What are the fire or smoke resistance requirements for floors?
Before our crossword getaway earlier in this series we talked about a number of topics related to fire-resistance and smoke-resistance for different wall assemblies in the built world.
We talked about the purpose behind fire-resistance-rated construction, Fire Partitions, Fire Barriers, and Fire Walls. We then covered smoke-resistant construction, and elaborated on Smoke Partitions and Smoke Barriers.
We talked about enclosures, but mostly in the context of wall construction.
In this segment, we’re looking at the horizontal plane.
What are the fire- and smoke-resistance requirements for floors?
First, we come back to a reoccurring topic in passive fire protection, and that is Continuity.
Floors are required to be continuous from exterior wall to exterior wall, with no breaks, gaps or openings except as specifically permitted in the Vertical Openings section of code. Unless we’re explicitly allowed to put an opening in a floor, it needs to be continuous. And that’s all the way from one exterior wall to the opposite exterior wall.
Things like HVAC shafts, interior exit stairs, and elevators are of course permitted, but they are strictly controlled. And yes, in some cases atriums are permitted.
The general rule is that floors must be continuous with no openings unless specifically permitted. We’re going to detail out the exceptions in our Vertical Openings videos.
By forming a continuous barrier with no breaks, this means that floors are effectively resistant to the passage of smoke.
Although they are not specifically classified as a Smoke Partition or Smoke Barrier, this continuity functionally limits the passage of smoke from floor to floor.
When floors intersect an exterior wall, there is even a requirement that the gap between the floor and the wall (called “the joint”) be sealed to resist the passage of smoke. This ensures that smoke will not spread from floor to floor via this opening at the exterior wall.
BALLOON FRAMING VS. PLATFORM FRAMING
If you’ve already completed our Building Structure series over on the suppression side of things, you know that historically, the first light-wood-frame buildings were built with a construction method we later termed as “balloon framing”. This method grew in the 1830s which used many more, but slimmer wood members, called studs in lieu of fewer and heavier timber posts.
Despite the economy savings, there was one major drawback. Wall cavities were uninterrupted, meaning a fire along an exterior wall could pass heat and smoke up vertically inside a wall cavity. This, acting like a chimney, would allow a fire to spread vertically up through the wall uninterrupted.
As we know from fire dynamics, fire spreads much quicker vertically than horizontally since convective and radiative heat transfer work together to pre-heat the area right above the flame and helps spread fire up vertically very quickly.
Around a century after balloon framing was introduced, it gave way to “platform” framing, where each floor level was framed individually. A floor or floor/ceiling assembly would be framed before the walls on an upper level would begin. From a fire perspective, this helps us break up those vertical cavities and delay fire spread between floors.
I digress on all of that to say that the continuity here serves a major purpose, and is backed by our historical experience with fire and the dangers that even small cavities can have on vertical fire spread.
Today’s codes carry on this tradition of requiring a floor level to be continuous from exterior wall to exterior wall, without gaps along the edge.
Now what about fire-rated floors? Are floors required to be fire-rated? If so, when?
First, this comes from the building’s Construction Type. In that series we identify and talk about the different Construction Types, but many of them carry a minimum fire-resistance rating requirement specifically for floors.
In Table 601 of the IBC, floor construction and it’s associated supporting construction is required to have a fire-resistance rating in Types I-A and I-B, Type II-A, Type III-A, Types IV, and Type V-A.
This fire-resistance rating requirement is driven simply by the Construction Type.
SUPPORTING FIRE BARRIERS
There’s also another trigger that could require a floor to be fire-resistance rated.
And that is when it is supporting a Fire Barrier. We talked about this concept earlier when discussing Fire Barriers. Generally, unless the floor is supporting an Incidental Hazard in Type II-B, the floor will need to have a rating at least as fire-resistant as the Fire Barrier which it supports.
Floor, floor/ceiling, roof/ceiling, and roof assemblies vary in fire resistance from no rating to 2 hours. Although 3- or 4-hour assemblies are achievable, they are quite rare.
A HORIZONTAL FLOOR ASSEMBLY
There are some unique instances where a floor assembly will carry a fire-resistance rating when it’s serving as a horizontal assembly. These are used to separate mixed occupancy buildings or separate fire areas. I want to cover this in some more detail, so we’re going to save this topic for our following video.
WHAT ABOUT SMOKE?
Whether a floor is fire-resistance-rated or not, floors form an impediment that limits the passage of smoke, and well, sometimes fire too, that limits fire from spreading to other floors in a building.
In the IBC, floor assemblies are detailed in Section 711.
So, what are the fire and smoke-resistive requirements for floors?
First, a floor must be continuous from exterior wall to exterior wall, without any gaps, unless the opening is specifically allowed by the building code.
A floor can have fire-resistance-rating requirements. These can be triggered by the Construction Type, by supporting a Fire or Smoke Barrier above, or by being a part of a Horizontal Assembly, which is the topic of our next segment.
I’m Steven Barrett, this is MeyerFire University.
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