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.