However, just as effective.
Instead of destroying all the foundational load bearing members (perimeter and core), "they" probably just destroyed a significant number of foundational core members which in itself had led to a bottom-up interior collpase of the building leaving only the perimeter shells to stand for some time before they too collapsed, though from their weakest points(impact floors) which gave us the appearance of a "top-down" collapse.
In other words, once the core columns had been taken out this caused the core to collapse and also it caused a transferrance of about 60% of the building's load to the perimeter columns. As this weight spread throughout the perimeter it finally came upon a "dead-end" or if you will, the impact zone.
This "dead-end"(impact zone) caused the transferrance of weight to bottleneck , which in itself caused the perimeter columns to fail once the added weight that was being transferred to it became unbearable.
Imagine if you will an empty soda can(no core) and then imagine a hole in the soda can, apply some weight, and try to imagine where that Soda Can will fail first.
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=125x112905#112915