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What is the purpose of smoke resistant construction?
INTRODUCTION In the first half of this series, we’ve focused on limiting fire spread. Meaning, how do we actively keep fire growth from moving from one space and into another? How do we keep fire within its room of origin? Outside of the flames itself, the major byproduct of combustion is more lethal than the fire itself. Smoke is more lethal than heat. In this and the next few videos, we’re going to turn our attention to smoke. SMOKE-RESISTANT CONSTRUCTION Fire-resistant construction is utilized to prevent fire from spreading from room to room, to keep fire in its room of origin. In a similar way, smoke resistant construction prevents smoke from spreading from room to room and contains the smoke to the room of origin, where the fire occurs. Now, aren’t these two concepts the same? Not exactly. The key distinction is between fire (the flame, the heat and the increasing amount of burning fuel) and smoke itself. SMOKE VS. HEAT RESISTANCE Let’s consider a trash can, say a 55-gallon can, filled with crumpled up paper and boxes and discarded plastics. Let’s place this trash can inside a small storage room within an office building. You might have seen this image before. If this were to start on fire, it would create a lot of smoke that would spread quickly but the fire would be contained to the trash can, at least initially. Smoke is created, its hot and less dense than the air, so it rises, and begins to separate out as it approaches the ceiling. How far will the smoke go? Well, if we have a door to this room, that is closed, which fits relatively tightly and there aren’t any other openings, then the smoke isn’t going to travel very far. The smoke is hot, but it’s not hot enough yet to ignite other material and travel beyond this room. Regardless of whether there are other combustibles in the room, or the type of firestop used along the joints in the wallboard, or whether the door is a hollow wood or steel door – this room, for now, is preventing the spread of smoke. Smoke is limited to the room of origin. FIRE GROWTH Now, let’s say the fire continues to grow. The heat from the fire still occurs, but the nearby materials are more easily ignited and start to catch fire due to convective and radiation heat transfer. The increase in temperature ignites nearby objects, the combustibles now add to the fuel load, more burns, and the temperature in the compartment continues to increase. Now, we have increased temperatures throughout and material that is actively burning and affecting the boundaries of this room. The room will continue to keep most of the smoke in the space until the surrounding walls or ceiling are compromised. THE DIFFERENCE If the enclosing surfaces are fire-resistant-rated, like we’ve talked about earlier in this series, then the wall assemblies and the floor/ceiling assembly above would have been tested to be able to withstand some amount of temperature for a relatively longer period of time. They are resistant to the heat transfer and high temperatures associated with fire. They are fire-resistant. If the enclosing surfaces are not fire-resistance-rated, then eventually the growth of the fire and the raised temperatures within will compromise some part of the enclosure, and the fire will spread beyond the boundaries of that room. Say that door is a hollow wood door that carries no listing and burns right through, and fire spreads out the door. Well, for some period of time, the room resisted the spread of smoke to areas outside the room. It’s possible that it could be considered smoke-resistant construction. But with a non-fire-resistant-rated wood door, the room is not consider fire-rated. See the difference here? Smoke-resistant construction is intended to prevent the spread of smoke, but not necessarily intended to oppose the effects of high temperature and heat transfer from a fire. This might beg the question; if a smoke resistant door can’t stand up to fire, what good is it? WHAT GOOD IS SMOKE-RESISTANCE? Well, preventing the spread of smoke has major benefits for life safety. Most of the fire deaths don’t occur because of high temperatures, they occur from the lethal effects of smoke. Smoke can be highly toxic, carrying lots of carcinogens that affect our respiratory system. Smoke can carry elevated temperatures and high humidity levels, which can burn our lungs when we breathe in hot humid air. It also decreases visibility, which affects our confidence in moving through smoke in a fire and it ability to properly navigate to exits to escape a building. So, smoke itself is a major fundamental hazard. More people die from smoke in a fire than from heat or burns. Because of this, preventing smoke spread can be an important dynamic in keeping buildings safe. Also, not all walls and partitions need to be able to withstand a fire for some prolonged period of time. We have other systems, like fire sprinklers, that when used can limit the heat and growth of a fire on their own. So there are plenty of situations where smoke-resistant construction will work hand-in-hand with sprinkler protection to limit the growth of a fire, and limit the spread of toxic smoke. SUMMARY Is fire-resistant construction better than smoke-resistant construction? Not necessarily. It might be best to consider these two things as separate and distinct requirements. Fire-resistant construction prevents the heat from spreading, whereas smoke-resistant construction prevents the smoke from spreading. In these next few segments, we’ll focus on containing the smoke in a fire, including our next video where we’ll introduce Smoke Partitions. I’m Steven Barrett, this is MeyerFire University.
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