Federal disaster investigators issued 31 recommendations to improve the safety of high-rises and emergency responses, but critics emerged.
Even veteran disaster investigators were stunned by the fall of the Twin Towers on September 11, 2001. The next thoughts of the researchers who probed the calamity, aside from outrage, were how and why it happened from an engineering perspective. Why did WTC 1 stand nearly twice as long as WTC 2 after the impact of the aircraft? How could World Trade Center Building 7, which a plane did not hit, stand for seven hours and then collapse? And could such building failures occur elsewhere? Answers to all these questions have not only solved mysteries but also have led to changes in international building codes that may help prevent future tragedies.