Getting rid of big scyscapers is no easy feat, so what do the experts have to say?
From How stuff works.com
You can demolish a stone wall with a sledgehammer, and it's fairly easy to level a five-story building using excavators and wrecking balls. But when you need to bring down a massive structure, say a 20-story skyscraper, you have to haul out the big guns. Explosive demolition is the preferred method for safely and efficiently demolishing larger structures. When a building is surrounded by other buildings, it may be necessary to "implode" the building, that is, make it collapse down into its footprint.
The key phrase is collapse down into its footprint, something which WTC 1,2 and 7 did remarkably well.
Some would say that fire and falling debris caused WTC 7's collapse but I beg to differ. If this was the case, why is there virtually no smoke or flames coming from the building (feel free to post a shot of the towering inferno that was WTC 7) and if falling debris was the culprit, why weren't buildings 6 and 4 also demolished, especially 6 as it is right next to the North Tower?
Take a look at these buildings which were bought down by explosives,
and now take a look at WTC 7 collapse.
For those of you who need a comparison here is a clip of a building collapse using no explosives clip 387kb
Buildings simply do not just fall down into their own footprints. They do it because it was planned that way.