On the morning of September 11, 2001, immediately after terrorists struck the Twin Towers, it looked as though the buildings might remain standing. While the plane crashes had taken huge chunks out of both towers, the overall structure seemed to be intact, at least to the observers on the ground and the millions of Americans watching the catastrophe on television. But within an hour, World Trade Center 2 had collapsed, followed by World Trade Center 1 only 40 minutes later.
In the weeks after the attacks, the Federal Emergency Management Agency (FEMA) and the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) assembled a team of scientists and engineers to investigate exactly how the buildings collapsed. Based on video, eyewitness accounts and debris analysis, the team formed a likely hypothesis of what happened, which they made public in April 2002. In August 2002, the National Institute of Standards and Technology (NIST), an agency in the U.S. Commerce Department, announced it would launch its own two-year, $16 million study of the collapse.
The following is a summary of the FEMA evaluation teams' findings, which were largely along the same lines as the numerous media evaluations in the weeks after the attacks. You can read the entire report at the FEMA Web site.
When the planes hit the two towers, the collisions damaged each building in two major ways:
- In each case, the force of the speeding plane knocked out a number of vertical columns around the building perimeter, damaged large sections of floor, sent furniture and plane wreckage flying through the offices and presumably damaged support columns in each building's core. Most likely, the initial impact also destroyed the sprinkler system on those floors. The evaluation team estimates that the first plane -- a 395,000-pound (180,000-kg) Boeing 767-200ER going about 470 miles per hour (756 kph) -- fractured as many as 36 perimeter support columns over a four-story area of WTC 1's north face. The connected floors partially collapsed, and the central core suffered undefined damage. The second plane, a Boeing 767-200ER flying at about 590 miles per hour (950 kph), inflicted similar damage on WTC 2. The collision fractured as many as 32 perimeter columns over a five-story area, collapsing sections of connected floor and damaging the central core.
- In each attack, the crash ignited the plane's' fuel supply, causing a massive fireball -- an expanding area of burning gas. While the ignited fuel didn't really explode, the fireball did spread fire down the side of the building, throughout the nearby floors and down interior shafts to lower floors. The investigators hypothesized that nearly all of the jet fuel was consumed in the initial fireball and first few minutes of the building fire, but it ignited enough office equipment, paper and building materials to keep the fire raging until the collapse.
Amazingly, the initial damage to the support structure was not enough to topple the building. The report, as well as a number of prominent engineers, have claimed that the majority of skyscrapers on the planet would have collapsed within seconds of such a collision. But the collisions did divert the entire vertical load of the buildings to the remaining columns, significantly increasing the structure's stress level.
Without any additional loads on the support structure, the report claims, the towers could have stayed up indefinitely. But the extreme heat of the fire, which might have been in excess of 2,000 degrees Fahrenheit (1,090 C) at some points, exerted tremendous stress on the perimeter columns, the core columns, and the floor trusses in between them.