http://www.nytimes.com/2001/10/02/science/scarred-steel-holds-clues-and-remedies.html?pagewanted=all
By KENNETH CHANG
Published: October 2, 2001

Two Wednesdays ago, on his first night in the city to collect scientific data on the collapsed World Trade Center buildings, Dr. Abolhassan Astaneh-Asl looked out the window of his room at the Tribeca Grand Hotel and saw a flatbed truck parked outside.

By chance, trucks hauling steel from the trade center site paused there for an hour or two before proceeding to the docks, where the steel was loaded onto barges.

Dr. Astaneh-Asl, a professor of structural engineering at the University of California at Berkeley, changed out of his nightclothes and went downstairs for a closer look. Over the next few nights, he cataloged 30 to 40 of the mighty beams and columns as trucks stopped in front of the hotel.

''I've found quite a number of interesting items,'' he said.

Dr. Astaneh-Asl hopes to conduct what is, in essence, an autopsy of the buildings felled by the terrorist attacks, to understand precisely how they fell apart.

Structural engineers regularly examine the remains of buildings, bridges and other structures damaged because of faulty construction, earthquakes -- or in rare cases, terrorism. Just as investigations of plane crashes lead to the design of safer airplanes, a look at the twisted steel will reveal the buildings' weak points and point to ways to fix them.

''It is important for engineers to be able to make the observations now that will lead to reduced building vulnerabilities in the future,'' said Dr. Priscilla P. Nelson, director of the division of civil and mechanical systems at the National Science Foundation.

Dr. Astaneh-Asl's project is one of eight financed by the National Science Foundation to study the World Trade Center disaster. He is also a member of a team assembled by the American Society of Civil Engineers to investigate the trade center site, and the society is dispatching a team to examine damage to the Pentagon.

One piece Dr. Astaneh-Asl saw was a charred horizontal I-beam from 7 World Trade Center, a 47-story skyscraper that collapsed from fire eight hours after the attacks. The beam, so named because its cross-section looks like a capital I, had clearly endured searing temperatures. Parts of the flat top of the I, once five-eighths of an inch thick, had vaporized.

Less clear was whether the beam had been charred after the collapse, as it lay in the pile of burning rubble, or whether it had been engulfed in the fire that led to the building's collapse, which would provide a more telling clue.

The answer lay in the beam's twisted shape. As weight pushed down, the center portion had buckled outward.

''This tells me it buckled while it was attached to the column,'' not as it fell, Dr. Astaneh-Asl said, adding, ''It had burned first, then buckled.''

Engineers believe they have a general understanding of why the World Trade Center towers fell. Fires stoked by the jetliners' fuel weakened the structural columns. When one floor collapsed, its weight collapsed the floor beneath it, starting a catastrophic series of failures.

But they do not know the details. How many of the interior steel columns were severed by the crashes of the jets? How hot did the fires burn? Did the jet fuel play a central role or would an ordinary fire have also led to collapse? Was the steel as strong as it was supposed to be?

By comparing the beam's specifications with architectural drawings, Dr. Astaneh-Asl said he would be able to tell roughly where the beam came from.

''I want to know which ones buckled and which ones did not,'' he said. ''That will lead you to the sequence of events. I can tell you exactly what happened there.''

For now, however, his impromptu inspections have ended. The trucks now take a different route, and there has been some concern whether there would be another chance before the steel is destroyed.

Dr. Astaneh-Asl and other engineers had assumed that the estimated 310,000 tons of steel columns and beams were being taken to Fresh Kills landfill in Staten Island with the rest of the debris, to be sifted by investigators.

But because the steel provides no clues to the criminal investigation, New York City started sending it to recyclers.

City officials, enmeshed with the more pressing priority of recovering bodies, did not realize that structural engineers would be keenly interested in the twisted metal. Because of crossed communications, a request by the civil engineers' society did not reach city officials until Friday.

The city's Department of Design and Construction has now approved the request, and James Rossberg, director of the civil engineering society's Structural Engineering Institute, said he hoped the team would be able to visit the site this week.

Robert Kelman, senior vice president and general manager of Hugo Neu Schnitzer East of Jersey City, one of the two companies that are recycling the steel, said the researchers were welcome to examine the several thousand tons of steel his company had received. ''The big beams that have obvious fire damage, we're putting aside for now,'' he said.

After collecting data, members of the research team plan to create a computer model that recreates the impacts, fires and collapses. They will then vary parameters in the computer model, like better fireproofing or thicker columns, that might have extended how long the towers remained standing.

''We anticipate we can come up with recommendations that might be done differently in the future,'' said the team's leader, Dr. W. Gene Corley, senior vice president of Construction Technologies Laboratories in Skokie, Ill.

Dr. Corley also led the society's investigation of the bombing of the Alfred P. Murrah Federal Building in Oklahoma City in 1995. There, for example, the upward force of the explosion could be seen in how it bent and cracked the topside of the concrete floor slabs. That investigation concluded that a construction design used to withstand earthquakes would have helped the building survive the blast, reducing the collapse as much as 85 percent.

At the Pentagon, the society's investigation, led by Dr. Paul F. Mlakar, an engineer with the Army Corps of Engineers, is interested in studying blast-resistant walls, added during a recent renovation, to see how well they fared. The jetliner hit at a spot roughly between a renovated section and one that had not yet been renovated.

The crash did not touch off a large explosion -- few of the concrete floors were cracked in the way the Murrah building floors were. ''We saw just a little bit of that in a couple of places, but not widespread,'' said Dr. Mlakar, who also worked on the Oklahoma City investigation.

The wreckage of the World Trade Center could also provide unique insight to what happens to building materials like concrete, steel and fireproofing insulation under extreme conditions that are not tested in the laboratory. Dr. Astaneh-Asl said that in some places, the fireproofing melted into a glassy residue.

Dr. Astaneh-Asl also hopes to send samples of steel to Dr. Claudia P. Ostertag, a professor of civil engineering at Berkeley, for metallurgical testing. Above about 1,600 degrees Fahrenheit, the crystal structure of steel shifts to a softer, weaker form.

It is not possible to see this change of crystal structure directly, but with an electron microscope it is possible to deduce how much a piece of steel has been heated for how long.

Molten steel does not solidify all at once, but as many little specks, or grains, that grow until they mesh together, each less than a hundredth of an inch wide. When the steel is heated again, the tiny grain structures merge. Thus, comparing the grain sizes in the charred steel beams with pristine samples, Dr. Ostertag believes she will be able to tell how much the fires heated the steel.

Both Dr. Corley and Dr. Astaneh-Asl said their data gathering efforts would not delay the recycling efforts by much, although they would like to set aside pieces that were near the impacts and fires.

The steel scrap is worth only a few million dollars, a tiny fraction of the billions of dollars the cleanup will cost, Dr. Astaneh-Asl said. The knowledge that can be gained from it could save lives in a future disaster.

''For the sake of those 6,000 people,'' he said, ''we should learn something about it.''

Photo: Charred I-beams can help tell how buildings fell on Sept. 11. (Dr. Abolhassan Astaneh-Asl)(pg. F2)