The Fall of Skyscrapers - Pushing the Limits: New Adventures in Engineering - page 109

The Fall of Skyscrapers
The terrorist attacks of September 11, 2001, did more than bring down the World Trade Center
towers. The collapse of those New York City megastructures, once the two tallest buildings in the
world, signaled the beginning of a new era in the planning, design, construction, and use of sky-
scrapers everywhere. For the foreseeable future, at least in the western world, supertall buildings
would be looked upon as potential terrorist targets, and the continued occupancy of signature sky-
scrapers by their prestige-seeking tenants would face increased scrutiny.
Since two separate hijacked airplanes loaded with jet fuel were crashed within about fifteen
minutes of each other into the two most prominent and symbolic structures of lower Manhattan, the
once reassuringly low numbers generated by probabilistic risk assessment seem irrelevant. What
happened in New York ceased being a hypothetical, incredible, or ignorable scenario. From then
on, structural engineers have had to be prepared to answer harder questions about how skyscrapers
will stand up to the impact of jumbo jets and, perhaps more important, how they will fare in the
ensuing conflagration. Architects have had to respond more to questions about stairwells and evac-
uation routes than to ones about facades and spires. Because of the nature of skyscrapers, neither
engineers nor architects are likely to be able to find answers that will satisfy everyone.
Although the idea of a skyscraper is modern, the inclination to build upward is not. The Pyramids,
with their broad bases, reached heights unapproached for the next four millennia. But even the great
Gothic cathedrals, crafted of bulky stone into an aesthetic of lightness and slenderness, are dwarfed
by the steel and reinforced-concrete structures of the twentieth century. It was modern building ma-
terials that made the true skyscraper structurally possible, but it was the mechanical device of the
elevator that made the skyscraper truly practical. Ironically, it is also the elevator that has had so
much to do with limiting the height of most tall buildings to about seventy or eighty stories. Above
that, elevator shafts occupy more than 25 percent of the volume of a conventional tall building, and
so the economics of renting out reduced floor space argues against investing in greater height.