Obstructed Construction: Truss Construction

MeyerFire University | FX108.51J

By Joe Meyer, PE

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RESOURCES

FX153 SERIES

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NOTES

• Notes Page [PDF]

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: Truss Construction

DEFINITION

Here we’re continuing in our series of Obstructed Construction which all stems from the Annex of NFPA 13. So, what is Truss Construction? 
 
Well first, just by the definition, NFPA 13 tells us we could have trusses of Wood or Steel.  

Second, the definitions is that truss construction refers to parallel or pitched chord members connected by open web members supporting a roof or floor deck with top and bottom members greater than 4-inches (100 mm) in depth.  

So, in my opinion, we still have a lot of tolerance here in the definition. 

The truss could be pitched or parallel. 

The truss can support a roof or a floor deck 

The truss has to have open web members 

And the top and bottom chord must be greater than 4-inches (or a 100 mm) in depth. 

CHORD THICKNESS

The only real restriction here is the thickness of the top and bottom chord. It’s not just a nominal 4 inches (or a 100 mm) that’s used in 2x4 construction, it’s a measurement of more than 4-inches (or a 100 mm) to qualify as truss construction. 

Pitched or parallel, roof or floor deck, that’s really less of a concern here because those qualifiers are pretty much going to cover every truss we have inside of a building. 

Where do we find chords that are this thick? If there are major structural loads, if we have some kind of bridges, or Skydeck, if we have cantilevers, or if we have long spans or heavy loads that require a whole lot of stiffness. Well, it’s possible to have a truss. What about the roofline of a theater? Or a gym? Or an arena? 

It’s possible that these are not simple and lean roof joists, but they’re actually constructed of larger trusses that here would qualify as Obstructed Construction.  

SPRINKLER DEFLECTOR EXAMPLE

So just like we discussed earlier in this series with bar joist construction, it’s not always easy to get dimensions and structural details if the structural engineer is delegating that portion of their work. The truss design just might not be flushed out until a fabricator gets involved and details everything out. 

If during design, we ask enough questions here and we could get our answer about that chord thickness. But in any event, it might be possible that our design or inspection work doesn’t really leave us in the dark. In other words, it doesn't matter so much whether we end up as obstructed or unobstructed construction in this case. Sprinkler deflectors, even for Obstructed Construction, have to be within 6 inches of the bottom of the top chord of the structure, So even in this case where we have large trusses, it’s only going to matter if that top chord is more than 6 inches deep. Because if it’s 6 inches deep and we got to be within 6 inches of the bottom of that, well, we still have to be 12 inches off the deck. 

So, if that top chord is greater than 6 inches deep, Obstructed Construction will apply and we can lower that sprinkler deflector further down so that we still want the 6 inches from the bottom of that chord but we’re further down from the roof deck. But, if that top chord is less than 6 inches, let’s say it’s a 5-inch top chord, we still have to be within 6 inches of the bottom of the chord so we’re gonna be 11 inches down from the deck. But that would be the same as Unobstructed Construction which requires us to be within 12 inches of the deck anyways. 

So here, even though we could meet the definition of obstructed construction with chords that are 5 inches deep. It’s really only going to matter, it’s only really going to give us an allowance if that top chord is more than 6 inches deep or we can start to locate the deflector lower and lower down from the deck. 

If it’s not, Unobstructed Construction is going to require us to be within 12-inches of the roof deck. 

SUMMARY

So, when do we quality as Obstructed Construction using this Truss example?

Well functionally, it’s when the chord members are more than 4 inches or a 100 mm deep. By definition, it’s when we have parallel or pitched chords, are serving a roof or floor, have the open web component, are using wood or steel, and have top and bottom chords that are more than 4-inches (100 mm) deep. 

Next and last in our series of Obstructed Construction is Bar Joist Construction.

I’m Joe Meyer, this is MeyerFire University.