Twin Towers Design Not At Fault

Twin Towers Design Not At Fault
After the most sophisticated building analysis in U.S. history, federal investigators have arrived at the clearest picture yet of the sequence of events that led to the collapse of the World Trade Center towers, largely ruling out a design flaw in the buildings as a central factor in the catastrophe.

Since the twin towers fell, questions have reverberated among families of victims and some fire-safety experts about whether insufficient fireproofing or an unusual weakness in the innovative, lightweight floors played a crucial role in the collapse.

Instead, the investigators tentatively conclude in nearly 500 pages of documents released Tuesday, the twin towers failed because the structural columns at the buildings' core, damaged by the impact of the airliners, buckled and shortened as the fires burned, gradually shifting more load to the tower's trademark exterior pin-striped columns. The exterior columns ultimately suffered such extraordinary stress and heat that they gave way.

The investigation -- based on an analysis of thousands of photographs and videos, an examination of nearly every element used to construct the towers and meticulous computer-enhanced modeling of the plane impacts and spreading fires -- is not yet complete. A final report by the National Institute of Standards and Technology is not scheduled to b