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Skyscrapers Then and Now
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Recommendations After September 11
Most experts agree that it is not practical to design skyscrapers to completely withstand devastating terrorist attacks, including being rammed by a fuel-laden jet airplane. However, the September 11 attack raised many questions about skyscraper safety. To address these concerns, experts in professional organizations such as the Council on Buildings and Urban Habitat and the Institution of Structural Engineers, have issued recommendations to improve the performance of skyscrapers during emergencies. The main suggestions include:
- Incorporate alternative load paths so that weight will automatically shift to another load-bearing element if one component is taken out. The World Trade Center towers remained standing immediately after the planes hit because weight no longer borne by damaged columns was transferred to the giant truss on the roof. The subsequent fires eventually weakened the steel in remaining columns and the truss, but the delay in the collapse gave many people -- an estimated 20,000 -- time to escape.
- Set evacuation strategies that can accommodate many people simultaneously and protect them from smoke and fire. These innovations in planning, while for the most part low-tech, can greatly improve safety compared with buildings of earlier design. This includes allowing inhabitants to escape by both stairs and elevators that have adequate space. The day after the September 11 attack, the Petronas Towers received a bomb threat, forcing an evacuation of the buildings. The evacuation took longer than expected, and experts at the Petronas Towers responded by designing and implementing a new escape plan using the elevators that halved the evacuation time to 20 minutes. Those unable to walk down far enough to escape the fire can go to a smoke-proof waiting area for rescue by a service elevator. In the Taipei 101 Tower, each floor includes emergency escape corridors leading to fire-safe rooms, and there are outdoor balconies every eight floors allowing refuge. Asian buildings also have specially reinforced fire or "bomba" elevators that are dedicated exclusively to emergency crews and fire departments, allowing rapid access to fires and avoiding conflict with those trying to evacuate.
U.S. elevator codes don't prescribe firefighter lifts. Instead, safety codes require elevators to automatically go to the ground floor if smoke is detected in the building. (Arriving firefighters can reactivate the cars using special keys.) This may change soon, however; the National Institute of Standards and Technology has come to preliminary conclusions that elevator use in emergency situations offers advantages to both occupants and firefighters. More research must be conducted before policy officially changes, however.
- Have both passive and active fire protection features. The passive features are mainly fire-resistant materials that will not be compromised by damage to the building. (At the World Trade Center, fireproofing material that had been sprayed onto the steel beams was knocked off by the impact of the planes and was thus ineffective). The active features mainly consist of sprinkler systems that automatically turn on during fires but should not be counted on for extreme events.
- Integrate building systems to provide on-site and remote information to the appropriate authorities. This allows both those on the scene and those at remote locations to get a complete picture of what is going on inside the building, where fires or damage has occurred, and where people may be trapped.
Others Join The Race to the Top
Whether any of the buildings listed in this article are truly the "world's tallest" is a matter of some debate, since the manner of measuring their height varies according to who you ask. Whether or not to include building-top extensions like antennas, spires, or flagpoles in the final height tally is a disputed matter. Some feel the height should be measured from the tallest occupied floor. Others argue that observation and communication towers -- generally regarded as structures since they are not habitable -- should be considered for the honor, rendering Toronto's 1,815-foot CN Tower the tallest building in the world.
In any case, it appears the competition for the tallest building will continue as other nations join in. A considerable number of even taller buildings are at various stages of planning, although it is impossible to foresee which ones will make it past the drafting table. Tentative plans have been made for the construction of a building in the Arabian Gulf state of Dubai that aims to be 800 meters high -- easily smashing all current records. A new prospect may end up bringing the title back to New York, if only temporarily. The new Freedom Tower that will replace the World Trade Center is to be 1,776 feet, or 541 meters. At this height, it would probably be the world's tallest building upon completion in 2008 or 2009. For a short while, at least, the race for the tallest building will have come full circle.
Jim Stallard is a New York-based science and humor writer who has been
published in SCIENCE Online, MCSWEENEY'S, MODERN HUMORIST, SWEET FANCY
MOSES, and MIGHT magazine.
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A model of the 101-story Taipei 101 Tower in the Republic of Taiwan. |
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