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6 · THE SINGAPORE ENGINEER Feb 2009
Fire Protection
Smoke Control by Pressurisation in Stairwells
and Elevator ShaftsDr Richard S Miller and Dr Don
Beasley, Department of Mechanical
Engineering, Clemson University, USA,
explore the application of elevator
shaft and stairwell shaft pressurisation
systems, as a means of preventing smoke
migration through the ‘stack eff ect’ in
tall buildings.
� is article is based on a summary
of the technical results recorded from
a study funded by the Smoke Safety
Council, USA.
INTRODUCTION
� e stack eff ect is created in tall building
shafts when there is a temperature
diff erence between the building interior
and the ambient. For a cold ambient, the
lower fl oors have a net negative pressure
diff erence while the upper fl oors show
a net positive pressure diff erence. In
physical terms, air is being entrained into
the shaft on lower fl oors and forced out
into the building on the upper fl oors.
� e conventional calculation of the
stack eff ect pressure diff erence is actually
the diff erence in pressures between the
elevator (or stairwell) shaft and the
outside world. In the absence of other
interior pressure barriers, this total stack
eff ect pressure diff erence comprises the
sum of the pressure diff erences across
the elevator (or stairwell) doors plus that
across the building exterior. � e primary
problem associated with the stack eff ect
in tall buildings, in relation to the current
study, is its infl uence on smoke migration
during fi res. In this regard, it is the ‘across
door’ portion of the stack eff ect pressure
diff erence that is directly related to smoke
migration and control. A fi re located on a
lower fl oor can cause substantial damage,
injury, and even death on upper fl oors
due to the smoke migration through
the elevator or stairwell shaft. � e most
well-known example of the occurrence of
this eff ect, is the fi re at the MGM Grand
Hotel and Casino, in 1980. A fi re broke
out in a ground fl oor restaurant, killing
85 people, with the majority of the deaths
taking place on the upper fl oors, due to
smoke inhalation.
A variety of smoke control techniques
has been proposed for both stairwell
and elevator shafts, primarily involving
enclosed vestibules or lobbies surrounding
the doorways. However, the current study
sought to investigate the feasibility of
solely using shaft pressurisation as a means
of preventing smoke migration in elevator
shafts.
Pressurisation systems for stairwells
have been used for some time. � e
available literature has shown stairwell
pressurisation systems to be feasible,
although the system performance can be
quite sensitive to several design parameters,
including the opening of stairwell doors.
In contrast, elevator shaft pressurisation
has only been recently approved by the
International Building Code (IBC) for
smoke prevention in elevator shafts, and
relatively little research has been done to-
date, although experimental measurements
in a fi re tower and a limited number of
simulation results, have been reported.
� e primary objectives of the current work
are to both illustrate the fundamental
diff erences between stairwell and elevator
shaft pressurisation systems, and to provide
input for future code changes. Eff ects of
the elevator and exterior building doors,
ambient temperature, fan location, and
shaft venting, on the pressurisation system
performance, are also addressed.
MODELLING APPROACH
� e article presents the results from an
investigation of stairwell and elevator
shaft pressurization, on potential smoke
distribution through the shaft eff ect.
All results were obtained via computer
simulations using the CONTAM software
developed by the Indoor Air Quality
and Ventilation Group at the National
Institute of Standards and Technologies,
USA. � e CONTAM software has been
used extensively for similar simulations of
air fl ow and for both stairwell and elevator
shaft pressurisation.
� e results are presented for a
30-storey building model. A schematic of
the building model’s typical fl oor plan is
provided in Figure 1 (not to scale) along
with prescribed leakages. � e building is
specifi ed as a 30-storey building with a
fl oor height of 9.85 ft (3.0 m) and a fl oor
area of 10,000 ft2 (930 m2). On each fl oor,
there are two stairwells located at opposite
corners of the building. In the centre of
the building, are two (open) elevator shafts
having four sets of elevators and elevator
doors. All interior building leakage areas
are based on typical values reported in
the literature. Each of the closed elevator
doors (four per shaft) has a leakage area
of 75 in2 (0.0484 m2). However, the fi rst
fl oor elevator doors have a 854 in2 (0.558
m2) leakage area modelling the elevator
doors being open with the car on that
fl oor. Each stairwell has a single door with
leakage area 16 in2 (0.0103 m2). Each
fl oor of the building has exterior leakages
calibrated with experimental data for
either a ‘residential’ (37-storey building in
Korea) or a ‘commercial’ (data measured
in a 20-storey bank in Boise, Idaho)
building.
