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Why the Towers Fell
NOVA News Minutes
Deconstructing the Towers' Collapse

(running time 01:23)

May 3, 2002

To many who witnessed the horrific attacks on the World Trade Center last September, it might appear obvious why the Twin Towers fell: They were struck at blinding speed by two massive aircraft carrying thousands of gallons of jet fuel, whose ignition generated a fire so searing that it weakened steel beams to the point of collapse.

But the exact nature of the buildings' implosion remains extremely complex. That is why it has taken a government-appointed team of some of the leading structural engineers in the country nearly eight months to release their official report, which is due out May 1st. NOVA's "Why the Towers Fell" explores the complexities of the collapse and the in-depth investigation that has taken place since September 11th.

The key to understanding starts with the unique structure of the towers. When he designed them in 1966, Leslie Robertson set out to create skyscrapers with more rentable office space than the current engineering techniques typically allowed. Standard skyscrapers of the day had support columns that were spaced evenly throughout the width of the building. Robertson decided instead to keep the columns in a main inner core (housing the elevators, emergency stairs, and other building services), and move the rest of the support columns to the exterior walls. This difference allowed the interior to have more open space. The only way to make it work was to connect the inner core and the exterior columns with a series of steel trusses below every floor. And it is the performance of these trusses that has come into question.

Like all the steel in the building, the trusses were protected from fire using a fire-retardant foam. But according to the findings of some of the forensic engineering teams -- which included researchers supported by the American Society of Civil Engineers (ASCE), the Federal Emergency Management Agency, the National Science Foundation, and the National Institute of Standards and Technology (NIST) -- each plane's impact blew the foam away. The naked steel was weakened by exposure to the intense heat of burning jet fuel. This could have caused the floor trusses to sag and fall.

Likewise, there may have been a similar problem with fire-protection for the emergency stairwells and elevators in the inner core. Those areas were surrounded by drywall -- a kind of Sheetrock that is especially fire resistant. But the drywall was so lightweight that the impact of each plane blew it away, exposing the stairwells and inner core supports to the intense fires.

It is impressive that the World Trade Center towers held up as long as they did after being attacked at full speed by Boeing 767 jets, because they were only designed to withstand a crash from the largest plane at the time: the smaller, slower Boeing 707. And according to Robertson, the 707's fuel load was not even considered at the time. Engineers hope that answering the question of exactly why these towers collapsed will help engineers make even safer skyscrapers in the future. ASCE will file its final report soon, and NIST has been asked to conduct a much broader investigation into the buildings' collapse.

Brad Kloza is a writer and producer for ScienCentral, Inc.
© ScienCentral, Inc.

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