Consulting-Specifying Engineer - January 2008 - (Page 33) to the descending side in the direction of occupants descending the stairs to facilitate traffic flow. Proximity egress signs and marking systems near the stairwell doors were placed less than 2 ft. above the floor to maximize visibility. The structural and life-safety features of 7 WTC go beyond current New York City building code requirements, parallel to many of the recommendations made by NIST, and in the design phase, anticipated many of the study’s recommendations. But “robust, hardened cores aren’t cheap,” said Carl Galioto, FAIA, technical architecture partner, SOM. “Wider stairs alone decrease the amount of leasable building space by 1% to 2% at 7 WTC. Hardened cores are not appropriate for every building,” he said. In short, most high-rise owners in the United States will not go to this same expense. “Teams need to assess the safety risk factors, such as location, building func- tion, and height, and then apply these strategies with discretion,” Galioto said. He suggested that jurisdictions encourage developers to include hardened cores and safety areas by deducting these spaces from the zoning areas so they can build more leasable area. The fire and life safety designs for WTC Tower 1, or Freedom Tower, the centerpiece of the new WTC complex under construction in lower Manhattan, take it even further than 7 WTC in moving building occupants to safety and providing first responders with access. New elevator models “The emergency access core [in 1 WTC, or the Freedom Tower] contains the service elevators of the building, with one of the service elevators as a swing car,” Galioto said. “The swing car, under normal operation, could be used as a service car but has a second entrance within the car that is used by firefighters only. The Above: Event space, 52nd floor of 7 WTC. Photo: Joe Woolhead, courtesy Silverstein Properties. Below: Exterior view of 7 WTC. Photo: Skidmore, Owings & Merrill LLP well set many trends for the future. These are the most advanced life safety design schemes to date, and are likely the safest buildings in North America. For example, in 7 WTC, a structure that replaced one of the buildings destroyed on Sept. 11, Skidmore, Owings & Merrill LLP (SOM), New York, included a core encased in reinforced cast-in-place concrete shear walls. The core has two 72-in. stairwells, a fire- and smoke-resistant refuge area on each floor, two-hour fire-rated doors, pressurized stair shafts, extra space at each stair landing for rescue assistance, and lighting systems with three power sources (normal, emergency, battery backup). Phosphorescent paint strips were applied throughout the core to improve visibility. Also, the stairwell doors are situated so that they open Consulting-Specifying Engineer • JANUARY 2008 33
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