AITHERSBURG, Md., Dec. 2 — Federal investigators said here Tuesday that new evidence supported earlier suggestions that the floor supports in the World Trade Center began failing in the minutes before the towers fell and might have played a major role in their collapse.
The investigators, who are carrying out a two-year, $16 million analysis of the collapses, made it clear that they had not yet settled on a final explanation. They said, though, that their findings gave new weight to a theory that the failure of the floors weakened the towers' internal structure to the point that the entire buildings came down.S. Shyam Sunder, who is leading the investigation for the National Institute of Standards and Technology in the Commerce Department, said, "We are seeing evidence of floors appearing to be sagging — or that had been damaged — prior to collapse." Still, Dr. Sunder said, "The relative role of the floors and the columns still remain to be determined in the collapse."
According to an alternative theory of the collapse, the planes that smashed into the towers damaged the towers' vertical structural columns so severely that the buildings were virtually certain to fall. In that view, none of the buildings' many structural novelties — the towers were daring engineering innovations in their day — would have played a significant role in the collapses.
Last spring, the standards institute found the first photographic evidence on the east face of the south tower that a single floor — with its lightweight support system, called a truss — had sagged in the minutes before it started collapsing. Now, detailed analysis of photos and videos has revealed at least three more sagging floors on that face, said William Pitts, a researcher at the institute's Building and Fire Research Laboratory.
In addition, Dr. Pitts said, sudden expansions of the fires across whole floors in each tower shortly before they fell suggested internal collapses — burning floors above suddenly giving way and spreading the blaze below.
Finally, an unexplained cascade of molten metal from the northeast corner of the south tower just before it collapsed might have started when a floor carrying pieces of one of the jetliners began to sag and fail. The metal was probably molten aluminum from the plane and could have come through the top of an 80th floor window as the floor above gave way, Dr. Pitts said.
"That's probably why it poured out — simply because it was dumped there," Dr. Pitts said. "The structural people really need to look at this carefully."
The investigators also said that newly disclosed Port Authority documents suggested that the towers were designed to withstand the kind of airplane strike that they suffered on Sept. 11.
Earlier statements by Port Authority officials and outside engineers involved in designing the buildings suggested that the designers considered an accidental crash only by slower aircraft, moving at less than 200 miles per hour. The newly disclosed documents, from the 1960's, show that the Port Authority considered aircraft moving at 600 m.p.h., slightly faster and therefore more destructive than the ones that did hit the towers, Dr. Sunder said.
The towers did withstand the plane strikes at first, allowing thousands of people to escape, but then the fires, stoked by burning jet fuel, softened the steel of the towers. Potentially challenging other statements by Port Authority engineers, Dr. Sunder said it was now uncertain whether the authority fully considered the fuel and its effects when it studied the towers' safety during the design phase.
"Whether the fuel was taken into account or not is an open question," Dr. Sunder said. It is also unclear, he said, "whether the extent of the loss of human life as a result of that" was taken into account.
The studies of the floor trusses and the design of the towers are just two elements of the investigation, which is carrying out computer calculations of the collapses, rebuilding pieces of the towers in order to test them in real fires, and piecing together a highly detailed chronology of the response to the attack.
In one set of laboratory tests concerning the floor trusses, researchers used earthquake simulators to violently shake assemblages much like the ceilings in the twin towers. The shaking was meant to simulate the impact of the aircraft.
The findings, said Richard Gann, a senior research scientist at the Building and Fire Research Laboratory, showed that many of the fire-protecting ceiling tiles near the impact probably crumbled, exposing the undersides of the trusses directly to the fires.