Kohn notes that the reasons for building lofty towers haven¿t changed: high land costs in congested cities, demanding economic needs (especially in fast-growing Asia) and the developers¿ egos. "A lot of great buildings get erected because somebody wants to make their mark on the skyline," he says. Kohn points to a pair of projects his firm has under way¿Union Square (Kowloon Station) in Hong Kong and the Shanghai World Financial Center, each of which will be more than 1,500 feet high (about 100 stories) when completed around 2007. Neither effort has been altered much since the September 11 assaults, he says, because of conservative building codes in China that make for strong structures.
New Ideas About Design
The attack did, however, lead engineers, architects and safety specialists to rethink high-rise design. Builders now favor more highly reinforced structures that "keep damaged buildings standing longer, so more people can escape," states Charles H. Thornton, chairman of the New York¿based firm Thornton-Tomasetti Engineers, which engineered Kuala Lumpur¿s Petronas Towers, the world¿s tallest at 1,483 feet. The focus is on halting chain reactions of failures set off by triggering events such as bombs, plane crashes or major fires.
Modern tall buildings are engineered so that the central core supports the weight or gravity load of the structure, whereas the surrounding exterior columns work like outriggers to keep the tower from overturning or sliding when exposed to hurricane-force winds or earthquakes. Meanwhile the floors tie the inner frame to the outer one, bracing the entire edifice.
In the case of the World Trade Center, which was a state-of-the-art design in the late 1960s, the steel-mesh exterior skeleton was highly robust, but the steel-truss floor framing turned out to be quite fragile, and the central core was not designed to handle significant lateral (sideways) loads, Thornton explains. When the planes hit the towers, they knocked out many internal and exterior support columns and dislodged much of the sprayed-on fire insulation that had protected the steel members. Although the remaining structure readily supported the new loads transferred to them when the columns were lost, it then had to contend with the insidious effects of the aviation-fuel fire that set all the flammable contents of the floors alight. "It was the intense fuel fire and the following inferno that led to the collapse," he says. The federally sponsored study of the disaster came to the same conclusion. (For an alternative view, see "Bending But Not Breaking.")
Thornton thinks that future mega-skyscraper designs are likely to make greater use of concrete. Reinforced with steel rods, it will be employed to make structural members. Concrete will also encase steel components, shoring them up and insulating them from fire. Strengthening the structure will raise construction costs, but not by much. "The reinforcement should add no more than 2 to 3 percent to the total job cost," the engineer says. And although concrete buildings tend to be markedly heavier and bulkier than steel ones, clever design can avoid the bunker look, according to architect Kohn.
More Safety Features
Architects plan to incorporate other safety features as well. Floors may be compartmentalized like naval vessels to stop the spread of frame failures and fire. Extremely strong load-transfer trusses inserted every 30 stories or so can isolate structural damage and avoid free-fall collapses. The progress of fires could be blocked by fireproof partitions and by ventilation systems that pressurize the floors both above and below the flames to contain smoke and heat, which would be vented out through exhaust shafts. Large water tanks at the tops of buildings could act as mass dampers to counteract any swaying from extreme lateral loads and as reservoirs for deluging fires.
A greater number of wider staircases better protected against the encroachment of fire and smoke are also likely. Designers will separate fire stairs so that the destruction of one does not mean the loss of the others. Independently ventilated and reinforced refuge floors or zones, where occupants could go to wait out a blaze, can be positioned every 15 stories or so. High-speed lifts for firefighters that can rise to the top of a building in a minute could be installed as well.
Despite these measures, however, experts emphasize that there must be a first line of defense in protecting skyscrapers¿namely, it must be to stop terrorist attacks from occurring at all.