ASHRAE LV-11-C058-2011 Smoke Control for Tall Buildings - An Integrated Approach to Life Safety.pdf

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1、 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 ha

2、s 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. Smoke Control for Tall Buildings An Integrate

3、d Approach to Life Safety Jeffrey Tubbs, PE, FSFPE Matthew Johann, PE Andrew Neviackas ASHRAE Member Abstract Smoke management systems have an important role in life safety programs for tall buildings. The 2009 Edition of the International Building Code (IBC) includes a comprehensive set of changes

4、that affect the design of tall buildings. Examples include requirements for fire fighter elevators, post-fire smoke purging, and the possibility of including evacuation elevators. As buildings increase in size, designers are turning to international practices to supplement the IBC. These new concept

5、s can significantly affect HVAC and emergency ventilation design. To be successful, emergency ventilation concepts need to be integrated into and work in concert with a coordinated life safety program. This paper presents these new requirements and concepts for developing smoke management systems th

6、at integrate into holistic life safety approaches for tall buildings. INTRODUCTION Tall buildings pose unique challenges for all aspects of design including smoke management systems. Many of todays tall buildings are being designed with more than 100 stories, some reaching 2,000 ft (610 m) or more i

7、nto 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 respond

8、ers 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 comprehensiv

9、e 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

10、 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

11、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 a

12、ccommodate 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 inher

13、ent imbalance will cause significant delays and crowding in stairs during simultaneous full evacuations. Providing a safe and LV-11-C058470 ASHRAE Transactions2011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Vol

14、ume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.smoke-free environment within these stairwells is therefore essential requirement for high-rise buildings. The code

15、 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. Tr

16、aditionally, 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 ta

17、sks. Special protection is needed for such elevators as discussed in greater detail in subsequent sections of this paper. (Tubbs and Meacham 2007) ENVIRONMENTAL CONSIDERATIONS In tall buildings, designing to account for stack and wind effects can prove to be challenging. The stack effect and the rev

18、erse 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

19、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, an

20、d 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 ca

21、n 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 win

22、dward 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 fo

23、r this phenomenon (Klote and Milke 2002, CTBUH 1992). INTEGRATED LIFE SAFETY STRATEGIES Effective smoke management designs integrate with and support the buildings fire and life safety strategy. Compartmentation, fire suppression, fire detection, evacuation, occupant notification, smoke management,

24、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 d

25、etection, 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 developi

26、ng 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 occupa

27、nts 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

28、 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 Internat

29、ional 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 2011 ASHRAE 471are designed

30、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 up

31、on 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

32、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

33、 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 o

34、r 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

35、possibly elevators. Current codes generally do not require the buildings 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 floor

36、s 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

37、 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 t

38、o 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 o

39、ccupants 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 instr

40、uctions 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 deforma

41、tions 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 vari

42、able 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 mi

43、ght 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 requ

44、ire smoke management systems. 472 ASHRAE TransactionsINTERNATIONAL 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 bui

45、ldings 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

46、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 smoke

47、proof 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

48、 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

49、 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