Meh. The flaw in the conspiracy theory isn't whether or not the steel melted; it's the fact that the steel didn't NEED to be melted. Steel loses a great deal of strength after it's been heated pasted 800 or so degrees, and becomes extremely pliable far before the point of it actually melting; otherwise, blacksmith's wouldn't have been able to work metal, and we'd have had to cast everything.
Therefor, the fires in the building, fed by aviation fuel and office furniture, carpeting, etc, didn't NEED to get hot enough to melt the steel in order to cause a loss of structural integrity; it just needed to get hot enough to cause the steel to lose its strength, which is quite a bit easier to do.