The steel is there to support a given weight or load.....as the steel get hotter it looses strength...it get weaker as it gets hotter.. it simply need to get hot enough to no longer have the strength to support the load on it.. does not even need to get even near melting.
Ice on a pond..you can walk on it but as it warms you can not longer walk on it .long before it melts
Last tower hit was first one to collapse..given same heat and fire from jet fuel....why?....if you think the steel needed to "melt" for a collapse then should it not happen in first hit tower first?
Answer... Last tower was hit lower...that equals ....more weight/load above fire... steel needed less to.just be weaken by heat to the point it no longer can support the load on it.... .I.E.the fire did not need to get as hot in the second tower before collapse...because it just about weakening...not melting,....the steel ..to the point it can no longer support the weight/load put on it