Consulting-Specifying Engineer - September 2008 - (Page 22)

Phased evacuation Typically high-rise structures and tall buildings, hospitals, large assembly facilities Critical features Combines critical features of evacuation or relocation with protect-in-place strategies, resilient automation sprinkler protection, fire-rated floor and shaft separations, protected exit stair shafts (pressurization and fire separation), and multi-channel floor-by-floor zoned notification to allow differing messages in different zones Figure 1 Phased evacuation strategies allow optimizing exit efficiency, as only those in immediate danger use the exits. Source: Arup nursing homes; and detention, correctional, and institutional facilities. These facilities typically employ horizontal exits or smoke barriers to allow evacuation from one fire compartment to another. The relocating strategy also can be used in tall buildings. In this scenario, occupants are relocated to lower floors. Thus, floor and shaft fire ratings are critical, as the lower floors create a safe area. In addition, the structural fire protection must match the expected hazards, as people will not be immediately escaping the building. Horizontal exits are convenient but often misunderstood exiting features. Horizontal exits allow occupants to move away from a fire or other incident to a protected area. Codes require a two-hour fire-resistive barrier to completely divide floors employing the horizontal exit concept into separate fire compartments. Upon crossing the boundary formed by these walls and door openings, occupants are considered to have exited the space, just as they would after crossing the threshold of an enclosed stairwell. Phased evacuation Phasing strategies combine evacuating or relocating a portion of the occupants—those who are in most danger— while allowing occupants remote from an incident to protect-in-place. This allows optimizing exit efficiency, as only those in immediate danger use the exits. A fundamental assumption is that an event will not affect occupants outside of the affected zone while occupants in the zone safely evacuate. Phased evacuation is appropriate in a wide range of facilities, including high-rise buildings, hospitals, and large assembly spaces. In hospitals, phasing may be necessary for larger incidents if such incidents might compromise adjacent evacuation zones. Large assembly spaces also may allow remote occupants to remain, while those closer to and intimate with the incident immediately escape. Assembly examples include large convention centers with multiple event halls; if the halls are appropriately separated, it may be possible to phase the evacuation using individual halls as evacuation zones. Phased evacuation is the traditional approach for high-rise buildings and is permitted by the International Building Code (IBC). Occupants on the event floor, and one or two floors above and below the event floor, are evacuated either to the exterior or to a lower floor. Occupants on other floors use a protect-in-place strategy. In theory, this allows occupants on the fire floor unobstructed use of the exit stairs, thus reducing the evacuation time for those on the affected floor or floors. This concept works well for traditional fire events, such as a sprinkler-controlled fire, because automatic suppression systems in high-rise buildings are designed with a degree of resilience and have proven to be effective in controlling or suppressing fires. Fire-rated floor separations, along with the systems in place, minimize the hazards and risks to occupants on the unaffected floors. When using the phased evacuation approach, it is important to notify occupants remote from the event floor as well as those in the evacuation floor zones. Occupants outside of the affected zone are notified of the incident and told to remain in place pending further instructions. Occupants not on the evacuation floors use the protect-in-place strategy. Thus, the features critical to protect-in-place strategies must be provided. Risk perception post-Sept. 11, 2001, adds complexity to phased evacuation. In the event of a disaster, occupants might ignore “standby” messages and instead immediately exit the building. People tend to trust live voice messages over recorded messages. However, building personnel who are allowed to provide live messages must be well trained so as not to create undue concern when giving the message. Designers may wish to consider these issues, as the events of concern, needs of the owner and occupants, and occupant characteristics differ from building to building. Simultaneous evacuation Simultaneous full building evacuation has been the norm for small buildings for many years. However, this strategy has emerged as an alternative solution for large, tall, or iconic facilities, particularly when designing for extreme events. In January 2008, Consulting-Specifying Engineer author Scott Siddens noted in “Rethinking high-rise egress, top to bottom” that one of the recommendations from the National Institute of Standards and Technology (NIST) Sept. 11 study called for tall buildings to be designed to accommodate timely full building evacuation. In most cases, buildings designed for extreme events may require immediate simultaneous evacuation of the entire building. 22 Consulting-Specifying Engineer • SEPTEMBER 2008

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Consulting-Specifying Engineer - September 2008 Contents Viewpoint Letters News M/E�Roundtable Selecting Appropriate Egress Strategies Commissioning On-Site Electrical Systems Mentoring Control Engineers and Technicians Casting Call for Cx Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - September 2008

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