Smoke control for tall buildings--an integrated approach to life safety.
Tall buildings pose unique challenges for all aspects of design including smoke management systems. Many of today's tall buildings are being designed with more than 100 stories, some reaching 2,000 ft (610 m) or more into the sky, yet many people may have difficulties in traversing down 40 floors or less through the stairways. Even tall buildings of modest height pose issues associated with increased stack effect, increased wind effects, difficulties with negotiating stairs, and difficulties for emergency responders to reach upper floors. The 2009 edition of the International Building Code (IBC) and international practices have evolved to address some of these concerns. This paper presents challenges, requirements, and current practices for developing smoke management system that integrate into comprehensive life safety programs.
LIFE SAFETY CONSIDERATIONS
In tall buildings, certain design challenges require special attention. Typical challenges can include: designing for the challenge of negotiating many flights of stairs, designing for large populations during simultaneous evacuations, and designing for fire fighter access. Negotiating down many flights of stairs in a tall building can be a difficult task for many, and nearly an impossible task for others. Evacuating wheelchair users and those with mobility impairments presents a special challenge in all buildings--this can be a very difficult task without the use of elevators or evacuation chairs. These challenges are more difficult in tall buildings. Consideration should be given to the inclusion of refuge or re-entry floors, as occupants may need an area to rest along the path to grade.
Code required evacuation widths have evolved to accommodate three to five floors simultaneously evacuating. If simultaneous full building evacuation from a tall building becomes necessary, it will involve many floors and a large number of people--much larger than that anticipated by current codes and many current building egress designs. This inherent imbalance will cause significant delays and crowding in stairs during simultaneous full evacuations. Providing a safe and smoke-free environment within these stairwells is therefore essential requirement for high-rise buildings. The code has also recognized that improved egress efficiency can be met with the inclusion of elevators during evacuation. These elevators can also be used as an accessible means of egress for occupants with disabilities.
During emergency events, emergency responders need access to the floors of concern. Traditionally, this meant responders needed to use exit stairs to reach the upper floors. Ascending many flights of stairs, while carrying needed response equipment, can be a difficult and slow operation. Fire fighter elevators can significantly help responders perform fire-fighting and life safety tasks. Special protection is needed for such elevators as discussed in greater detail in subsequent sections of this paper. (Tubbs and Meacham 2007)
In tall buildings, designing to account for stack and wind effects can prove to be challenging. The stack effect and the reverse stack effect occur due to temperature differences between the building and outside air. Buildings tend to leak air to the exterior through construction cracks, ventilation systems, and other exterior openings. If the exterior temperature is lower than the interior temperature, the interior air will rise through stair shafts, elevator shafts, and similar floor-to-floor connections and flow to the exterior on the upper floors. This flow of air is called the stack effect. These flows can cause smoke from fires to quickly spread to the upper floors of a tall building through vents, stairs, and other shafts, affecting occupants in areas remote from the fire. The process reverses in the summer months.
Wind velocity typically increases with building height. In tall buildings, this can be significant, as wind imposes pressures on the exterior. Wind pressures affect interior air flows and can be particularly problematic for buildings with operable windows or other similar vent openings. These pressures can be the source of considerable air movement in buildings - this condition can also cause smoke from fires to spread to areas remote from the fire. Wind pressures are positive on the windward side and negative on the leeward side. Since wind-driven air flows in and around buildings are complicated, pressures need to be carefully considered. In many cases, computer analysis, along with scale model wind tunnel testing, may be necessary to fully appreciate and appropriately account for this phenomenon (Klote and Milke 2002, CTBUH 1992).
INTEGRATED LIFE SAFETY STRATEGIES
Effective smoke management designs integrate with and support the building's fire and life safety strategy. Compartmentation, fire suppression, fire detection, evacuation, occupant notification, smoke management, emergency power, and structural fire protection need to work in concert to support the strategy. In tall buildings, it is particularly important for the strategy to support the specific evacuation concepts. Too often, life safety systems are designed without a clear understanding of the integrated detection, notification, and evacuation concepts - sometimes the evacuation concepts are developed in isolation as part of finalizing the fire alarm sequence of operations matrix, rather than being a design goal that integrates with, compliments, and guides the life safety systems design. When developing comprehensive life safety strategies for tall buildings, evacuation concepts need to be determined early in the design process. This allows the safety systems to be designed with a common goal, informing and guiding decisions throughout the design process.
