When I was still an architecture student, a teacher told me, "We learn more from buildings that fall down than from buildings that stand up". What he meant was that construction is as much the result of experience as of theory. Although structural design follows established formulas, the actual performance of a building is complicated by the passage of time, the behavior of users, the natural elements—and unnatural s. All are difficult to simulate. Buildings, unlike cars, can"t be crash-tasted. The first important lesson of the World Trade Center collapse is that tall buildings can withstand the impact of a large jetliner. The twin towers were supported by 59 perimeter columns on each side. Although about 30 of these columns, extending from four to six floors, were destroyed in each building by the impact, initially both towers remained standing. Even so, the death toll(代价) was appalling—2,235 people lost their lives. I was once asked how tall buildings should be designed given what we"d learned from the World Trade Center collapse. My answer was, "Lower". The question of when a tall building becomes unsafe is easy to answer. Common aerial fire-fighting ladders in use today are 100 feet high and can reach to about the 10th floor, so fires in buildings up to 10 stories high can be fought from the exterior(外部). Fighting fires and evacuating occupants above that height depend on fire stairs. The taller the building, the longer it will take for firefighters to climb to the scene of the fire. So the answer to the safety question is "Lower than 10 stories". Then why don"t cities impose lower height limits A 60-story office building does not have six times as much rentable space as a 10-story building. However, all things being equal, such a building will produce four times more revenue and four times more in property taxes. So cutting building heights would mean cutting city budgets. The most important lesson of the World Trade Center collapse is not that we should stop building tall buildings but that we have misjudged their cost. We did the same thing when we underestimated the cost of hurtling along a highway in a steel box at 70 miles per hour. It took many years before seat belts, air bags, radial tires, and antilock brakes became commonplace. At first, cars simply were too slow to warrant concern. Later, manufacturers resisted these expensive devices, arguing that consumers would not pay for safety. Now we do willingly. Why are there still high buildings, or even skyscrs in many cities