There was roughly 200,000 tons of steel in the building columns. If we roughly calculate it will be about 1800 tons per floor. How much fuel do you think need to heat up in an open environment to melt the just that much metal which is used in the floor where airplane hit the building?
I have an example. In Pakistan steel mills the maximum production of pig iron was 1760 ton per day per furnace (in ideal condition), which is in 24 hours continuous operation of one blast furnace consuming lot more fuel than the fuel in an airplane in an air tight closed environment. The raw material itself is ignitable in a blast furnace while the metal of the WTC was not even directly exposed to the fire. The critical temperature of steel is 724 Celsius while the temperature in blast furnace required to be 1900 Celsius. There are more questions before we assume that the metal was melted or even weakened.
If you analyze there are several factors like fuel, time of heating, environment etc. So the first hypothesis that the heat melted the steel on a floor of WTC is not correct and house card effect comes later, we even don't need to analyze that.