How High Will They Build?

Faster-setting concrete mixes have erased the advantages of steel (a steel skeleton intrudes less into floor space than bulky concrete, and steel buildings can be erected more quickly, though they have to be fireproofed). Composite approaches, such as steel skeletons wrapped in concrete, are efficient, strong and fire-resistant.



If there is no structural limit, is there a safety limit? Typhoons, earthquakes, terrorist attacks -- and fires caused by any of those -- are real risks in Asia.



Mitigation of movement is a basic challenge in all tall-building design. Early high-rises were made stiff to resist wind, but the approach shifted long ago to buildings with give, incorporating tuned active and pendulum dampers that help absorb and counteract wind or seismic forces. The John Hancock Tower in Boston uses a big block of concrete floating in a bed of oil; computer-controlled hydraulics push it around to counter the building's sway. Sydney's Chifley Tower employs a pendulum, "a giant block of concrete hanging by wires," Klemencic says. "It's quite dramatic." Taipei 101 will feature the world's largest passive tuned mass damper, an 800-ton sphere 18 feet across that will swing like a pendulum from the 92nd floor in the view of restaurant-goers.



Fire may be a thornier problem than jolt and sway: It was fire that brought down the WTC, not the impact of the jetliners. Here KPF architect Eric Howeler argues that some Asian countries may lead the West in building codes and technological strategies. Basic construction is very different from that employed for the WTC in the 1960s. The New York towers were steel cages, with floors suspended from beams, and a post-collapse enquiry suggested that when fireproofing material on the steel blew off on impact, exposing beams directly to melting heat, it might have sped the collapse. A steel-concrete composite tower might be inherently safer.



Every 25 floors, Hong Kong buildings must have a refuge floor -- empty and designed to resist smoke accumulation. Many stairwells are pressurized.