A troubling and enduring story line among 9/11 truthers is that the Twin Towers were brought down, not by an act of Islamic terrorists, but by a deliberately planned act of the Federal government that was intended to incite the American populace to a) support a war for oil in the Mideast, b) permit themselves to be manipulated into complacency by fascist stormtroopers, or c) some other silly bullshit. One of the central tenants of this "argument" is that the fires started by the planes crashing into the Twin Towers could not have been hot enough to melt the structural steel that was used to make the frame of these buildings.
Years later, the estimable Charlie Sheen famously came out and said, "As I watched the Twin Towers come down, I remember saying, 'Call me crazy (Author's note: It's hard to imagine more prescient words ever being uttered), but did that look like a controlled demolition?'"
This silliness has persisted since 9/11 and survives primarily because anyone who knows anything about metallurgy knows that the argument is so moronic that even bothering to respond is likely to make one stupider. However, fools rush in...so, here are the facts:
Steel is an almost magical material; a mixture of iron and a tiny bit of carbon, which if heat-treated a particular way, makes a material that is harder than crap but very brittle and, if treated another way, makes something that is so soft that most of us could bend in their hands. For the most part, this process is reversible; you can take a hard, brittle piece of steel, re-heat-treat it and turn it into a bend-in-your-hands metal noodle. Note - because this is important - heat-treating steel is done at temperatures much, much lower than the melting point. I could launch into a long, involved discussion of how the strength of steel is largely determined by its microstructure (a combination of the size and shape of the iron crystals that make up the steel and how the carbon is distributed among these crystals) and that this microstructure changes radically with temperature, but I won't. For the purposes of this discussion, it is only important to know that steel loses strength as it is heated up.
Strength versus temperature data for structural steel is pretty easy to come by. Here, for instance. What is this graph telling us? Most importantly, it says that structural steel heated to 1200 deg F has only 20% of the strength it has at room temperature. Note that steel melts at 2795 deg F, so at less than half of its melting point, steel is already 5 times weaker than it is at room temperature.
Next question: Exactly how hot is 1200 deg F? As any Boy Scout who has spent time playing with campfires - and all Boy Scouts, of course, were primarily Boy Scouts so they could do just that - will tell you, a reasonably hot campfire will melt an aluminum can. Aluminum, as it turns out, melts at 1220 deg F. Consequently, a reasonably robust campfire will be hot enough to seriously weaken structural steel. As a historical aside, this fact was used to advantage by William Sherman's Federal army during its march through Georgia. Any Confederate railroads they came across were torn up, the wooden ties piled up and torched, and then the steel rails laid over the fire. Once the rails had heated up, men would pick up the ends of the rail and bend it around a nearby tree trunk.
Back to the Twin Towers, terrorists crashed planes into them with essentially full fuel tanks. For a 767 this is 23,980 gallons of Jet-A. Twenty-four thousand gallons of jet fuel burning is more than a reasonably hot campfire. In fact, simulations run by the NIST in their report on the WTC collapse put the maximum structural column temperature 60 minutes "after impact" at 1396 deg F. At this temperature, structural steel is down to ~10% of its room temperature strength...the Twin Towers collapsed due to temperature-induced creep and plastic failure, period. End of story. Sorry, truthers, go sell crazy somewhere else.
Of course, there's no need to take the word of an anonymous blogger on this. Take a look at the National Institute of Standards and Technology reports on the WTC collapse...it's all there, post-mortems on the recovered structural members, mechanical and thermal simulations, analysis, and enough other "techy" stuff to drive a much-needed wooden stake into the heart of the truther vampire.