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JERSEY CITY, N.J. (AP) — A structural engineer examining the twisted bones of the World Trade Center said Friday he has tentatively concluded the towers collapsed because of intense fires fanned by jet fuel. 

The interior steel remained supportive after the crash, only buckling when the fire exceeded 1,000 degrees, Abolhassan Astaneh-Asl said. 

“The impact did nothing,” Abolhassan Astaneh-Asl said, pointing to a massive interior column from the south tower that he believes remained standing even after three-quarters of it was sliced away by a jet part. 

He expects that research will examine whether a tougher skin for future skyscrapers might be useful in deterring similar assaults. A steel-concrete composite exterior, for example, might crumple a plane and keep more fuel outside, he suggested. 

“This building could not fight. It was just innocently standing there and somebody shot it,” said Astaneh-Asl, a professor of structural engineering at the University of California, Berkeley. 

Pieces of one of the planes — the largest the size of a business envelope — were found imbedded in steel facade columns of one tower on the opposite side from where they entered, Astaneh-Asl said. 

“The airplane did not do much damage,” he said, showing bolts and fasteners that suggest to him the towers were well designed and well constructed. 

Once beams supporting the floors began to buckle in the fire, the floors pancaked and that brought the towers down, he said. 

Astanah-Asl, under a grant from the National Science Foundation, plans to build a computer model of the towers from data gleaned in the mounds of steel at ground zero and at a massive scrap yard in New Jersey. 

The computer model will examine if the buckling could have been prevented and whether additional fireproofing would withstand even a fully fueled airliner, he said. The steel had protection for about three hours of an office fire, he said. 

“All scenarios will be looked at,” Astaneh-Asl said. 

Workers at the Hugo Neu Schnitzer East yard in Jersey City, just a mile across the Hudson River from the Trade Center, saw the towers fall. 

They have now been trained to alert Astanah-Asl to possible clues as they labor around the clock with torches and heavy machinery, cutting into steel panels up to 6 inches thick. 

Eventually, nearly all of the estimated 310,000 tons of Trade Center steel will be diced and sent to mills, where it will be melted and forged again, Hugo Neu general manager Robert A. Kelman said. 

The south tower interior column with a chunk missing was probably hit by the jet’s nose or an engine, and was between floors 47 and 78, he said, adding that the location will eventually be pinpointed. 

The hollow rectangular column has four sides (16 and 32 inches wide), composed of steel 1.5 inches thick. Analysis of the sheared face would help determine how fast the jet was traveling at that time, Astaneh-Asl said. 

Astaneh-Asl, 53, led a team that studied damage to the San Francisco-Oakland Bay Bridge after the 1989 Bay Area earthquake, and has investigated methods to make buildings bomb resistant.