buildings began to dramatically change the skylines in major cities more
than a century ago. Technological advances made it possible for people
to effectively use spaces at heights far above grade level. Tall
buildings provide challenges for the designers of fire protection
systems not found in other buildings.
Like previous editions, the 2012 edition of NFPA 101, the Life Safety Code1
allows building designers to use performance based options in designing
the egress system in the building. The performance criterion is given
in Section 5.2.2. Based on this section of the code, the designer must
consider the different fires that could occur in the building, how these
fires will impact tenability, and how long the occupants will require
to safely evacuate the building.
using this approach, all of the assumptions and design methods must be
included in the simulation of the evacuation. This means that the egress
system designer must develop assumptions about how the population is
expected to behave during the evacuation of a tall building. These
assumptions then have to be applied to the calculation using data that
What is not directly
stated by NFPA 101 is that the egress system designer needs to
understand the source of the data and how it applies to tall buildings.
Some behaviors might be insignificant for someone going down a single
flight of stairs, but become more significant as the travel distances
become much larger.
solution is to apply safety factors to the design. With only a limited
understanding of the data, a large safety factor may be required so as
to not subject the building occupants to undo risk.
article will look at components of the evacuation time of occupants in
tall buildings and the assumptions that are made by egress system
designers. The focus will be on the movement to and within the stairs as
well as the data used to develop an estimate of the descent rate. How
the data was collected relative to its application for use in tall
buildings will be analyzed. Finally, other egress options will be
FIRST ASSUMPTION: TIME REQUIRED TO START EVACUATING
egress system designer needs to consider two sets of conditions in
parallel. On one side, there is the fire growth and tenability in the
different building areas. On the other side, there are the building
occupants that need to get to a place of safety. For people remote from
the fire, they need to receive some cue (e.g., smell smoke, see flames,
or hear an emergency announcement) before they will start to evacuate.
Occupants remote from the ignition location may require some time before
they start to evacuate. In a tall building, direct observation of fire
cues might not be possible for occupants located many floors away and/or
on the opposite side of the building. In tall buildings, it is likely
that many of the building occupants will not become aware of the need to
evacuate until the fire alarm system activates.
egress system designer could add the time for the alarm to activate to
the time calculated for egress. In this case, the assumption is that all
of the occupants hear the alarm and immediately start toward the exit.
It is based on all people perceiving the alarm, paying attention to it,
comprehending what it means, realizing that it applies to them, and then
deciding to evacuate. Research has shown that many people do not
recognize the temporal-three signal as applying to fires.2 Even when people in tall buildings do realize that there is an emergency, they have reported doing other tasks.3
using the pre-evacuation times from tables, using the average value
creates two potential limitations. First, the data has only been
collected from a relatively small number of incidents. Training and
other unknown variables could cause these times to be too short or too
long. More data is needed to be able to fully understand what the most
appropriate values are. Second, NFPA 101 requires that all occupants
remote from ignition be protected from untenable conditions. If some
vulnerable populations require more pre-evacuation time, using the
average value will leave them at risk for not being able to evacuate
before conditions become untenable.
SECOND ASSUMPTION: MOVEMENT WITHIN THE STAIRS
the evacuation of a tall building, stairs are intended to allow people
to descend and leave the building. While there is some travel distance
on the floor of origin, NFPA 101 limits that travel distance. After
descending fewer than 11 floors, the building occupants have travelled
further within the stair than before they reached it.4
descent times in tall buildings can be substantial. While the stairs
can usually be considered safe, a poor estimate of how people descend
could lead to crowded conditions that prevent people from the floors
from entering the stairs.
One equation that has been used for calculation of movement on stairs is found in the SFPE Handbook of Fire Protection Engineering.5 The Handbook does not provide any limitations on the applicability of the results. For densities greater than 0.54 persons/m2 and less than 3.8 persons/m2, the SFPE Handbook equation is:
S = k - akD (Equation 1)
S =Speed along the line of travel (m/s or ft/min)
D =Density (persons/m2 or persons/ft2)
k =constant for four different riser and tread combinations
a =empirical constant (0.266 when calculating m/s, 2.86 when calculating ft/min)
For densities less than 0.54 persons/m2, the people are able to travel at their free speed (the speed at 0.54 persons/m2). For densities greater than 3.8 persons/m2, the flow comes to a stop.
on this formula and no limitations, it is then possible to predict the
evacuation time. In order to do so, there are several more assumptions
that are made.