� e two models take account of the
presence or absence of openable windows,
balcony doors, and other features
associated with the two specifi c buildings
used for the calibration. � e parameters
are not meant to imply that all residential
buildings will have more porous skins
than other buildings. It is noted that the
ground fl oors have larger leakage areas
than those on upper fl oors, due to the
presence of exterior doors and/or other
unique features. � e building temperature
is maintained at 70° F (21° C) on all fl oors.
Figure 1: Schematic representation of the 30-storey building fl oor plan. External leakages correspond to either a residential (R) or commercial (C) building model.
THE SINGAPORE ENGINEER Feb 2009 · 7
Fire Protection
No wind is present.
Each building model also has a roof
level with only the stairwells and elevator
shafts (where there are fans, they are
installed here). Elevator shaft and stairwell
shaft pressurisation are considered by
pressurising the shafts until a specifi ed
minimum pressure diff erence across any
elevator or stairwell door is achieved.
� is corresponds to a minimum pressure
diff erence of +0.05 inch water (+12.5
Pa) across elevator doors or +0.15 inch
water (37 Pa) across stairwell doors. � e
process is iterated with a model for the
average shaft air temperature based on
turbulent duct theory along with the
Dittus Boelter heat transfer correlation.
For elevator shaft pressurisation, the
elevator cars are on the fi rst fl oor with all
doors in the open positions and with all
stairwell doors closed, unless otherwise
specifi ed (where there is no pressurisation
system, all elevator doors are in the closed
position). For stairwell pressurisation,
all elevator doors and all stairwell doors
are in the closed positions. Simulations
are conducted for both pressurised
shafts and for non-pressurised shafts for
comparisons. Both ‘cold day’ (−12° C, 10°
F) and ‘hot day’ (38° C, 100° F) conditions
are considered.
RESULTS
One fi nding of the study is the importance
of the elevator or stairwell air temperature
on the simulation results. On very warm
or very cold days, the ambient air is at a
substantially diff erent temperature than
that of the building, and this diff erence
aff ects the pressure profi les within the
shaft. A detailed model for the average
shaft temperature was therefore derived
from turbulent duct fl ow theory and
incorporated into the simulations.
Stairwell pressurisation only
Results for stairwell shaft pressurisation
only, are presented in Figure 2, for the
residential and commercial building
models. Stairwell pressurisation is
observed to work well within the limits
allowed by the current code limits of
.
� e results of the simulations show that
the system calibration is highly sensitive
to the ambient temperature. � e location
of the minimum pressure diff erence across
Figure 2: Pressure diff erence across the stairwell doors as a function of the fl oor number for a stairwell only pressurisation system: (a) residential building with no pressurisation, (b) residential building with pressurisation, (c) commercial building with no pressurisation, and (d) commercial building with pressurisation.
a) b)
c) d)
8 · THE SINGAPORE ENGINEER Feb 2009
Fire Protection
stairwell doors is the ground fl oor when
the ambient temperature is less than
the building temperature. However, the
minimum pressure diff erence occurs on
the top fl oor when the ambient is warmer
than the building temperature. Fan
output is also dependent on the ambient
temperature.
Eff ects of the exterior building door position
While not particularly sensitive, stairwell
pressurisation systems can be aff ected
by the position of the building’s exterior
doors. Figure 3 illustrates the eff ects of
the ground fl oor exterior door position on
the system performance for the residential
building model, on a cold day. Results are
presented for systems that are calibrated
with the exterior building doors either
closed [Figure 3(a)] or open [Figure 3(b)].
! e eff ect of then opening (or closing)
the exterior doors, is also shown in each
fi gure. Required fan speeds also change
and are approximately 15% larger for
the open door cases. To understand the
results, the system should be considered
calibrated with the building doors closed.
As the doors are opened, the slightly
pressurised building loses pressure to the
ambient. ! e eff ect is felt throughout the
building as the elevator shafts provide a
relatively unrestricted fl ow of air between
fl oors. ! e resulting pressure diff erence
across the fi rst fl oor stairwell doors, is
slightly reduced below the code-specifi ed
minimum, but remains large enough to
inhibit smoke penetration. As discussed
below, a very diff erent behaviour results
with elevator shaft pressurisation systems,
as the stairwells are relatively well sealed
and do not provide an equivalent route
for air fl ow between fl oors.