Several strategies for protecting occupants and supporting safe evacuation in high-rise structures are in use to varying degrees throughout the world. Some of these strategies place greater reliance on active fire safety systems, such as smoke management systems. This section discusses common life safety strategies for high-rise buildings and the fire safety systems that they rely upon.
Traditional High-Rise Evacuation
For many years, typical evacuation schemes for high-rise buildings included evacuating the 'fire' floor - or the floor experiencing a fire incident - along with one or two floors above and below the 'fire' floor. The International Building Code requires five total floors in high-rise buildings to be simultaneously notified to initiate evacuation. Occupants on other floors use a defend-in-place strategy. This concept has worked well for traditional events because automatic suppression systems are designed with a degree of resilience and have proven to be effective in controlling fires.
In addition to crediting fire suppression systems with the ability to minimize the spread of a fire away from the point of origin, both horizontally and vertically, a traditional high-rise evacuation approach relies upon several other strategies. These strategies include fire resistance rated floor/ceiling assemblies to provide compartmentation between floors to reduce spread of fires to provide more time for occupants to evacuate or defend in place. Also important are stairwell protection strategies. Stairwells are protected using both fire resistance rated construction and smoke management strategies to help prevent smoke from the fire floor from spreading to the stair or to other unaffected floors. Without this protection, smoke can migrate from the point of origin into the stairwell, effecting occupants attempting to evacuate. Stair smoke management strategies are discussed in more detail later in this article.
Tall Building Evacuation Strategies
Tall buildings often require evacuation strategies that extend beyond the traditional approach. A description of these concepts follows. Note that some or all of these may be used together for a given building.
* Simultaneous Full Building Evacuation. Some events may require simultaneous full evacuation of the building. This means that a large number of people will need to move through the egress components, including protected corridors, stairs, and possibly elevators. Current codes generally do not require the building's egress systems to support simultaneous full evacuation, so delays will occur if this is necessary. Phasing of evacuation may assist with congestion in the stairs - this involves evacuating specific floors while asking other floors to remain in place. Since phasing strategies require occupants to defend-in-place while waiting to escape, they do not help to reduce the overall evacuation time. Additional fire and life safety systems may be required. Often, refuge floors or protected refuge areas are provided to allow occupants to rest during their descent. Smoke management strategies need to support these concepts.
* Evacuation Elevators. Protected elevators provide an alternative to walking down many flights of stairs. With appropriate design, elevators can be designed to achieve a range of goals: (1) it may be possible to allow elevator evacuation for a large segment of the building population; (2) with good training and stringent controls, it may be possible to limit the use of elevators to those who cannot walk down stairs; (3) elevators can provide more rapid descent from refuge floors or a sky lobby, allowing occupants to exit to the refuge floor, then to choose to use the elevator from that floor or continue down the stairs. In order to support use for evacuation, elevators must be protected and must be provided with appropriate fire and life safety features, signage and way-finding provisions, and instructions and support for situational awareness. Also, an appropriate and effective evacuation plan is required. In addition to various protection strategies aimed at hardening the elevator shaft, protecting it from the infiltration of suppression water, and resisting the effects of structural deformations that may occur during a fire event, systems for protecting evacuation elevators from the effects of smoke must be considered. Elevator shaft pressurization, along with protected elevator lobbies, can accomplish this goal.
* Event-Specific Evacuation. Event-specific evacuation strategies use variable actions based upon pre-planned strategies. Emergency situations need to be assessed, and the expected conditions will dictate the specific actions and strategies required. For example, consider a fire event on a single floor of a tall building. In this case, relocating and defending-in-place might be appropriate. For the same building, a severe weather event may necessitate full building evacuation using elevators and stairs. Depending on the identified events and response strategies, a wide range of fire and life safety systems or strategies may be required; many of these strategies require smoke management systems.
INTERNATIONAL BUILDING CODE
The International Building Code (IBC) has been widely adopted throughout many jurisdictions in the United States and the world. The IBC has included requirements relating to the pressurization of stairs in high-rise buildings since the first edition. The requirements related to tall buildings have evolved to include requirements for protecting elevators and for removing smoke after an incident (ICC 2009).
