It is important that professionals involved with smoke control know about the myth of panic. Movies, television, and the press often present an unrealistic image that panicked behavior is common in fire situations. Numerous post-fire studies have shown that panic does not occur often.^1,2,3 People usually respond adaptively to a fire incident and often are altruistic in their behavior.

This article will address fire-evacuation analysis related to smoke-control applications. The most common types of smoke-control systems are pressurized stairwells, pressurized elevators, zoned smoke control, and atrium smoke control. Evacuation analysis often is needed for smoke-control systems, but is essential for atrium smoke-control systems that rely on smoke filling. With smoke-filling systems, occupants should have sufficient time to leave an atrium before smoke descends upon them.

During building fires, elevators almost always are taken out of service, and vertical evacuation takes place via stairways. In a few situations, elevators are used for evacuation, but this article will address only vertical evacuation via stairways. (For information about calculating evacuation time via elevators, see “Design of Smoke Control Systems for Elevator Fire Evacuation Including Wind Effects”⁴ and “Protected Elevators for Egress and Access During Fires in Tall Buildings.”⁵)

The primary approach to estimating evacuation time consists of a hydraulic analogy that simulates people as fluid particles. This analogy can be generated with and without the consideration of human behavior. (For more detailed information about estimating evacuation time, see “SFPE Handbook of Fire Protection Engineering”¹ and “Principles of Smoke Management.”⁶) However, many computer models can be used to estimate evacuation time with and without the consideration of human behavior. A National Institute of Standards and Technology study examined 28 of these computer models.⁷

TOTAL EVACUATION TIME

Evacuation time consists of pre-movement time and movement time. For example:

t_t = t_pm + ռt_m

where:

t_t = total evacuation time (in minutes)

t_pm = pre-movement time (in minutes)

t_m = modeled evacuation time for an egress route (in minutes)

ռ = evacuation efficiency

The approach described in this article assumes that people follow a directed route to their destination, typically the outdoors or an area of refuge. Such a directed route does not account for the possibilities of proceeding in the “wrong” direction (e.g., traveling in circles or being blocked by smoke or fire). For this reason, an efficiency factor commonly is added to the modeled evacuation time.

There is no consensus about what value should be used for evacuation efficiency, but 1.5 is the minimum. For many applications, an efficiency value of 2 would be more reasonable. For example, a value of 2 might be considered for a building with two egress paths, one of which might be blocked by smoke or fire.

PRE-MOVEMENT TIME

Pre-movement time is the time from the ignition of a fire to a person's becoming aware of it and deciding to move. There are many ways a person can become aware of a fire. Some common cues of a fire include seeing flame, smoke, or the fire department; feeling heat; smelling smoke; hearing noise or a fire alarm; being told; and loss of electrical power. For a detailed discussion of the ways that occupants become aware of a fire, see “SFPE Handbook of Fire Protection Engineering.”¹

People often wait for some time after hearing a fire alarm to respond. Part of the reason for such waiting is experience with false alarms. It is reasonable to expect much less waiting time when people become aware of a fire by seeing fire or smoke.

A study of human responses to various types of fire-alarm signals in drills was conducted at mid-afternoon in a London subway station.⁸ Video cameras recorded individuals' responses, and interviews supplemented the video recording. Five types of alarms were used in the study (Table 1). Alarms were initiated 5 sec after a train arrived at the station. It can be seen from Table 2 that pre-movement time amounted to as much as 9 min for an alarm bell only, but was much less with verbal announcements. However, caution needs to be exercised with the use of verbal announcements, as they involve complex human-behavior issues.

MODELED EVACUATION TIME

Approaches that can be used to calculate evacuation time include component-by-component analysis and constrained-flow analysis. Component-by-component analysis involves a determination of the time it takes for a population to traverse each egress component. The density of flow along each egress component must be determined to measure velocity and flow rate. Component-by-component analysis can be used to analyze complex flow situations involving merging, diverging, and converging flows.

Component-by-component analysis is more complicated and time-consuming than constrained-flow analysis. If a component-by-component analysis is required, a computer-generated evacuation model should be considered.

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