Engineers who studied the World Trade Center after the September 11 attacks tell why the Twin Towers stood as long as they did, and why they eventually collapsed. For the most current information on New York's World Trade Center, be sure to visit our WTC Index.
New York's World Trade Center Twin Towers were designed to withstand fire and hurricane-force winds. Some engineers believed the Twin Towers could even survive impact from a Boeing 707. But no engineer or architect could have anticipated the terrorist attack that turned the Twin Towers to rubble, and experts often don't agree what steps might have been taken to make the buildings stronger. For the most recent findings and detailed technical information, be sure to visit the World Trade Center Coverage page at icivilengineer.
How did the Twin Towers fall?1. Impact from the Terrorist Planes
When Boeing jets piloted by terrorists struck the Twin Towers, some 10,000 gallons (38 kiloliters) of jet fuel fed an enormous fireball. But, the impact of the planes and the burst of flames did not make the Towers collapse right away. Like most buildings, the Twin Towers had redundant design. The term redundant design means that when one system fails, another carries the load. Each of the Twin Towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. When some columns were damaged, others could still support the building.
2. Heat from the Fires
3. Collapsing Floors
Why did the collapsed towers look so flat?Before the terrorist attack, the Twin Towers were 110 stories tall. Constructed of lightweight steel around a central core, they were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.
Could the World Trade Center have been made stronger?In a report produced by the Federal Emergency Management Agency (FEMA), the American Society of Civil Engineers (ASCE), and other organizations, experts concluded that no skyscraper could have withstood the impact of the terrorist airplanes. Further, the experts warned that it would not be "technically feasible" to design a building that could survive this type of terrorist attack. Instead, engineers and architects are suggesting that we focus our efforts on designing better warning and evacuation systems so that we can save more people inside the buildings.