Unobstructed Construction: Smooth Ceiling

MeyerFire University | FX108.52D

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(3) 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: Smooth Ceiling Construction 

DEFINITIONS

Here we’re exploring our third example of Unobstructed Construction, and that is smooth ceiling construction. 

Now, Unobstructed Construction is a type of construction for a ceiling or roof deck that generally does not impact heat movement or sprinkler flow. That's in the definition of unobstructed construction. We covered that already. 

Now today, this one seems pretty straightforward from the top, smooth ceiling construction, but let’s go into a little more detail. The Annex of NFPA 13 gives us eight examples of what is considered a smooth ceiling. Now, these are examples. You're trying to describe areas that would be considered unobstructed construction at the end of this whole effort.  

We're trying to determine whether we're unobstructed or whether we're obstructed construction. Because those set the rules for sprinkler spacing and sprinkler height, that's the end goal. Going through each of these in detail is good. It's a good training exercise. It's good for exposure. And it's good for our awareness so that we know what the annex is saying. We know what the committee has decided to put out there and support us in determining whether we're unobstructed or obstructed construction. 

But don't get caught up in all the detail here. At the end of the day, we're only trying to determine whether the ceiling or roof deck is going to be considered unobstructed or obstructed. That's it. That's really the end goal. It doesn't matter as much whether we have the second example or the fifth example today. Again, the end goal is to determine unobstructed or obstructed construction. 

These are all examples of unobstructed construction. And again, today covering smooth ceiling construction.  

EXAMPLES

So our first example, flat slab, pan-type reinforced concrete. The flat slab is what we would typically associate with the top of a concrete pour. That's the finished, you know, smooth concrete floor, for example. But what about the pan description? There's not a definition of pan in NFPA 13 and just remember, we're still in the annex portion of the standard anyways. 

If a concrete slab has deep pans, those are going to affect the heat movement and sprinkler flow. That would throw us into obstructed construction territory. So where do we draw the line? Well, go back to the original definition of obstructed and unobstructed construction. If we have deep pockets, the only way that we would have unobstructed construction here is if the structural members are more than 7 1/2 feet or 2.3 meters apart on center, and that's a measurement that's on center in both directions, X and Y. These pockets have to be pretty big and wide to be considered unobstructed construction. This also with the footnote that's at the end of this unobstructed construction definition includes standard mill construction, which we'll cover in our next video. 

Our second example of smooth ceiling construction is a continuous smooth bay formed by wood concrete or steel beams spaced more than 7 ½ feet, 2.3 meters on center beams that are supported by column skirts or trusses. So, when we have very wide bays with continuous and smooth decks or ceilings, we're considering that unobstructed construction. That's consistent with the definition of unobstructed construction that we've already covered. 

Another example, smooth monolithic ceilings of at least three-quarter inch, 19 millimeters of plaster. That's on the top of metal lath, or a combination of materials of equivalent fire resistance rating that's attached to the underside of wood joist, trusses, or bar joist. This is a very specific example of plaster on a particular construction method. But I think the description here is fairly straightforward. 

Next one, open-web-type steel beams, regardless of spacing. Now we've talked about bar joist before, but here we're saying any spacing of open-web-types steal beams. Like we've covered in prior videos, we just need to be sure that the top and bottom chords aren't very thick where they're more than four inches, which would start to impact heat flow in water distribution. The term open-web means that we have openings in the web portions. But here, they talk about steel beams. They're not necessarily saying joists, they're not necessarily saying bar joists. I'm trying to read in between the lines here, open-web-type steel beams. I still think we're coming back to this concept of open-web joists. 

Our next example is smooth shell type roofs, such as folded plates, hyperbolic paraboloid saddles, domes, and long barrel shells. Wooh. Quite a bit to digest here. What we're talking about are fairly complex shapes, but with similar themes, continuous and smooth ceilings. Many historic large dome features that have these shapes really try to embellish that structure. They try to show it off. And so, it's not often in those cases that we're gonna find ourselves putting sprinklers beneath those features. In general, we're gonna try and use some type of performance design or use other fire protection measures and mitigations instead of hanging sprinklers and pipe below these fantastic structures. 

Our next example is suspended ceilings of combustible or non-combustible construction. Commercial office cube heaven. Here we have what is our most common ceiling type for commercial structures in the United States. That's suspended ceilings. They could go by ACT suspended ceilings. Some people call them T-bar. It's our most common for commercial spaces. These are considered unobstructed, no real surprise here, which drives sprinkler spacing and height requirements. 

Our last example that we're given for smooth ceiling construction is smooth monolithic ceilings with a fire resistance, less than what we identified just earlier and attached to the underside of wood joists, wood trusses, and bar joists. So here, if we have a smooth monolithic, meaning a continuous or seamless material that's attached to the underside of joists or trusses, well, we have unobstructed construction. 

SUMMARY

I don't know if we have a whole lot of controversy with this segment and this concept, but at the end of the day, again, what we're trying to establish are examples of smooth ceiling construction that counts towards unobstructed construction. 

End of the day, we just need to know unobstructed or obstructed construction. With unobstructed construction, heat and sprinkler flow are generally not going to be impacted by the forms that the ceiling or roof deck. So, sprinkler spacing and height are affected accordingly. 

In our next segment, we're gonna cover standard mill construction, which is our next example of unobstructed construction. 

I’m Joe Meyer, this is MeyerFire University.