Elevator shaft pressurisation only
Results for elevator shaft pressurisation are
presented in Figure 4, for the residential
and commercial building models. Several
of the major potential problems with
elevator shaft pressurisation systems are
illustrated. Elevator shaft pressurisation
is markedly diff erent from stairwell
shaft pressurisation. Required fan
fl ow rates are approximately 50 times
larger for elevator shaft pressurisation
systems than for stairwell pressurisation
systems. ! e current code limits of
Figure 3: Pressure diff erences across stairwell doors as a function of the fl oor number for the residential building model. Data are for a stairwell only pressurisation system calibrated with the exterior building doors in either the (a) closed or (b) open position, and show the eff ects of opening or closing
the exterior building doors. All data are for ‘cold day’ conditions (10°F).
a) b)
a) b)
c) d)
Figure 4: Pressure diff erence across the elevator doors as a function of the fl oor number for an‘elevator shaft only’ pressurisation system: (a) residential building with no pressurisation, (b)residential building with pressurisation, (c) commercial building with no pressurisation, and (d)commercial building with pressurisation.
are also impossible to meet. Furthermore,
pressure diff erences across upper fl oor
elevator doors far exceed any reasonable
limits for proper door functioning.
! e reasons for the resulting ‘across
elevator’ door pressure diff erences can
be explained. Air is forced into the shaft
from the roof and some is ‘lost’ along the
way through the closed elevator doors
and into the building interior. However,
a relatively large fl ow rate is needed to
achieve the +0.05 inch water (+12.5 Pa)
pressure diff erence across the fi rst fl oor
open elevator doors, due to their much
larger leakage areas. As the ground fl oor
elevator doors are open and have relatively
large leakage areas, this required fl ow rate
can be considerable.
! e air fl owing into the fi rst fl oor
from the shaft then pressurises the fi rst
fl oor until the fl ow rate out of the fi rst
fl oor (through exterior and stairwell
THE SINGAPORE ENGINEER Feb 2009 · 9
Fire Protection
leakages) equilibrates with the fl ow rate
entering through the elevator shafts. � e
second fl oor interior building pressure is
much less than that on the fi rst fl oor as
the closed stairwell doors have a relatively
small leakage area (in cases where there is
a coupled stairwell pressurisation system,
no air would be allowed to fl ow into the
stairwell shaft).
However, the pressure within the
shaft only varies hydrostatically, and
so is only slightly lower at the second
fl oor. � erefore, the ‘across elevator’
door pressure diff erence is increased
substantially on the second fl oor (as well
as all remaining fl oors). � is pressurisation
of the ground fl oor is due to the large
open door leakage areas and is the primary
eff ect distinguishing stairwell and elevator
shaft pressurisation systems. � e eff ect
is enhanced as the fi rst fl oor leakage
becomes smaller for the commercial
building model (and vanishes if the fi rst
fl oor exterior door is open). � e outside
temperature has relatively little infl uence
on the fi nal system pressure diff erences.
However, signifi cantly diff erent fan fl ow
rates are required based on the exterior
temperature (fl ow rates are approximately
10% larger on hot days than on cold days).
� erefore, a system calibrated and tested
during one season, may have signifi cantly
diff erent behaviour during other seasons.
Eff ects of the exterior building door position
One possibly tempting means of
overcoming the large pressure diff erences
across elevator doors, is to calibrate
Figure 5: Pressure diff erences across elevator doors as a function of the fl oor number for the residential building model. Data are for an ‘elevator shaft only’ pressurisation system calibrated with the exterior building doors in either the (a) closed or (b) open position and show the eff ects of opening or closing the
exterior building doors. All data are for ‘cold day’ conditions (10° F).
the system with the exterior ground
fl oor building doors open. � is would
eliminate the overpressure on the
ground fl oor. However, in contrast to
stairwell pressurisation systems, elevator
shaft pressurisation systems are much
more sensitive to the ground fl oor door
position. Figure 5 shows the eff ects of
the exterior building door positions for
the residential building model on a cold
day, with elevator shaft pressurisation.
System performance is considered for
systems calibrated with the exterior doors
in either the open or closed positions. A
set of double doors propped wide open, is
modelled with a 42 ft2 (3.90 m2) leakage
area on the ground fl oor. � e eff ects of
then changing the exterior door position
are also included in the fi gure.
