Examples of obstructed construction, beam and girder.
Today we’re expanding on how we define and determine the construction type of a ceiling. In our last segment we introduced how we define ceiling construction types, which are Obstructed and Unobstructed Construction. This categorization impacts what rules we use to properly locate fire sprinklers in a building.
Here, we’re going to explore examples of obstructed construction and how we identify them under NFPA 13.
First, as a quick recap, Obstructed Construction is an assembly where members impede heat flow or water distribution that affects the ability of sprinklers to control or suppress a fire.
Essentially, if a built-up form will significantly affect how heat flows or how sprinklers fight a fire, then our situation would be considered Obstructed Construction.
CODE MANDATES & ANNEX
That’s the extent of the code-mandated qualifications out of NFPA 13. The definitions in Chapter 3 are all of the enforceable material in the standard that addresses whether we categorize an area as Obstructed versus Unobstructed.
The Annex of NFPA 13, which is not enforceable but rather includes explanatory material to help supplement the standard, gives us much more clarity on what exactly would be considered Obstructed and what is not.
Here, in the Annex material that corresponds with the definition of Obstructed Construction in Chapter 3, we have a list of examples that qualify as Obstructed Construction. In this series we’re going to talk in more detail about each of these, using the 2022 Edition of NFPA 13.
While much of this has remained consistent for some time. Be sure to check the applicable edition for your project, and I might suggest, also check the very latest version to see if any clarifications or changes have been made.
BEAM & GIRDER
First example of Obstructed Construction is Beam and Girder Construction.
This style of construction includes both combustible and noncombustible structure supporting roofs or floors.
It includes wood beams of 4-inches (100 mm) or greater nominal thickness, steel beams, or concrete beams which are spaced 3-7.5 feet apart (900 mm to 2.3 m) which are supported on or framed into girders.
Now Joe I know we talked about structure types, but what does this mean? Let’s break it down.
BEAM TYPE & SPACING
First, let’s start with the wood beams. 4-inch nominal thickness means larger solid members that are 3.5-inches or greater in width. This style does not include a 2x10 solid wood joist, for instance, because the width is only 2-inches nominal (or 1.5-inch actual). Having a 4-inch or greater thickness usually means we’re talking about large solid timber members.
We also here have steel beams and concrete beams as well, although these don’t carry and specific stipulations.
The second dimension they give us is beams that are 3 to 7.5 feet apart (900 mm to 2.3 meters) on center. That gives us a limited range that would qualify as this Beam and Girder construction.
The last item here is where they talk about framing on or framing into girders. What does that mean?
GIRDER & GIRDER FRAMING
We covered girders in our structural series, but a girder is a type of structural member that collects beams or joists. Girders are stronger, thicker, and many times deeper than a typical beam because it is collecting the loads from all of those beams.
When the codes says framing on a girder, that would be where the beam rests on top of the girder. Here, there could be an open pocket right above the girder where heat could flow.
When the code says framing into a girder, that would be a connection where the top of the girder and the beams are relatively aligned. In this arrangement, heat is not going to flow past the girder in the direction of the beams.
Any of these arrangements we described would match NFPA 13’s description of beam and girder construction, and therefore be our first example of obstructed construction.
LIMIT TO VOLUME?
Let’s dive in on a few common challenges around these definitions for our beam and girder example.
Is there a limit to the volume or area in these channels?
No. There is no requirement that the beams frame into the girders, or that a maximum pocket size must be provided. There can be openings above the girder, or even with use of an open-web truss to pick up the loads from the beams.
WHAT ABOUT OPEN POCKETS?
Can we still consider this Obstructed Construction if there are open pockets above the girder, or using an open-web truss?
Yes, and for the same reason. NFPA 13 here isn’t stipulating that these gaps have to be solid. As an informal interpretation by the National Fire Sprinkler Association pointed out in 2007, if the tops of the beams are at the ceiling or roof deck, then the sprinkler’s ability to collect heat and distribute water is going to be affected by these beams. That meets the definition of Obstructed Construction.
WHAT ABOUT SPACING OVER 7.5-FT?
What about if our beams are more than 7.5 feet (2.3 meters) on center, could we still have this be considered Obstructed Construction?
Man, lots of good questions today.
Yes, possibly. If the beams are framed into the girder, forming a pocket without any penetrations, and this pocket is no more than 300 sqft in area (or 28 square meters), then it would be described as “Panel Construction”. Panel construction is another example of Obstructed Construction that we’ll cover in this series.
What if my situation is slightly different? What if we have smaller beams inside larger beams?
The annex material, including the examples here, are advisory. They are explanatory material to help provide some clarity, and help support quality decision making.
At the root of the issue with different situations, is how will heat move and how will water be distributed? Does the arrangement materially affect either of these two things?
Next in our series, we will cover Concrete Tee Construction.
I'm Joe Meyer, this is MeyerFire University.
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