Traditional High-Rise Requirements
The IBC has required exit stairways in high-rise buildings to be protected as smokeproof enclosures or pressurized stairways since its inception. These requirements were derived directly from the legacy model codes. Smokeproof enclosures are composed of an enclosed interior exit stairway in conjunction with either open exterior balconies or ventilated vestibules. The smokeproof enclosure and associated vestibules must consist of 2-hour fire resistant construction and the stairway must be positively pressurized to a minimum 0.10 inches of water (25 Pa) relative to the vestibule with all stairwell doors closed. The vestibule is required to be ventilated such that it is supplied with at least one air change per minute; the exhaust capacity is required to be at least 150 percent of supply.
As an alternative to smokeproof enclosures, pressurized stairways can be used in sprinkler protected buildings without incorporating the ventilated vestibule. Such stairways need to be pressurized to a minimum of 0.10 inches of water (25 Pa) to a maximum of 0.35 inches of water (87.5 Pa) with all stairway doors closed. The design must also consider maximum stack effect conditions. Prior to the 2009 edition of the IBC, a minimum pressure differential of 0.15 inches of water (37.5 Pa) was required.
In tall buildings, the stack effect and wind are more prominent and systems need to be designed to compensate for expected conditions. Designing stair pressurization systems to balance stack effects can require detailed assessment and unique solutions such as dividing stairs into several compartments throughout the building height. In these systems, there is a greater dependence upon multiple injection strategies and a greater likelihood of system failure when stairwell doors are open. There is also a greater potential to over-pressurize stairwells and elevators due to increased fan capacities, among other issues. Smokeproof enclosures can provide a more robust solution to prevent pressure drops due to the effect of open doors. Because the ventilated vestibules are also designed to have a net exhaust, this will result in smaller fan capacities within the stairwells.
In addition to protected stairs, elevators that serve more than three stories are required to be equipped with enclosed elevator lobbies on all levels served by the elevator except the level(s) of exit discharge. The lobbies need to be protected with fire partitions and rated lobby doors. Smoke partitions can be used for the elevator lobbies where the building is equipped with an automatic sprinkler system. The code also allows for elevator hoistways to be pressurized in lieu of providing enclosed elevator lobbies. The hoistway is then required to be pressurized to a minimum of 0.10 inches of water (25 Pa) and a maximum of 0.25 inches of water (62.5 Pa) in relation to adjacent occupied spaces on all floors.
Historically, the model codes required zoned pressurization systems for high-rise buildings. This includes a 'sandwich' approach - pressurizing the floors above and below the fire floor and exhausting the fire floor. Alternatively, the system can be limited to exhausting the fire floor. This approach is required by the California Building Code, but not by the IBC.
The 2009 edition of the IBC allows for passenger elevators to be used for evacuation purposes in high-rise buildings. Within buildings over 420 ft (128 m), an additional egress stair is required. If two stairs are required by the egress provisions of the code, three would be required in buildings over 420 ft (128 m) tall. Evacuation elevators are specially designed elevators that allow continued use during emergencies which are allowed to substitute for the additional stair requirement. These additional stairs or specially designed elevators can speed evacuation of tall buildings. In order to utilize elevators for evacuation, certain requirements must be met in accordance with the IBC. These requirements are summarized in Table 1.
Reports indicate that 16% of occupants of Tower Two of the World Trade Center escaped through the elevators before the second plane struck the building (Averill 2002).
Table 1: IBC Evacuation Elevators Requirements (ICC 2009)
Evacuation plan detailing evacuation elevator system usage
High-hazard areas prohibited in facilities with evacuation elevators
Emergency voice alarm and communication system throughout the facility
Audible and visible notification appliances in each elevator lobby
An automatic sprinkler system is required throughout the facility
Monitoring of automatic sprinkler control valves and waterflow-initiating devices
Protection for hoistway and equipment from automatic sprinkler system water
Fire-rated and smoke-protected hoistway enclosure and elevator lobby
Direct access to an exit enclosure (stairway )from lobby
Lobby doors vision panel (allow visual observation before entering/exiting lobby)
Lobby sized to accommodate 25 percent of the floor (or area) occupant load
Lobby capacity is required to be based on 3.0 [ft.sup.2] (0.28 [m.sup.2]) per person
Lobby is required to provide one wheel chair space per 50 occupants served
Elevator lobby doors are required to be automatic-closing
Elevator evacuation system status indicator in lobby
Signage to identify self-evacuation elevators
Two-way communication system between elevator lobby and fire command center
Two-way communication system instructions in elevator lobby
Elevator system monitoring to indicate car floor location and direction of travel
Elevator car occupancy status indicator
Phase I Emergency Recall remote control located in fire command center
Stand-by electrical power for elevator equipment, and ventilating and cooling equipment
Status indicator of normal and standby power in fire command center
One-hour protection for power and control wires and cables
When using evacuation elevators, designers need to determine how these systems will be operated during an emergency. Designers need to consider if the elevators will serve occupants on the affected floor, or if the elevators will be used to shuttle occupants from a refuge floor. IBC evacuation elevator requirements provide a higher level of protection to allow the elevators to serve the affected floors. These concepts are illustrated in Figure 1.
