smoke control and fire evacuation
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Smoke ControlTRANSCRIPT
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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:
tt = tpm + ռtm
where:
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tt = total evacuation time (in minutes)
tpm = pre-movement time (in minutes)
tm = 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.
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TABLE 1. Types of fire-alarm signals used in drills in a London subway station.
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.
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TABLE 2. Responses to various fire-alarm signals in a London subway station.
MODELED EVACUATION TIME
Approaches that can be used to calculate evacuation time include component-by-
component ana lysis 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, acomputer-generated evacuation model should be considered.
CONSTRAINED-FLOW ANALYSIS
The constrained-flow approach is appropriate for an egress system that has a point at
which a line of waiting p eople forms. Considering that the constraint is the exterior
stairwell door and all occupants start their evacuation simultaneously at time zero, the
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modeled evacuation time is estimated as:
tm = ta + tc
where:
tm = modeled evacuation time for an egress route (in minutes)
ta = time for first person to arrive at the exterior stairwell door (in minutes)
tc = time for population to pass through the exterior stairwell door (in minutes)
If all stairwell doors are the same width, the constraint will be at the exterior stairwell
door. If the width of the exterior door is less than the sum of the widths of the interior
stairwell doors that are used for evacuation, the exterior door still is the constraint.
Considering that the first person to reach the exterior stairwell door is walking down thestairs from the floor above, the time to reach the exterior door is about 0.5 min. The time
for the population to pass through the exterior stairwell is estimated as:
TABLE 3. The time it takes for a person to pass through the exterior
stairwell door depends on the door's width
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tc = ßP
where:
ß = time for a person to pass through the exterior stair door (in minutes)
P = population evacuating through the stairs
The time it takes for a person to pass through the exterior stairwell door, ß, depends on
the door's width (Table 3).
EXAMPLE CALCULATION
Let's determine the evacuation time for a five-story building with two stairways and 200
people on each floor (Figure 1). The doors leading into and out of the stairways are 32-in.
wide. The pre-movement time, tpm, is estimated at 8 min, and the evacuation efficiency,
ռ, is taken to be 2.
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FIGURE 1. Evacuation paths for a five-story building with two stairways
and 200 people on each floor
The people on the first floor would exit directly to the outside without using the stairs.
The population of the remaining floors using the two stairways is calculated as:
4(200) = 800 people
It is considered that half of the people would use one stairway, and half would use the
other. Therefore, the population using a stairway is P = 400. As seen in Table 3, ß = 0.025
for a 32-in.-wide exterior stairwell door. The time for population to pass through the
exterior stair is estimated as:
tc = ßP = 0.025(400) = 10 min
The modeled evacuation time is:
tm = ta + tc = 0.5 + 10 = 10.5 min
The total evacuation time is:
tt = tpm + ռtm = 8 + 2(10.5) = 29 min
SUMMARY
Evacuation time can be estimated by a hydraulic analogy to simulate people as fluid
particles. It is important that estimates of evacuation time include pre-movement timeand evacuation efficiency. The constrained-flow approach is appropriate for a large
number of buildings in which the exterior stairwell door is the flow constraint. For more
complex egress routes, a component-by-component analysis can be done, and
information about this kind of analysis is available from other sources. For such
complicated routes, readers may consider using a computer-evacuation model.
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REFERENCES
A smoke-control consultant, John H. Klote, DSc, PE, developed and conducts a series of
smoke-control seminars for the Society of Fire Protection Engineers. For 19 years, he
conducted fire research for the National Institute of Standards and Technology. He is
1. Society of Fire Protection Engineers. (2002). SFPE handbook of fire protection
engineering. Quincy, MA: National Fire Protection Association.
2. Quarentelli, E.L. (1979, October). Five papers from the panel session on panic.
Paper presented at the Second International Seminar on Human Behavior in Fire
Emergencies, Washington, DC.
3. Keating, J. (1982). The myth of panic. Fire Journal, 3, 57-61, 147 .
4. Klote, J.H. (1995, April). Design of smoke control systems for elevator fire
evacuation including wind effects. Paper presented at Elevators, Fire, and
Accessibility, Second Symposium, N ew York, NY.
5. Bukowski, R.W. (2003, October). Protected elevators for egress and access during
fires in tall buildings. Paper presented at the CIB-CTBUH Conference on Tall
Buildings, Kuala Lumpur, Malaysia.
6. Klote, J.H., & Milke, J.A. (2002). Principles of smoke management . Atlanta:
American Society of Heating, Refrigerating and Air-Conditioning Engineers.
7. Kuligowski, E.D. (2004, June). Review of 28 egress models. Paper presented at the
Workshop on Bu ilding Occupant Movement During Fire Emergencies,
Gaithersburg, MD.
8. Proulx, G., & Sime, J.D. (1991, July). To prevent ‘panic’ in an underground emergency: Why not tell people the truth. Paper presented at the Third
International Symposium on Fire Safety Science, Edinburgh, Scotland.
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