System performance and fan
requirements change dramatically based
on the exterior doors. If the system is
calibrated with the exterior doors in the
closed position, a relatively large fan
speed is required (larger by in excess of
300% than if the doors are open). If the
exterior doors are then propped open,
the minimum pressure diff erence is still
maintained across all elevator doors
(although prohibitively large pressure
diff erences exist). In contrast, if the
system is calibrated with the exterior
doors open and then operated with the
doors closed, there will be essentially no
pressure diff erence across the ground fl oor
elevator doors. In this situation, there is
a high probability of smoke entering the
shaft if a fi re is present on the ground
fl oor. In summary, the only way to achieve
a potentially code-compliant system
performance is with the ground fl oor
doors open. However, the system will fail
if it is operated with the doors closed as
may very well be the case in an actual fi re
situation.
Eff ects of other parameters
Further studies have also examined the
eff ects of the fan location, secondary
pressurisation systems, multiple injection
points, and the eff ects of various louvre/
vent systems to alleviate overpressures.
� e results clearly show that each of these
approaches is incapable of alleviating the
above problems. Since the elevator shaft
is relatively wide, it experiences negligible
frictional resistance and the shaft pressure
simply equilibrates to pressure changes
as would occur in a large tank. � e shaft
pressure is therefore independent of the
fan location or to multiple injection
points. Louvres and vents are similarly
incapable of properly controlling the shaft
pressure distribution because they are
only capable of uniformly changing the
pressure in the entire shaft. � erefore, any
reduction in the maximum shaft pressure
due to a roof (or otherwise located) vent
or louvre, simply shifts the entire pressure
distribution within the shaft evenly.
� is results in the minimum +0.05 inch
(+12.5 Pa) being violated as the fi rst fl oor
pressure diff erence drops. For example,
if a louvre system that allows 2,500 ft3/
min (70 m3/min) of air to fl ow from the
top of the shaft, is installed, the fan speed
a) b)
10 · THE SINGAPORE ENGINEER Feb 2009
Fire Protection
would need to be increased by the same
2,500 ft3/min (70 m3/min) to compensate
and to re-acquire the minimum pressure
diff erence across the ground fl oor elevator
doors. � e net eff ect is to re-acquire the
original pressure profi le but with a larger
fan requirement. Relying on transients is
also ineff ective. An analysis shows that the
system response time to changes in door
positions, fan fl ow rates, etc. is ~5 sec for
the current model building.
Coupled stairwell and elevator shaft
pressurisation
Results for simulations of the building
models with coupled stairwell and
elevator shaft pressurisation systems
(that is, operating simultaneously and
jointly aff ecting the building fl oor
pressures) are presented in Figure 6 for
the residential and commercial building
models. � e simulation results illustrate
an additional and very serious problem
for stairwell pressurisation systems if
used in conjunction with an elevator
shaft pressurisation system. � e addition
of the elevator shaft system results in an
additional fl ow of air into the building
on all fl oors. � is raises the pressure of
the building interior and would result in
negative pressure diff erences across the
stairwell doors, if the stairwell-only fan
speeds were used. � erefore, substantial
modifi cation of existing stairwell
pressurisation would be required if an
elevator system were later installed.
Furthermore, another problem occurs
after the stairwell system is re-calibrated
to acquire a minimum +0.15 inch water
(+37 Pa) pressure diff erence across any
stairwell doors. In this case, a phenomenon
occurs, as was observed previously for the
elevator shaft pressurisation systems. � e
overpressure on the fi rst fl oor as compared
to the second fl oor of the building, also
creates very large pressure diff erences
across all upper fl oor stairwell doors.
� ese pressure diff erences are far too large
for proper stairwell door functioning. For
example, if a 7 ft x 3 ft (approx 1 m x
2 m) stairwell door has a 1.5 inch water
Figure 6: Pressure diff erences across either stairwell or elevator doors as a function of the fl oor number for coupled stairwell and elevator shaft pressurisation systems: (a) residential building, stairwell doors, (b) residential building, elevator doors, (c) commercial building, stairwell doors, and (d) commercial building, elevator doors.
a) b)
c) d)
THE SINGAPORE ENGINEER Feb 2009 · 11
Fire Protection
(approx 375 Pa) pressure diff erence, this
would require a force of approximately
165 lbf (approx 750 N) to open the
door! � ese results show that in addition
to the problems described previously for
stand-alone systems, an elevator shaft
pressurisation system will also make the
standard stairwell pressurisation system
fail.
Eff ects of the elevator door position
Simulations were also conducted
assuming that the system is calibrated
with all the elevator doors in the closed
position. � e results are similar to those
described above, for the exterior building
door position. Both the stairwell and the
elevator pressure diff erences are within
reasonable limits when the elevator
doors are closed. However, if the elevator
doors are opened on the ground fl oor,
the minimum pressure diff erence nearly
vanishes on the ground fl oor. � is occurs
also for the stairwell doors on the ground
fl oor. In this case, there is a strong potential
for smoke to enter either shaft, if present
on the ground fl oor.
