The Unique Challenges Of AtriaAtria present some very unique design concerns for fire protection engineers. Because atria interconnect a number of floor spaces within a building, these areas are in contrast to the principle of floor-to-floor fire compartmentalization that would normally limit the spread and movement of fire and smoke. Limiting the spread of smoke is a very important aspect of ensuring life safety under any circumstances; however, society places even more emphasis on containment for a school environment where the primary occupants are children between the ages of 5 and 18.
As has previously been stated, the main objective of a smoke management system is to accomplish a high level of life safety and minimize property losses. Specifically, the objectives of a smoke management system are:
- Providing a safe, tenable means of egress for building occupants;
- Providing an adequate level of visibility for firefighting and occupants; and
- Removing smoke.
Smoke management system design can be divided into four main components: controlling the fire, exhausting the smoke, separation of the adjacent building spaces, and providing negative pressures. While all of these components are vital to the design of the system, here we will be focusing mainly on the smoke exhaust.
Exhaust ConcernsA fire generating smoke within an enclosed space will create what is known as "smoke filling" at the ceiling and a resulting smoke layer. It is the height of this smoke layer that is important when designing the smoke management system. In order to establish a high level of life safety within an atrium space, it is ideal to keep the smoke layer at the highest possible level; this will limit the amount of smoke that occupants will encounter while exiting the building.
There are a number of means to help prevent the smoke layer from descending to a height of 10 ft above the highest walking surface. A few such means include incorporating the use of mechanical venting at the roof of an atrium, and the use of the ceiling height and shape to create a smoke reservoir.
Venting is good means to begin exhausting some of the smoke from the area, and creating a smoke reservoir will allow for additional time for the smoke layer to descend. However, these alone are not necessarily ideal to prevent the smoke layer from building up underneath the ceiling and descending to the floor, mainly because the required exhaust rate of the smoke through the venting is significantly less than the rate of smoke that is being produced by the fire.
In order to effectively prevent the smoke layer from descending to a hazardous level, it is generally necessary to incorporate the use of an exhaust fan that will be able to exhaust the smoke at a rate at least equal to the rate of smoke production. In many cases these fans are required to be quite large, and thus are rather costly, but they are the most effective means by which to exhaust smoke from an atrium, and therefore are the main component in a smoke management system.
It's In The CodeMany of the building codes have, in the past, specifically required rate of smoke exhaust in terms of air changes per hour. However, recent testing and research have determined that a prescriptive number of air changes may not be appropriate for all smoke management system designs; therefore, many of the modern building codes are converting to a performance-based design approach to smoke management system design.
To determine the appropriate size of the exhaust fan for any atrium situation, the equations in NFPA 92 B, "Smoke Management for Malls, Atria and Large Areas," should be used. These calculations will determine the size of the exhaust fan necessary, based on the amount of smoke produced from a design fire and the volume of the atrium space.
In conclusion, in a situation where an atrium has been incorporated into the design of a school, there is an increased life safety concern to protect the occupants from fire and smoke. Therefore, it becomes necessary to properly design a smoke management system in accordance with modern design criteria such as NFPA 92B, "Smoke Management for Malls, Atria and Large Areas." ES