does bar joist construction show up in obstructed and unobstructed construction examples? 

Well, first, check out our videos on those, the links should be below. We got into a lot more detail in each of those two scenarios.

FIREPROOFING
 
Second, bar joist construction with fireproofing is when the fireproofing is applied onto the bar joists and obstructs more than 30% of the bar joist profile. That's clearly a different situation than just open bar joists that we’re talking about today. 

But what about that example of obstructed construction that is just listed as bar joist construction?

THICK CHORDS

Well, if you remember in that example, bar joist construction is considered obstructed when wood or steel joists have top and bottom chord members that are greater than 4-inches or a 100 mm in depth. When those top and bottom chords are really thick, NFPA 13 is considering them to be obstructed. They’re impacting the heat flow and the water distribution. 

UNOBSTRUCTED BAR JOISTS

So, what’s our scenario today that brings about unobstructed construction for bar joists? Well, first let's hit the definition. 

To be considered unobstructed, bar joists should have steel tube or bar webs (even if the chords are wood), and the top and bottom chords can’t exceed 4-inches or a 100 mm in depth. 

So, like we looked at before, it’s really all about those chords. If the chords are more than 4 inches deep, we’re obstructed and NFPA 13 lays out that example nicely. If we’re 4 inches or less, we’re unobstructed and we meet the definition that we have today. 

WOOD CHORDS

Well, what if those chords are wood? Does that still meet today’s definition? 

Yes, it does. If the chords are wood but there are metal tubes or bars for the web members, and those wood chords are 4 inches or less, then we’re unobstructed meeting the definition we’re seeing here today. 

BAR JOIST PROFILE

Now, what about that original definition of unobstructed construction? Are bar joists really 70% open?

Well, we dove into this with a test case in our video on bar joists with fireproofing. There, we were trying to figure out what that level of openness was and we found that just a straight 14-inch-deep bar joist was about 27% obstructed before the fireproofing was applied. So then when we applied the fireproofing, it was more than 30% obstructed. But even in that case, if the web members are angle iron and not tubes, they’re still going to generally be more than 70% open.  

The only exception I can see is where we have an exceptionally squat joist, where the depth is very shallow. Think about something like 6 inches or 8 inches deep. The depth of the chords will start to dominate that profile and start to obstruct quite a bit of that profile. This would be unusual, though. Joists that are that shallow are just not gonna be as strong as a joist that is 12 or 14 inches deep. 

That's why when we see a typical joist, we're seeing something that's in the ballpark of about a foot deep. If a joist were gonna be that squat, what would just go with a beam? A solid beam or something like that. The other aspect of the original definition on unobstructed construction is that opening. The minimum dimension of the opening has to be greater than the depth of the joist. Well, if you have an open truss, well, if you have an open bar joist, you're gonna have a minimum dimension. Also on that original definition, the depth of the joist can't exceed the least dimension of the openings. Now I'm thinking, just reading between the lines here, what they're trying to prevent is a joist that is otherwise solid, but has round holes poked into them. Those are gonna be mostly solid and honestly, they'd be over a 30% obstructed profile anyways.

INSULATION

Now let’s say we have insulation between the joists, and that insulation is covering up some of the top chord. Would we need to count the hidden part of that chord in our 70% openness calculation? 

Well, the NFSA say, the National Fire Sprinkler Association had a writeup on this back in 2018. The informal interpretation synopsis was that only the portion of the chord that is below the insulation would really need to be considered, because that is what is impacting heat flow and sprinkler distribution. 

SUMMARY

So, for today in summary, if we have bar joists that have steel web members even if the chords on top and bottom are wood, we will be considered unobstructed construction as long as those chords aren’t more than 4 inches deep and the joist profile is at least 70% open. 

Now next in this series we’ll discuss open grid ceilings which is our second example of unobstructed construction. 

I’m Joe Meyer, this is MeyerFire University.