THIRD ASSUMPTION: THE ORIGINS OF THE SFPE HANDBOOK EQUATION APPLY TO TALL BUILDINGS
For travel down stairs, the Handbook
equation is based primarily on the work of two researchers from the
1960s and 1970s. The equation comes mainly from the work of Pauls and
The work of Fruin6
primarily involved pedestrian planning for horizontal egress and
ingress components. For level surfaces, he developed six "levels of
service" (A to F ) to qualitatively explain the ability of people to
choose their walking speed at different densities. He extended his
observations by observing two different stairs. One of the stairs was
indoors and the other was an outdoor stadium.
Again the "levels of service" ranged from Level A (below 0.54 persons/m2),
where people are free to choose their own speed to Level F (above 2.70
persons/m2), where the descent is reduced to a shuffling pace. In
neither case was it reported that the building occupants were in tall
In the 1960s and 1970s, Pauls7 observed evacuations of 58 tall buildings in Canada with a range of riser and tread dimensions. These buildings were up to 20 stories in height, but most were shorter. In his study, he looked at building averages and a limited number of spot measurements. From this data, he proposed that the descent speed could be calculated based on:
S =1.08- 0.29D (Equation 2)
S =Speed along the line of travel (m/s)
D =Density (persons/m2)
in Equation 1, the constants for the metric units and 17.8 cm riser
height and 27.9 cm tread depth are used, the two equations are
Using the same data, Pauls8
later reported that most of the stairs in his study had 17.8 cm riser
heights and 27.9 cm tread depths. He theorized that people might descend
stairs at different rates depending on the riser height and tread
depth. With his theoretical equation, he calculated what the different
speeds might be for four different combinations. He also explicitly
stated that the values were not based on actual data and should not be
used in practice.
Based on these three pieces of research, Nelson and MacClennan9 developed Equation 1. When the density was less than 0.54 persons/m2,they
used the findings of Fruin6 to determine the free movement speed. The
subsequent speed values for the 17.8 cm riser height and 27.9 cm tread
depth case was based on the work of Pauls.7
The 3.8 persons/m2 end point was based on where the graph crossed the x-axis. It is at a much greater density than Fruin6 gave for level of service F and well beyond the maximum density observed by Pauls.7 For the other three k values, Nelson and MacClennan9 used the theoretical values that Pauls8 had said should not be used in practice. These other k values came from the assumptions made by Pauls and not from data that had actually been collected.
It should be noted that Pauls7 and Fruin6 did not measure density in the same manner. Pauls7
identified a boundary layer that people leave between themselves and
walls. His density measurements are based on the effective width. The
previous approach used the entire area. Thus, value from Fruin6 should have been adjusted to be comparable to the measurements of Pauls.7
FOURTH ASSUMPTION: THE SFPE HANDBOOK EQUATION APPLIES IN ALL CASES
There are seven issues that challenge the assumption that Equation 1 is valid for use in tall buildings:
- The reliance on averages could lead to underestimating times for vulnerable populations.
- The basis on density rather than human interactions might not match reality.
- The untested k values might not be valid.
- For buildings over 20 floors (and possibly less due to sample size issues), the buildings are taller than those used to collect the original data.
- The population considered might not be representative of the earlier population.
- The measurement methods used might not be consistent.
- The equation can be applied to densities that were not observed.
1 is primarily a regression equation that was developed using averaged
values. While this can give an approximation of the mean value, it does
not give any indication of the scatter of the data. In order to develop
an appropriate safety factor, the expected minimum movement speeds need
to be known. This is especially true if those minimum values apply to a
particular subpopulation. If that subpopulation will always move slower
than average, it is not conservative to apply the average value to them.
the intent to protect all occupants not intimate with ignition, relying
on just average values could lead to vulnerable populations not having
sufficient time to evacuate. For example, Boyce, Shields, and Silcock10 found that people with varying levels of physical impairments required greater time to descend stairs.
underlying assumption of Equation 1 is that people behave like a fluid.
