>>No still-frame building EVER colapsed as result of fire. At the point and the time of of impact temperature might have reached
>>melting point of (naked) steel beams. But jet fuel melting steel beams all the way through thick cement of support columns
>>and then building colapsing at free fall speed is totally banana (pancake) story.
>
>http://www.tms.org/pubs/journals/JOM/0112/Eagar/eagar-0112.html ?

So if it was the clips which gave, how come they all failed at the same time. Try to hold a large book horizontally, holding it on your fingertips at the edges. Or even better, on two desks. Then slowly move the desks away from each other. Repeat this a thousand times, while recording a video. Let me know the number of times the book fell straight down, perfectly parallel with its initial position.

IMO, if it was the clips that failed because of buckling, there's no way the floors would be falling straight down. The side which failed first would go down first. We'd either see them piled down on one side, or stuck diagonally around the central core. The floors falling so neatly down is just not something you'd expect to be caused by random malfunctions of whatever harness was holding them in place, not even once. Three times is a tad too implausible.