Structural steel melts at about 1,500°C. Ordinary building and jet-fuel fires reach up to about 1,100°C, and NIST reported top air temperatures of about 1,000°C in the towers. NIST says plainly: “In no instance did NIST report that steel in the WTC towers melted due to the fires.” [1]
What matters is that bare steel at 1,000°C softens and keeps only about 10 percent of its strength at room temperature. Steel stripped of its fireproofing could reach the air temperature in the time the fires burned. The result was yielding and buckling, not melting. [1]
The jet fuel itself mostly burned off in fireballs within minutes. It spread the fires; office furniture, paper and fittings kept them going. [1]
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