NEW YORK - After Sept. 11 and the destruction of the World Trade Center, observers asked whether developers would have the stomach to take on such massive, high-rise projects again.
The last several years have shown that the will to take on the sky is still there and picking up speed - thanks to a strong real estate market worldwide, demand for housing and office space, and a healthy bit of ego.
The result is a tall-building boom in pockets of the United States and Britain, and especially in newer skyscraper frontiers: the quickly expanding desert emirate of Dubai and the newly rich cities of China, where developers are building attention-getting designs that are breaking height records.
Fourteen of the world's 50 highest buildings have been completed since the attacks, the majority of those in just the last two years, according to Emporis, a firm based in Hamburg, Germany, that tracks international building. About 40 percent of the top 200 have been completed since 2000.
"After Sept. 11, I kept asking: 'Does this mean we'll build shorter buildings?' and I guess the answer is 'No,' " said Bill Hudnut, a fellow at the Urban Land Institute in Washington, and former mayor of Indianapolis.
Experts say the booming real estate market is the major - but not the only - reason for the skyscraper surge.
"Tall buildings are a matter of ego. Tall buildings are a sign of success," said George Efstathiou, managing partner at Skidmore, Owings & Merrill, the architecture firm behind the new Freedom Tower in downtown Manhattan, and the Burj Dubai, soon to be the world's largest "ultratall" building.
Builders of the Burj Dubai said earlier this month that the tower had reached the height of America's tallest building, Chicago's Sears Tower, and still has a long way to go. Its ultimate height is a secret, but developers say it will be at least 2,300 feet, surpassing the current world's tallest building - the Taipei 101 at 1,671 feet - in July.
And at the end of last month, the developer of what would surpass the Sears Tower to become the tallest building in the United States - at 2,000 feet - got zoning approval for the Chicago Spire condominium tower.
The skyscraper construction boom is not based on hubris, said Carol Willis, founder and director of the Skyscraper Museum in New York.
The newer buildings are innovative designs - frequently slender, and curved, twisted and torqued. Where older buildings were frequently big, blocky office buildings, the newer ones are residential or mixed-use, have less space, and will be less trouble to rent out, Willis said.
People "assume the reason for building is that there is competition driving it," Willis said. "But financial institutions scrutinize every deal. They only do it if they think it will pay back."
Building in Europe and America is slower than the frenzy in Dubai and China, where seven of the world's top 10 buildings are situated.
Financing is tougher to come by than in the fast-growing economies of the Middle East and Asia, where builders are less likely to need to borrow and where there tend to be fewer government regulations. There is also less interest by America's traditional skyscraper cities of New York and Chicago to keep competing with the up-and-comers.
But U.S. cities are reviving their downtowns, demand is growing, and there is frequently nowhere to go but up.
"There's a lot of vibrancy in downtowns you wouldn't have expected a decade or two ago," as residents move back to revived city centers, said Kermit Baker, the chief economist for the American Institute of Architects.
In Philadelphia, construction continues on what will be the city's tallest building, the Comcast Center on John F. Kennedy Boulevard between 17th and 18th Streets. The 975-foot office tower, designed by Robert A.M. Stern Associates, will house the new corporate headquarters of cable-TV giant Comcast Corp.
Last year, Seattle city officials overturned a rule that had limited the city's vertical growth for 15 years.
In Boston, usually identified with its colonial 18th-century architecture, the mayor is backing an 80-story office building that would be 15 or 20 stories higher than the city's current highest building. And Chicago, Miami, Las Vegas and Denver are experiencing construction booms not seen in a decade.
"You build vertical because you get a better floor area ratio," Hudnut, the Urban Land Institute fellow, said. "You can cram a lot more people into a 100-story building than into a 10-story building."
But Willis, the Skyscraper Museum's founder, said what goes up will eventually slow down.
"If you look at any economic cycle of construction, back through the 20th century, buildings always appear in cycles," she said, "and the tallest usually appear before a crash."