Obstructed Construction: Steel Purlins

MeyerFire University | FX108.51H

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: Steel Purlins 

INTRO

We’re continuing on our series of Obstructed Construction. We’ve covered Beam and Girder, Concrete Tee, Composite Wood Joists, Panel Construction, Semi-Mill Construction, Wood Joist Construction, and Bar Joist with Fireproofing. Today it’s all about Steel Purlins.  

What is a steel purlin? 

WHAT IS A PURLIN?

A purlin is a horizontal structural members that supports the weight of the roof deck. The roof deck is the flat surface that works like a plate to accept a load along any point and transfer that load to the nearest purlin.  

Purlins can be any structural material; they’re commonly wood or steel.  

Steel purlins are very common in metal buildings, or also called pre-engineered metal buildings. Without a purlin, there would be no support and nowhere to attach the sheeting or roof deck onto. 

STEEL PURLINS

Steel purlins are made from cold-form steel and they are thin enough to put screws through. The shape is made by pressing or rolling steel from thin sheets into the desired shape. Cold-formed steel is less expensive to produce and easier to work with than hot-formed steel.  

Steel purlins are usually hot dip galvanized with a coating, which is similar to other lightweight structural steel products, like steel studs. The galvanized coating gives a zinc protection layer against exposed environments from corrosion. 

FIRE SPRINKLER PIPE & PURLINS

Purlins run perpendicular to the main structural frame. For sprinklered buildings, the fire sprinkler pipe often is hung directly from the purlins. Supporting large pipe on these can be difficult, as the purlins are thin members designed only to accept so much load.  

For larger sprinkler mains, structural engineers may often require that mains be hung from the main structural frames, or when supported by purlins, be trapezed so that any one main is hung from two nearby purlins.  

Purlins often come in C- and Z- shapes, with a profile just like the letters.

OBSTRUCTED CONSTRUCTION & STEEL PURLINS

Thanks Joe, but what about Obstructed Construction? The Annex of NFPA 13 gives us the definition of when Steel Purlins qualify our ceiling or roof deck as Obstructed Construction. The eighth example is Steel Purlin Construction, which refers to a span with straight or tapered columns and frames that support C- or Z- purlins greater than 4-inches (100 mm) in depth and spaced up to 7-1/2 ft (2.3 m) on center. 

EXCEPTIONS

Now every pre-engineered metal building doesn’t necessarily fall into this category.  

Standard purlin sizes usually start at 4-inches deep, and the definition here specifically says greater than 4-inches deep. So just the presence of purlins alone isn’t enough to kick us over into Obstructed Construction. 

Also, we have a limit of 7-1/2 feet (or 2.3m) between purlins. If our purlin spacing turns out to be greater than this, we also wouldn’t qualify as Obstructed Construction based upon the presence of the purlins.  

EXAMPLE

Why does it matter? You guessed it; obstructed construction affects our sprinkler layout; height and spacing.  

Now if we only have 6-inch deep purlins, can we still meet unobstructed construction rules? Sure. Just because we have Obstructed Construction, there’s nothing stopping us from keeping the sprinkler deflector within 12-inches of the roof deck. 

Also, as part of obstructed construction, our sprinkler is still not allowed to be more than 6-inches below structure. This means that even if we qualify for Steel Purlin Obstructed Construction, we can’t be more than 6-inches below the bottom of the purlins. The rule is 1-6” below structure and up to 22-inches below the ceiling or roof deck. So here the distance below structure is more restrictive. 

With a 6-inch purlin, that means our sprinkler deflector cannot be more than 6-inches below the bottom of the purlin, or 12-inches below the deck. 

That’s about a wrap on Steel Purlin Obstructed Construction. Our next segment will cover the next example which is Truss Construction.  

I'm Joe Meyer, this is MeyerFire University.