comparison to low-rise buildings, very tall buildings have several
inherent attributes that increase the variety, probability, and severity
of potential fire events, including: higher occupant loads; longer
evacuation times; access issues for responding fire departments;
potential water pressure/availability issues; pronounced stack effect;
the potential for multiple concurrent occupancies to be present; and
their iconic, high -visibility nature. Specifying adequate smoke control
provisions during the design of very tall buildings plays a key role in
addressing several of these issues.
control" in the broadest sense simply means controlling the movement of
smoke throughout a building via passive and active means. The
installation of fire and smoke barriers with protected openings is a
form of passive smoke control. Arguably, automatic sprinkler systems
provide a form of smoke control by limiting the size and growth rate of
fires and by cooling the smoke and thereby reducing buoyancy and
will focus on engineered smoke control systems in very tall buildings,
which use mechanical means to produce pressure differentials across
barriers to inhibit smoke spread. It will discuss the code trends of the
past and some of the relevant design considerations for smoke control
in very tall buildings of the future.
Common types of engineered smoke control systems in high rises and very tall buildings include:
- Atrium smoke control systems – It is common for very tall buildings to contain one or more atria or even to contain a covered mall or other large-volume space.1 Most consensus codes in the United States require a smoke management system for multiple-story atria. This would typically consist of a mechanical smoke exhaust system to extract smoke from the top of the atrium with low-level, low velocity make-up air at the bottom of the atrium in order to maintain the smoke layer above the occupied areas and their associated means of egress for a specified time period.
- Stair pressurization – Most building codes require high-rise buildings to be provided with smokeproof exit stairs or stair pressurization.2 The International Building Code (IBC) allows three methods for compliance: exterior stair balconies, mechanically ventilated stair vestibules, or stair pressurization. Mechanical stair pressurization is the most commonly used approach to meet this requirement for high-rise buildings in the United States.
- Pressurized elevator hoistways – The IBC permits the omission of elevator lobbies if a hoistway pressurization system is provided. In order to meet the aesthetic and functional needs of the building, this design option is often chosen.
- Post-fire smoke removal – Post-fire smoke removal systems are intended to facilitate smoke removal during post-fire salvage and overhaul operations. To meet this requirement, the IBC and various previous codes have allowed operable windows/panels, or mechanical equipment capable of providing a prescribed number of air changes per hour.
- Zoned smoke control system – Defined as a smoke control system that divides the building into separate smoke control zones and creates pressure differentials to inhibit smoke spread. Mechanical exhaust is provided for the areas containing smoke and pressurization is provided for the other contiguous zones. In a high-rise building, zones may consist of entire floors, but sometimes floors are subdivided into multiple zones. An example is the pressure "sandwich" effect in which the fire floor is exhausted and the floors above and below are pressurized. These systems have the drawback of being ver y complex due to the necessary coordination with other HVAC equipment, controls, operational matrices, and so on.
building and fire codes usually required at least one exit stair in
buildings over a certain height to meet requirements for "smokeproof
towers." For example, the 1927 Edition of the Uniform Building Code (UBC)
required buildings five or more stories in height to be provided with
at least one exit meeting the requirements for smokeproof towers.3 Early editions of the BOCA Basic Building Code
(BOCA) dating prior to the 1960 Edition required at least one
smokeproof stair tower in buildings over 75 ft (23 meters) in height.
The requirements for these smokeproof towers were met by constructing
exterior stair balconies or stair vestibules with openings to the
In 1975, the BOCA Basic Building Code included a dedicated section on high-rise buildings, which included smoke control requirements. Allowable smoke control methods in that edition included: HVAC equipment designed to exhaust air to the outside, operable panels or windows for natural ventilation, tempered glass windows, or, a continuous shaft used to mechanically exhaust one air change per minute from within the building. In 1990, BOCA provided prescriptive requirements for zoned smoke control systems in high rises based on a required number of air changes per hour. However, operable windows/panels were still permitted as an alternative. In 1993, zoned smoke control requirements were removed entirely from the BOCA high rise requirements. The 1993 BOCA Commentary indicated that:
"high-rise buildings have life safety systems including: sprinklers, pressurized stairs, control of the HVAC system, and smokeproof enclosures. In the absence of floor openings, these systems, along with the story-to-story compartmentation provided by continuous floor construction, provide an acceptable level of safety. As such, smoke control is not a consideration unless the building also contains an atrium."4
BOCA eliminated all high rise zoned smoke control requirements in 1993,
the UBC requirements went in the opposite direction. The 1994 UBC
required an engineering-based zoned smoke control system in all high
rises with the stated purpose of providing a tenable environment during
an evacuation. Stack effect, temperature, effect of fire, wind, and HVAC
systems all needed to be evaluated. These requirements remained in the
UBC through its final edition in 2000.