Instead of calibrating the pressurisation
system with either the exterior building
doors propped open or with the elevator
doors closed, it may be proposed that the
system be calibrated by simply ignoring
the pressure diff erences across the open
ground fl oor elevator doors. Aside
from the ignored pressure diff erences,
Figure 7: Pressure diff erences across elevator doors as a function of the fl oor number for theresidential building model with coupled stairwell and elevator shaft pressurisation on a cold day. Pressure diff erences across the open elevator doors on the ground fl oor are ignored for system calibration. ! e results show the eff ects of having two elevator cars move to the 15th fl oor with open elevator doors.
all systems are essentially able to meet
the specifi cations. Both the stairwell
doors and the elevator doors experience
reasonable pressure diff erences. However,
the open elevator doors may still be
problematic. In all cases, these pressure
diff erences are essentially null. In the
event that smoke was present on the
ground fl oor and the elevator doors were
opened, it could be forced into the lower
level fl oors either just above the ground
fl oor or into the lower (basement) levels.
Occupants may be forced to evacuate
towards the fi re containing fl oor. In
contrast, a pressurisation fan mounted
on or below the ground fl oor would
prove catastrophic as the smoke would
be blown throughout the entire building.
Another potential problem with a system
calibrated, ignoring the open elevator
door pressure diff erences, is illustrated
in Figure 7. In this case, the calibrated
system for the residential building model
on the cold day conditions is examined.
� e calibrated building model is altered as
follows: two of the elevator doors from a
single shaft are now closed on the ground
fl oor and the same two doors are opened
on the 15th fl oor (mimicking the eff ects
of two cars in use by either fi re fi ghters
or building occupants). In this case,
the pressure diff erence across all of the
elevator doors is lost on the 15th fl oor,
as air from the shaft pressurises the fl oor.
� e results show that if the elevators are
brought to a smoke containing fl oor, there
is a high probability of smoke entering the
shaft. In this case, the fan pressurisation
system would actively distribute the
smoke throughout the building (and at a
higher rate than the stack eff ect the system
was originally designed to overcome).
� erefore it is recommended that pressure
diff erences across open elevator doors,
should not be ignored, if there is any
potential for elevator usage during a fi re
situation.
Eff ects of the building height and
number of elevator cars
� e results to this point have shown
that a robustly operating elevator shaft
pressurisation system with reasonable
pressure diff erences across elevator doors
is nearly impossible to design in the
30-storey building model if (1) these
pressure diff erences apply to both open
and closed elevator doors, and (2) if the
system must function properly when the
ground fl oor exterior building doors are
closed. Additional simulations have shown
that these results are not directly aff ected
by the building height but are directly
aff ected by the number of elevator cars and
shafts. � erefore, while tall buildings may
have the characteristics that produce large
‘across elevator’ door pressure diff erences
(due to larger numbers of elevator cars),
it is not the building height directly that
causes the behaviour observed in the
study.
CONCLUSIONS
� e operation of stairwell shaft
pressurisation systems was found to be much
simpler than elevator shaft pressurisation
systems (and quite feasible). In contrast,
elevator shaft pressurisation was found to
require substantially larger fan fl ow rates
to achieve the required minimum pressure
diff erences. Prohibitively large pressure
diff erences across upper fl oor elevator
doors were found for all cases where the
exterior building doors are kept closed,
and open elevator doors contribute to the
minimum pressure diff erence.
� e elevator shaft system also
catastrophically interferes with the
stairwell pressurisation system in these
cases. In contrast, systems calibrated
with either the exterior building doors
open, all elevator doors in the closed
position, or ignoring the open elevator
door pressure diff erences, were all
found to maintain reasonable ‘across
door’ pressure diff erences on all fl oors
(stairwell and elevator). However, each
of these will lead to situations in which
nearly null ‘across elevator’ door pressure
diff erences on some fl oors, could allow
smoke to enter the shaft and be actively
distributed throughout the building. Fan
location, vents and louvres were all found
to be ineff ective as a means of controlling
the shaft pressures. Little eff ect of the
ambient temperature was observed on the
fi nal elevator door pressure diff erences.
However, signifi cantly diff erent fan speeds
are required.
(More information on the study, and on
the references cited in the summary of the
technical results, may be obtained from Dr
Richard S Miller at rm@clemson.edu)
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