1. Slow onset with large visible deformations

Kenny argued:

"In 40 minutes steel can reach 900C when exposed to 1000C fires. So the towers stood pretty long with hindsight.

Let's assume for a moment that the WTC steel could have been heated to 1000C in 40 minutes, which I doubt (x piece of steel in y condition is not the same thing as the WTC building). Steven Jones makes it clear that even NIST doesn't believe that the fires were at that temperature for more then 15 to 20 minutes:
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Both WTC 1 and WTC 2 were stable after the aircraft impact, standing for 102 min and 56 min, respectively. The global analyses with structural impact damage showed that both towers had considerable reserve capacity. This was confirmed by analysis of the post-impact vibration of WTC 2… where the damaged tower oscillated at a period nearly equal to the first mode period calculated for the undamaged structure. (NIST, 2005, p. 144; emphasis added.)
At any given location, the duration of [air, not steel] temperatures near 1,000C was about 15 min to 20 min. The rest of the time, the calculated temperatures were near 500oC or below.” (NIST, 2005, p. 127, emphasis added.)
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http://physics911.net/stevenjones

Kenny also argued:

"Visible deformations took place with the considerable bowing of the tower. Nothing else can explain this but extreme fire temperatures."

I've heard that the supposed bowing may have simply been refraction of light, but even assuming it wasn't, I know of no evidence that the office fires could have done it.