What is the difference between obstructed and unobstructed construction?
The way that we classify the construction of ceilings is really important. Sprinklers have to be located within the hot gas layer that develops near the ceiling during a fire. The only way is sprinkler activates is when that hot gas layer moves across the fusible link and heats it up to the point that it expands fractures and releases the water.
If a sprinkler is not located within that hot gas layer then it won't respond in a timely manner.
Also, the spray from a sprinkler needs to reach the burning hazard. If we can't reach combustible surfaces then we can't cool the fire and the sprinkler system will be compromised.
The way which we determine where that sprinkler is allowed to be located depends upon the ceiling construction type. NFPA 13 defines two construction types for ceilings, which then dictates a whole number of other requirements.
Starting with the 1991 edition of NFPA 13, the committee started lumping together various construction methods and defining them in two simple ways. What they wanted to address was really two key elements;
The first is how well hot gases move along the ceiling layer and will that be impacted by structural members and the second is how will sprinkler discharge be affected by those same structural members?
They've addressed this by defining two types of construction.
The first is obstructed construction, or a type of construction that will impact the flow of heat and could affect the water discharging from sprinklers. In NFPA 13 we consider this to be obstructed construction.
Construction that does not impact the movement of heat or water discharge from a sprinkler is considered unobstructed construction. These terms become really important because they govern both the height whereas sprinkler is allowed to be placed, and the spacing rules between sprinklers.
Under NFPA 13 obstructed construction is defined as Panel construction and other construction where beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire.
We will talk about this more in this series, but some examples of obstructed construction could include solid steel beams in pre-engineered metal buildings, solid lumber in residential construction, panel construction, trusses with flanges more than 4-inches (100mm) in depth, and concrete tees for parking garages.
Unobstructed construction is Construction where beams, trusses, or other members do not impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire. Unobstructed construction has horizontal structural members that are not solid, where the openings are at least 70 percent of the cross-section area and the depth of the member does not exceed the least dimension of the openings, or all construction types, with the exception of panel construction, where the spacing of structural members exceeds 7-1⁄2 ft (2.3 m) on center.
We’ll get into this in more detail, but unobstructed construction could include acoustical ceiling tiles in an “open-office” plan, open-grid ceilings, smooth level ceilings, or even trusses with chords not more than 4-inches (100 mm) in depth.
When we are first teaching sprinkler spacing to people new to the industry, It's all too easy just to say that we have a flat horizontal ceiling and a light hazard office so sprinkler spacing is 15 feet by 15 feet, or 4.6 meters by 4.6 meters. if your sprinklers are not more than 15 feet apart then you're good. And that does work for a light hazard office building, but that's assuming a number of things. That's assuming that the construction type is noncombustible, hydraulically calculated, light hazard, of penton torn up right, and that we have a flat and smooth ceiling.
In this series well we want to begin to establish it is the correct process for how we go about locating sprinklers within a space. It's not as simple as 130 or 225 square feet per sprinkler, and the term obstructed and unobstructed construction plays a lot into that.
That is our very short introduction to what obstructed and unobstructed construction is. in our next two segments we're going to show examples of unobstructed and obstructed construction and began to talk about how those different construction styles impact sprinkler placement.
I'm Joe Meyer, this is MeyerFire University.
12/15/2022 09:05:58 am
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