consensus definition for "very tall building" has not yet been
determined. Like its predecessor codes, the IBC defines "high rise" as a
building with an occupied floor(s) 75 ft (23 m) or more above the
lowest level of fire department vehicle access. The IBC provides several
additional requirements for high-rise buildings, including extra
requirements for stair enclosures. If stair pressurization is used as an
alternative to a smokeproof stair enclosure, the stair must be
positively pressurized to a minimum of 0.10 inches of water (25 Pa) and
maximum of 0.35 inches of water (87 Pa), relative to the building
internal pressure, with all stairwell doors closed. The stair vestibule
(if provided) is required to be ventilated such that it is supplied with
at least one air change per minute with an exhaust capacity of at least
150 percent of supply.
editions of the IBC have included additional requirements for buildings
exceeding 420 ft (128 meters) in height, such as increases in the
minimum construction type permitted, increases in required fire
resistance ratings, and requirements for more robust stair and elevator
shaft construction. However, no additional smoke control system
requirements were added. Thus, regardless of the height of the building,
the IBC has never required a zoned smoke control system due to building
height alone. However, since 2009, the IBC has incorporated language
that requires the provision of a natural or mechanical system or method
to assist with smoke removal during post-fire incident clean-up
operations in high-rise buildings.
Middle East is home to many of the world’s tallest buildings. The
United Arab Emirates (UAE) Ministry of the Interior published the UAE Fire and Life Safety Code of Practice in 2011, which is now a mandatory compliance document for new projects submitted for approval to Civil Defense.5
The code contains a chapter on smoke control. For all high-rise
buildings, the UAE code requires pressurized stairs and a zone - type
smoke control system for egress corridors.
is also home to many of the world’s tallest buildings. Requirements for
tall buildings in China, Hong Kong, and other countries in that region
are similar to the U.S. codes. One exception is that the Chinese codes
require refuge floors to be located every 20 stories.6 These
stories must remain dedicated for refuge use and must be open to the
exterior. Buildings over 780 ft (238 m) in height must have additional
protection, but such protection is undefined. Smoke control or
pressurization is also required in stairs and in corridors in buildings
over 105 ft (32 m) in height.7
The Society of Fire Protection Engineers created the first edition of a new guidance document entitled: Engineering Guide – Fire Safety for Very Tall Buildings.8
new guide is a useful reference for professionals designing very tall
buildings using performance-based fire protection engineering concepts.
The guide also includes a section on smoke control.
CONSIDERATIONS DURING DESIGNS
tall buildings are subjected to the same forces that drive smoke
movement within any other building (buoyancy, expansion, ventilation
systems, elevator piston effect, stack effect, and wind). However, by
nature of their height, very tall buildings warrant additional attention
to the following features:
Stack effect – Stack effect and reverse stack effect are terms that describe the vertical air movement within a building resulting from air density differences between the building interior and exterior or between two interior spaces.9 These flows can cause smoke from fires to spread between floors of tall buildings through vents, stairs, and other shafts. The pressure difference due to stack effect can be expressed by:10
Δp =3840 (1/T0 -1/Ts)h
Δp =pressure difference [Pa]
T0=absolute temperature of outside air [degrees K]
Ts=absolute temperature of air inside shaft [degrees K]
h=distance above neutral plane [m]
1 shows the impact that shaft height can have on stack effect induced
pressure differentials, given a constant temperature differential.
|Building Height||Outside Temp [°F] (°C)||Inside Temp [°F] (°C)||ΔP [in. w.c.] (Pa)|
75 ft (23 m)
300 ft (91 m)
984 ft (300 m)
2133 ft (650 m)
Table 1. Stack Effect Induced Pressure Differentials by Building Height
Compared to the pressure differences generally created by smoke control systems, the stack effect in very tall buildings can be significant. It should be noted that the pressure differentials shown in Table 1 are calculated between a shaft and the outside of a building, whereas smoke control systems are intended to create pressure differentials between areas within a building. In any case, stack effect can create large pressure differentials that can lead to dangerous smoke movement in a building, make it difficult to open doors, or have an adverse effect on smoke control systems.
