Examples of Obstructed Construction: Concrete Tees
Here we’re continuing on our series on Obstructed and Unobstructed Construction. In this segment we’re talking about another example of obstructed construction which NFPA 13 gives us, and that is Concrete Tee Construction.
Now thus far we’ve established that Obstructed Construction is an assembly where structural members impact either heat flow or water distribution from a sprinkler. The qualifications for what is Obstructed Construction consists only of the definition in NFPA 13.
However, we have annex material which helps to further clarify examples of what is to be Obstructed Construction.
In that annex portion, our second example is called Concrete Tee Construction.
CONCRETE TEE CONSTRUCTION
This example is a bit more straightforward than the first example; here Concrete Tee Construction just refers to solid concrete structural members with a leg (or stem) that has a nominal thickness less than the nominal height.
That is, the width of the stem has a nominal thickness less than the height. This really isn’t much of a restriction at all; many if not all integrated floor or roof concrete beams would qualify under this definition.
CONCRETE TEE EXAMPLE
A concrete tee might not be difficult to picture if you’ve parked in any urban parking garage. Concrete tees are often deep and have the ability to carry very heavy loads. This example of obstructed construction gives us no dimension limitations, other than a stem having a thickness less than the height.
SINGLE VS. DOUBLE TEE
Also of note is that we don’t distinguish here between a single concrete tee, or a double tee. A double tee is a structural member that has our two “T” shaped beams poured as one solid beam. In either case, we’re considering this to be Obstructed Construction.
How does this impact sprinkler spacing?
Concrete Tee Construction is considered Obstructed Construction, but NFPA 13 also carries specific rules for sprinklers with Concrete Tee Construction. We’ll get into this in more specific terms later, but essentially for Concrete Tees with very deep stems and where the stems are spaced 3-ft to 7.5-feet (0.91 to 2.3 m) apart, sprinklers are allowed to be located at or above 1-inch from the bottom of the stem.
Normally, sprinklers in obstructed construction can only get down 22 inches below a ceiling or roof deck, but this requirement specifically does not apply for concrete tees that meet these limits.
Why do we allow this additional depth?
In general, concrete in a thicker form is better able to withstand the heat generated from a fire better than thin steel members or wood.
That’s our rundown for concrete tee construction as one of the examples of Obstructed Construction. In our next segment we’re going to look specifically at Composite Wood Joist Construction.
I'm Joe Meyer, this is MeyerFire University.
Leave a Reply.
Sentry Page Protection
Aaron Johnson, CFEI
Al Yakel, SET
Chris Campbell, PE
Chris Logan, CFPS, RSE
David Stacy, PE
Ed Henderson, PE
Joe Meyer, PE