New theory on WTC collapse exonerates construction technique

If a new report on the cause of the World Trade Center’s collapse holds up, history may not force the towers’ unusual construction method to shoulder some of the blame for the high death toll following the terrorist attacks.

Engineering experts commissioned in a $4 billion insurance case formed conclusions that contradict a federal investigation that blamed the failure of lightweight floor supports for the buildings’ fall.

The new analysis contends the heat from the fires that raged inside the towers weakened the buildings’ vertical steel columns, making them unable to support the upper sections of the structures even with all floors intact.

Weidlinger Associates, a Manhattan engineering firm, used a computer program called Flex, which it developed with the Pentagon to study the blast effects of bombs, to recreate the events of Sept. 11, 2001.

The program allowed engineers to approximate the number of columns instantly severed when the planes struck the towers. They then used photographs and videos to study the spread of fires inside the building.

Hat trusses atop the buildings connected their cores to exterior columns and, at first, allowed the towers to continue standing by transferring loads from damaged columns to undamaged ones, the report claims. But heat from the fires eventually weakened the columns.

The north tower stood the longest, Weidlinger engineers say, because it was hit dead center and the undamaged columns at the four corners of the building acted like the legs of a table. The south tower, on the other hand, was like a table with two weak legs on one side and therefore fell first.

Larry Silverstein, who held the lease on the World Trade Center, paid for the report, which is to be used in lawsuits filed against him and his landlord, the Port Authority of New York and New Jersey. Skeptics of the report’s theory question whether a study financed by Silverstein could be impartial.