In order to limit
the stack effect, shafts in very tall buildings should be interrupted at
regular intervals. For example, mechanical shafts can be capped every
so often. Stair shafts can be interrupted with transfer passageways or
Wind – Like stack effect, wind on the outside of a building can also create pressure differentials, which can lead to smoke movement within a building. This pressure differential is a complex phenomenon and depends not only on wind velocity but also on the building geometry and can vary locally over a wall surface.8 Average wind velocity increases with height above the ground, so wind becomes more of a concern in very tall buildings. In buildings that are tightly constructed with all windows closed (typical of modern high rises), the effect of wind on air movement inside the building is small.9 However, wind can become a much more critical factor if a window is broken during a fire event.
tunnel testing or computer modeling of very tall buildings may be
necessary to evaluate the effects of wind induced pressure differentials
on smoke movement within the building and the impact on any
smoke-control systems.11 Window breakage scenarios should be considered.
– The term "piston effect" refers to the transient pressure
differentials created by the movement of an elevator car. Very tall
buildings experience piston effect due to the large number of elevators
employed and the high speeds at which the elevators travel. Providing
elevator hoistway ventilation can reduce the piston effect, but this
phenomenon needs to be evaluated during the design of smoke control
systems for very tall buildings.
– Heating, ventilation, and air conditioning (HVAC) systems can play a
role in interior smoke spread by extracting and recirculating smoke from
the fire area or by pressurizing the fire area and forcing smoke into
adjacent building areas. The building codes address this issue by
requiring HVAC fan shutdown upon smoke detection in the ductwork and the
installation of fire/smoke dampers at fire-resistance rated shaft
enclosures, fire barriers, and smoke barriers as a way to reduce smoke
spread in a building. Full integration of HVAC systems with any
engineered smoke control system is critical.
Long Egress Times –
Prescriptively designed stairs are usually required to be sized based
on the highest occupant load of any single floor the stairs serve (or
multiple floors, if merging occurs). In very tall buildings, the issue
is that the stairs are not sized for a simultaneous total building
evacuation. Significant delays and queuing can occur in the stairs
during simultaneous total evacuations.12 Therefore,
tenability inside the stair needs to be maintained for long periods of
time. Pressurizing the stair is a well-recognized method of maintaining
tenability. Long egress times and queuing will result in stair doors
remaining open, which will have an impact on the required fan size
needed to maintain the necessary pressure differential.
Refuge Areas –
Even during a staged evacuation, many occupants are not capable of
walking continuously down dozens of flights of stairs. In very tall
buildings, occupant refuge areas located at certain intervals vertically
along the path of egress to grade may be necessary. These refuge areas
should be considered an extension of the pressurized stairs and
protected accordingly. The stairs serving these refuge areas can be
offset from one another to help obstruct vertical smoke spread through
the stair. Connecting these refuge areas to multiple stairs would
provide alternative routes in case one stair becomes unusable during an
emergency. Interrupting the stairs also limits the stack effect by
limiting the shaft height.
Erik Anderson, P.E., is with Koffel Associates, Inc.
- Super tall buildings--special smoke control requirements. ASHRAE Transactions, January 2011.
- International Building Code, International Code Council, Washington, DC, 2012.
- Uniform Building Code, International Council of Building Officials, Whittier, CA.
- BOCA National Building Code, Building Officials Code Administrators International, Country Club Hills, IL.
- UAE Fire and Life Safety Code of Practice, General Headquarters of Civil Defence, Ministry of the Interior, United Arab Emirates, 2011.
- Brief Review on Forced-Ventilation Requirements in China Fire Codes. International Journal on Engineering Performance-Based Fire Codes, Volume 5, Number 1, pp. 7-19, 2003.
- Code for Fire Protection Design of Tall Buildings (GB50045-95, 1997), Ministry of Public Security, Ministry of Construction, P.R. China (1997).
- Engineering Guide – Fire Safety for Very Tall Buildings, International Code Council, Washington DC, 2013.
- Klote, J., Milke, J., Turnbull, P., Kashef A. & Ferreira, M., Handbook of Smoke Control Engineering, American Society of Heating Refrigeration and Air-Conditioning, Atlanta, GA, 2012.
- Klote, J. "Smoke Control," SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Quincy, MA, 2008.
- Cernak, A., Wind Tunnel Studies of Buildings and Structures, American Society of Civil Engineers, Reston, VA, 1999.
- ASHRAE Research Project RP1203, "Tenability and Open Doors in Pressurized Stairs," 2002.