Creative Speculation

LOL. Unlike the unproven and rather laughable hypothesis that random office fires can cause the total collapse of modern steel skyscrapers, thermate is a very real thing. But thanks for the laugh!

Thermate composition is a thermite one enriched with a salt-based oxidizer (usually nitrates, e.g. barium nitrate, or peroxides). In contrast with thermites, thermates burn with evolution of flame and gases. The presence of the oxidizer makes the mixture easier to ignite and improves penetration of target by the burning composition, as the evolved gas is projecting the molten slag and providing mechanical agitation.[18] This mechanism makes thermate more suitable than thermite for incendiary purposes and for emergency destruction of sensitive equipment (e.g. cryptographic devices), as thermite's effect is more localized.

http://en.wikipedia.org/wiki/Thermite#Thermates

Thermate has military applications and is capable of cutting through tank armor.

Because thermate burns at higher temperatures than ordinary thermite,[1] it has useful military applications in cutting through tank armor or other hardened military vehicles or bunkers. As with thermite, thermate's ability to burn without an external supply of oxygen renders it useful for underwater incendiary devices.

http://en.wikipedia.org/wiki/Thermate

Thermate, thermite whatever you want to call it is much more plausible way to take down modern steel high-rise than office fires.