[FIGURE 1 OMITTED]
Fire Fighters Elevators
The 2009 edition of the IBC first introduced the requirement for fire service access elevators. A minimum of one fire service access elevator is required to be provided in buildings where an occupied floor level is located more than 120 ft (36.6 m) above the lowest level of fire department vehicle access. The intent of these elevators is to shuttle fire fighters and emergency equipment as quickly as possible to the fire floor. Some fire tactics dictate shuttling to the floor below, or several floors below, the fire floor. Requirements for fire fighter elevators are outlined in Table 2.
Table 2: Fire Fighter Elevator Requirements (ICC 2009)
Every floor required to be accessed by the fire fighter elevator
Fire-rated and smoke-protected hoistway enclosure and elevator lobby
The lobby shall have direct access to an exit enclosure
Lobby doors shall be a minimum 3/4-hour fire rated and shall be smoke tight
Each lobby shall be a minimum 150 [ft.sup.2] (13.9 [m.sup.2]) with a minimum dimension of 8.0 ft (2.4 m)
A class I standpipe hose connection shall be provided in the associated exit enclosure
Elevator shall be continuously monitored at the fire command center
Normal and Level 1 standby power shall be provided for elevator equipment, hoistway lighting, machine room ventilation/cooling and elevator controller cooling equipment
One-hour protection for power and control wires and cables
Smoke Overhaul/Removal Systems
The 2009 edition of the IBC also includes requirements for a smoke removal system within high-rise buildings for post-fire salvage and overhaul operations. This system can be comprised of either natural or mechanical ventilation. Manually operable windows or panels can be provided around the perimeter of each floor at intervals not more than 50 ft (15.2 m) apart. These windows must provide at least 40 ft (3.7 [m.sup.2]) per 50 linear feet (15.2 m) of perimeter. Exceptions to this requirement allow for group R-1 occupancies to be provided with 2.0 [ft.sup.2] (0.2 [m.sup.2]) of venting area on the exterior wall of every sleeping unit. Windows that are easily breakable by the fire department are also permitted. A mechanical exhaust system providing four air changes per hour exhausting directly to the exterior is also permitted. The code also allows for other approved means to be considered.
Refuge Floors and Areas
To address the difficulties experienced by some occupants when negotiating stairs, tall buildings can incorporate the use of refuge floors or areas to allow occupants an opportunity to rest during evacuation. These are typically required within China, Hong Kong and other Pacific Rim countries. Refuge areas need to be protected from the effects of fire and smoke. Since refuge areas are essentially an extension of the stairwell, they should be provided with protective measures similar to those required for stairs. This can be accomplished through exterior openings or pressurization systems.
When refuge floors or areas are provided, stairwells are often offset at these locations to help prevent vertical smoke spread through the stair. Occupants evacuating from above discharge into the refuge area and must re-enter a stair at a different location to continue down. This provides an alternative route if a stair becomes unusable during an emergency. Figure 2 illustrates a possible configuration for a pressurized stairwell and refuge area. Refuge floors or areas can occupy significant space. Since refuge floors or areas need to dedicated spaces used for no other purposes, this approach imposes a cost premium. The IBC provides more cost-effective options, such as allowing floor re-entry or protected stair vestibules that serve as areas of refuge.
[FIGURE 2 OMITTED]
Pressurization systems are provided to contain smoke to areas or to keep smoke out of areas. The IBC requires a minimum pressure difference of 0.05 inches of water (12.5 Pa) across smoke barriers to prevent the spread of smoke in sprinkler protected buildings. For stairwell pressurization systems, the IBC requires a minimum pressure difference of 0.10 inches of water (25 Pa).