The flow rate out is a constant and the people do not interact in any
way other than the density; no one person will slow down the other
people around them. Pauls8specifically addressed this point
by noting that people passed slower individuals to keep the ultimate
flow in line with the expected results. However, Shields, et al.11
found that occupants were unwilling to pass a wheelchair user being
assisted down the stairs (approximately 40 cm available to pass) and
Proulx, et al.12 found that occupants using the handrail or
with disabled occupants ahead of them did not pass slower moving
occupants. Finally, Shields, et al.13 found that people
moving behind a slower moving occupant chose not to pass. Even beyond
the considerations of the vulnerable populations, people will interact
as they descend. For example, Jones and Hewitt14 discussed groups forming during evacuations and how those people interacted both before and during their descent.
better understanding of these interactions could result in an improved
understanding on the amount of time that people will require to descend.
However, assuming that the slower moving people will just be passed is
Another potential limitation with Equation 1 is the k value that is used. While the work of Templer15
indicates that there could be differences in speed based on riser
heights and tread depths, it is unknown if the k-values in Equation 1
are accurate. Applying the equation to any situation other than a 17.8
cm riser height and 27.9 cm tread depth is outside the scope of the data
that was collected. How much of an error this will make in the final
predicted value is unknown.
scope of the data could also limit effects that would manifest
themselves as people descended greater distances. The Joint Committee16 believed that fatigue would start to play a role when there were no merging flows, and Galea and Blake17
reported instances where fatigue was caused by footwear. Equation 1
does not have any difference in speed caused by fatigue. Based on the
equation, a person descending from the top of a hundred story building
would never slow down. If fatigue is an effect, then Equation 1 presents
an optimistic estimation of speed on stairs in tall buildings.
have also been raised about the applicability of data collected nearly
half a century ago on the population of today. Pauls, Fruin, and Zupan18
were unsure about whether the changing demographics of the population
would cause descent speeds to be slower. It is important to note that
the researchers whose work enabled the creation of Equation 1 questioned
whether it was still applicable or not.
Hoskins and Milke4
explain the different methods to measure occupant density that have
been used by previous researchers and include a method for calculating
landing distances not done for Equation 1. Also, related to the previous
issue about the k values, Hoskins19 has proposed a
method for equating densities on different tread dimensions, and when
landings are included, to make equations applicable to more stair
configurations. However, this method needs to be validated using more
The final potential problem
that can arise when using equation 1 for tall buildings is to have
theoretical conditions that do not match reality. The maximum density
does not match the observations of Fruin6 or any observation made by Pauls7. Any calculations that involve the highest density conditions may not be accurate.
seven of the limitations come back to one central point when
considering people movement in tall buildings: Equation 1 could be
accurate. How accurate is unknown and thus requires safety factors.
After all, in smaller buildings, an estimated time that is off by a few
seconds per floor results in errors that fall within the level of the
noise of the data. As the buildings get taller, those seconds can become
minutes if not tens of minutes. The errors can then rise above the
level of the noise.
USE OF COMPUTER MODELS
of the issues involving Equation 1 apply to the use of the computer
models. When a model is used, the system designer needs to be aware of
the limitations of the model, the basis of the calculations, and how the
default settings alter the results. Simply using the default settings
might not provide accurate results for evacuations from tall buildings
for all of the reasons that applied to Equation 1.
travel time down stairs required for vulnerable populations could be
substantial, or they might not be able to descend the stairs at all. The
2012 edition of NFPA 1011 allows the use of elevators for occupant -
controlled egress prior to phase 1 emergency recall. This should help to
meet the goal of protecting all building occupants not intimate with
ignition in tall buildings.
Bryan Hoskins is with Oklahoma State University.
