|
Article Excerpt
INTRODUCTION
The invention of elevators in the mid 1800's, together with the need for more urban real estate, resulted in the construction of high-rise buildings in major cities across the United States and throughout the world. Tall buildings solve demanding urban space problems. Often, these buildings create unique, iconic structures, such as New York City's Empire State Building. At the same time, these structures pose particular egress challenges. Exiting via stairway systems can involve negotiating twenty, fifty, or over a hundred flights of stairs-a difficult task for many, and an impossible task for some. This difficulty is compounded by the number of occupants in these structures, the stack effect and fire fighting operations, as well as changing demographics, and risk perception and human behavior in a post September 11th environment. This paper presents design challenges, and suggests concepts and strategies for tall buildings.
TALL BUILDINGS VERSUS HIGH-RISE STRUCTURES
In the United States, high-rise facilities are typically defined as structures with occupied floors 75 ft or more above the lowest level of fire department vehicle access. This definition is based on the reach of 100 ft aerial fire apparatus, and accounts for typical setbacks (ICC 2006; NFPA 2006a). Figure 1 depicts the measurement of a high-rise building on a sloping site. While the site in this figure can be approached by the fire department from multiple directions, the definition requires that the height to the highest occupied floor be measured from the lowest level of fire department access. This definition is, for the most part, accepted throughout the United States, even though all jurisdictions do not have aerial fire apparatus, and many site configurations would limit the reach of such aerial fire apparatus. Some jurisdictions enforce stringent criteria - for example a height of 55 ft is used to define high-rise structures in many cities in Orange County California (Anaheim 2008).
[FIGURE 1 OMITTED]
Tall buildings are a subset of high-rise buildings. At present, there is no universally accepted definition of when a high-rise building is deemed a tall building. The building design and construction community has recently questioned whether tall buildings should have the same set of requirements as buildings minimally meeting the high-rise definition. Clearly, the risks and hazards associated with a ten story building are different than those for an eighty or one-hundred story building. This difference is even more evident for buildings rivaling the tallest in the world.
PRESCRIPTIVE CODE REQUIREMENTS
In general, the codes require the following generic features for high-rise buildings: a central command station (fire command center), fire pumps, automatic fire sprinklers, supervision for fire sprinkler system valves, a Class I stand-pipe system, fire detection, emergency communication, two-way fire-fighter's communication (firefighters' phones), emergency lighting, and emergency power. Emergency power is required to supply the fire alarms system, fire pump(s), central control station equipment, one elevator serving all floors, switched service for all elevators, mechanical equipment for smoke proof enclosures, and mechanical equipment for atria. Table 1 lists systems and features required for the central control station. This list is generic; the specific applicable code should be consulted, as different occupancies and jurisdictions have differing requirements (ICC 2006; NFPA, 2006a). High-rise buildings also require smoke-proof egress enclosures; both the International Building Code (IBC) (ICC 2006) and the Life Safety Code (NFPA 2006b) allow the following three options:
1. Pressurized stairs
2. Exterior open vestibules leading into the stairs
3. Vestibules for all openings into stairs with vestibule smoke purging
Table 1. Central Control Station Requirements in High-Rise Buildings (ICC 2006; NFPA 2006a)
Voice alarm system
Two-way fire department communication
Fire detection and alarm system
Elevator floor indication and operation indicators
Sprinkler valve and water-flow indicators
Emergency power status indicators
Controls for automatic stair door unlocking
Controls for smoke management systems
Fire pump status indicators
Telephone with controlled access to the public telephone system
The codes are evolving to respond to some of the concerns with tall buildings. For example, International Building Code, 2006 edition, eliminated the high-rise structural fire protection reduction in buildings over 420 ft tall. While not limited to high-rise buildings and tall structures, a change to the areas of refuge requirements will impact these facilities; areas of refuge are required within the 2006 edition of the International Building Code to serve all accessible stairways and elevators. In addition, the following changes are included within the International Building Code, 2007 Supplement (ICC 2007): (1) the bond strength of structural fire protection has been enhanced; (2) a firefighter elevator with enhanced features is required for buildings over 120 ft tall; (3) an additional stair is required for buildings over 420 ft tall; and (4) exit pathway marking is required for all high-rise buildings.
Several groups are also developing requirements for elevators when used for evacuation and for firefighter access of tall buildings. Evacuation elevators are further discussed in Table 2.
Table 2. Evacuation Strategies (Tubbs and Meacham 2008; Tubbs and Jacoby 2006) Relocation and The combination of relocation and defend-in-place Defend-in-Place is a traditional approach for high-rise buildings. Occupants on the fire floor, the two floors above it, and the two floors below it are evacuated, either to the exterior or, in very tall buildings, to another floor. Occupants on other floors use a defend-in-place strategy....
|