In many Middle Eastern jurisdictions, the requirements are more onerous. In such jurisdictions, a minimum pressure differential of 0.20 inches of water (50 Pa) is required across all pressurized stairs and fire fighter lifts for high rise buildings. In addition, some jurisdictions require what is informally classified as "cascading pressurization systems." The intent of this strategy is to protect the primary means of egress and fire department access with the highest level of pressurization (0.20 inches of water or 50 Pa) relative to the adjacent rooms and spaces. Through natural leakage paths or additional mechanical ventilation, the elevator lobbies or corridors are required to be pressurized to a lower pressurization (typically 0.10 inches of water or 25 Pa) measured relative to the adjacent rooms or spaces including the tenant areas. Figure 3 illustrates this concept.
[FIGURE 3 OMITTED]
Stairwell pressurization system design typically performed in the United States usually considers all stairwell doors to be closed as required by the IBC. NFPA 92A, Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences (NFPA 2009) requires one door to be considered open when designing stairwell pressurization systems. In some Middle East jurisdictions, it is not unusual for the authority to require a minimum of three doors open within the analysis.
The inclusion of three open doors in the analysis results in added design challenges and significant increases to fan capacities. In addition, the added fan capacities increase the likelihood of door opening force limitations being exceeded, which can prevent occupants from entering the stairwells. To counteract these negative results, barometric dampers or pressure sensor systems can be used. Barometric dampers are typically not practical in tall buildings. Pressure sensors can be used to determine when a specified pressure threshold has been exceeded. The sensors can then ramp down variable speed fans or shut down single speed fans. Although the primary engineering and analysis approaches for these systems are similar to those used to develop stairwell pressurization systems in the United States, the additional requirements enforced by the Middle Eastern authorities can result in more complicated but often more innovative solutions.
The 2009 edition of the International Building Code now identifies tall buildings as warranting special attention for fire and life safety professionals. International practices continue to evolve and some are becoming more relevant to tall building designers in the United States. From these sources, a range of life safety strategies is available to deal with the unique challenges posed by tall buildings. Smoke management systems continue to be a key component in the protection of these facilities. As with other building and structural systems for tall buildings, engineers must seek innovative solutions to develop safe and effective protection strategies. As very tall buildings become increasingly common, innovative life safety solutions will continue to evolve; some of these solutions are now reflected in the 2009 IBC.
Averill, J. (2005) "Occupant Behavior, Egress, and Emergency Communications." Federal Building and Fire Safety Investigation of the World Trade Center Disaster, National Institute of Standards and Technology, Gaithersburg, MD.
CTBUH (2004). Emergency Evacuation Elevator Systems Guide. Council on Tall Buildings in Urban Habitat, Chicago, IL.
ICC (2009). International Building Code, 2009 Edition, International Code Council, Country Club Hills, IL.
Klote, J. and Milke, J. (2002). Principles of Smoke Management, American Society of Heating Refrigeration and Air-Conditioning, Atlanta, GA.
Klote, J., Deal, S., Donoghue, E., Leven, B., Groner, N. (1993). "Fire Evacuation by Elevators," Elevator World 41.
NFPA (2009). "Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences," National Fire Protection Association, Quincy, MA.
Tubbs, J. and Meacham, B. (2007). Egress Design Solutions: A Guide to Evacuation and Crowd Management Planning, John Wiley and Sons, Hoboken, NJ.
Jeffrey Tubbs, PE, FSFPE
Matthew Johann, PE
Jeff Tubbs is an Associate Principal with Arup where he leads the Boston Fire Engineering Group, and the Americas Fire and the Global Evacuation and Human Behavior skills networks. Jeff Chairs TC 5.6 and serves as the Handbook Chair for TC 5.9. Matt Johann is a Senior Engineer with Arup where he has led the development of unique and pragmatic smoke management concepts in the US and internationally for nearly nine years. Andrew Neviackas is a Fire Specialist with Arup where he has assisted international teams to develop smoke management approaches.
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|Author:||Tubbs, Jeffrey; Johann, Matthew; Neviackas, Andrew|
|Date:||Jan 1, 2011|
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