A structural engineer who was a member of the team assembled by the American Society of Civil Engineers to investigate the World Trade Center site after 9/11 has described numerous phenomena indicating extremely high temperatures suffered by the WTC structural steel. This appears to be further evidence that high-temperature explosives, such as thermate, were used to bring down the towers.
Dr. Abolhassan Astaneh-Asl is a professor of civil and environmental engineering at the University of California at Berkeley, who specializes in studying structural damage done by earthquakes and terrorist bombings. He flew to New York on September 19, 2001 to conduct a two-week reconnaissance of the collapsed towers, hoping to gain an understanding of how they'd come down. He was able to examine numerous pieces of steel taken from Ground Zero. 
He said the towers were exceptionally well designed and built, describing the WTC as "the best-designed building I have ever seen."  Yet the structural steel had suffered unusual warping and other major damage:
- Astaneh-Asl said that steel flanges "had been reduced from an inch thick to paper thin." 
- At a recycling center in New Jersey, he saw 10-ton steel beams from the towers that "looked like giant sticks of twisted licorice."  He showed the San Francisco Chronicle a "banana-shaped, rust-colored piece of steel" that had somehow "twisted like toffee during the terrorist attack." 
- He noted the way steel from the WTC had bent at several connection points that had joined the floors to the vertical columns. He described the connections as being smoothly warped, saying, "If you remember the Salvador Dali paintings with the clocks that are kind of melted--it's kind of like that." He added, "That could only happen if you get steel yellow hot or white hot--perhaps around 2,000 degrees." 
- In an interview in 2007, Astaneh-Asl recalled, "I saw melting of girders in [the] World Trade Center." 
- He found a foot-long twisted shard of steel that was "like a piece of bread, but it was high-strength steel." He commented, "I haven't seen anything like this [before]." 
- He came across "severely scorched [steel] members from 40 or so floors below the points of impact [by the planes]." 
- The fireproofing that had been used to protect the WTC steel also showed evidence of extreme conditions. In some places it had "melted into a glassy residue." 
- Astaneh-Asl saw a charred I-beam from WTC Building 7--a 47-story skyscraper that collapsed late in the afternoon of 9/11, even though no plane hit it. "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." 
These observations indicate that the World Trade Center steel was subjected to very high temperatures. Yet, while postulating that the towers collapsed due to fire (and without the use of explosives), even Thomas Eagar--an engineering professor at the Massachusetts Institute of Technology--admitted, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel."  One must conclude that the phenomena observed by Astaneh-Asl are therefore highly suspicious.
It may well have been because it offered such revealing clues that the remaining structural steel from the World Trade Center was so rapidly destroyed, being shipped abroad as scrap to be melted down and recycled. CBS News described: "The [cleanup] operation--which began days after the collapse, okayed by then-New York Mayor Rudy Giuliani--goes on 24 hours a day, seven days a week. As a result, Astaneh has almost certainly missed seeing crucial pieces before they were cut up and sent overseas."  As the New York Times reported, the steel scrap was worth "only a few million dollars, a tiny fraction of the billions of dollars the cleanup" was going to cost. Yet the knowledge that could have been gained from it "could save lives in a future disaster."