- NFPA 101, Life Safety Code, National Fire Protection Association, Quincy, MA, 2012.
- Proulx, G., and LaRoche, C. "Recollection, Identification, and Perceived Urgency of the Temporal- Three Evacuation Signal," Journal of Fire Protection Engineering. Vol. 13, No. 1, pp. 67-82, 2003
- Kuligowski, E. and Hoskins, B. "Analysis of Occupant Behavior," Pedestrian and Evacuation Dynamics, 2010 Conference, Springer, New York, 2011. pp. 685-698, 2011.
- Hoskins, B. and Milke, J. "Differences in Measurement Methods for Travel Distance and Area for Estimates of Occupant Speed on Stairs," Fire Safety Journal, Vol. 48, pp. 49-57, 2012.
- Gwynne, S. and Rosenbaum, E. "Employing the Hydraulic Model in Assessing Emergency Movement," The SFPE Handbook of Fire Protection Engineering National Fire Protection Association, Quincy, MA, 2008.
- Fruin, J. Pedestrian Planning and Design, Metropolitan Association of Urban Designers and Environmental Planners, Inc., New York, 1971.
- Pauls, J. "Building Evacuation: Research Findings and Recommendations." Fires and Human Behaviour, John Wiley & Sons, New York, pp. 251-275, 1980.
- Pauls, J. "The Movement of People in Buildings and Design Solutions for Means of Egress." Fire Technology, Vol. 20, Issue 1, pp. 27-47, 1984.
- Nelson, H. and MacLennan, H. "Emergency Movement," SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Quincy, MA, pp. 286-295, 1995.
- Boyce, K., Shields, T., and Silcock, G. "Toward the Characterization of Building Occupancies for Fire Safety Engineering: Capabilities of Disabled People Moving Horizontally and on an Incline," Fire Technology, Vol. 35, No. I, pp. 51-67, 1999.
- Shields, T., Boyce, K., Silcock, G., and Dunne, B. "The Impact of a Wheelchair Bound Evacuee on the Speed and Flow of Evacuees in a Stairway during an Uncontrolled Unannounced Evacuation." Journal of Applied Fire Science, Vol. 7, No. 1, pp. 29-39, 1997.
- Proulx, G., Bénichou, N., Hum, J., and Restivo, K. "Evaluation of the Effectiveness of Different Photoluminescent Stairwell Installations for the Evacuation of Office Building Occupants," National Research Council of Canada, Research Report 232, 2007.
- Shields, T., Boyce, K., and McConnell, N. "The Behaviour and Evacuation Experiences of WTC 9/11 Evacuees with Self-Designated Mobility Impairments," Fire Safety Journal, Vol. 44, pp. 881-893, 2009.
- Jones, B. and Hewitt, J. "Leadership and Group Formation in High-Rise Building Evacuation," Fire Safety Science: Proceedings of the 1st International Symposium, International Association for Fire Safety Science, London, 1985.
- Templer, J. "Stair Shape and Human Movement," Ph.D. Dissertation, Columbia University, 1974.
- Joint Committee of the Building Research Board of the Department of Scientific and Industrial Research and the Fire Offices' Committee, "Fire Grading of Buildings – Part III – Precautions Relating to Personal Safety," Post-war Building Studies Number 29, pp. 22-95, Her Majesty's Stationary Office, London, 1952.
- Galea, E. and Blake, S. "Collection and Analysis of Human Behaviour Data Appearing in the Mass Media Relating to the Evacuation of the World Trade Centre Towers of 11 September 2001," Office of the Deputy Prime Minister, London, 2004.
- Pauls, J., Fruin, J. and Zupan, J. "Minimum Stair Width for Evacuation, Overtaking Movement and Counterflow – Technical Bases and Suggestions for the Past, Present, and Future," Pedestrian and Evacuation Dynamics, Springer- Verlag, Heidelberg, pp. 57-69, 2007.
- Hoskins, B. "Effective Density Measurement Methods on Stairs," Proceedings of the 5th International Symposium on Behaviour in Fire, Interscience Communications, London, pp. 182-193, 2012.