In this case, you had MASSIVE transformers short and gas mains ignite and literally incinerate anything next to them, making the fire burn in the whole building, instead of simply localized burning in the upper floors. WTC was not made mostly out of concrete, but steel, like the two towers. All the steel needed to do was be weakened enough to lose structural integrity, and since it was burning in the core of the building, it collapsed inwards.
The towers collapsed for the same reasons, and they fell into their footprint because of momentum. The upper floors weighed a shit ton, and once that started falling, it crushed the floors below, which then crushed the floors below, etc, etc.
Really, what you conspiracy theorists need to prove to us is what you would have expected to see differently, not just assume that they couldn't have collapsed the way they did. If two 747's full of fuel collided with towers that tall at the level they, did, what would you have expected to see? And don't just say it, prove it, make models, test it. Besides, if you look at the videos, the towers don't fall precisely into their footprints, they actually crumple and fall inwards, then take the rest with them.
You guys have no ground to stand on until you prove scientifically that they couldn't have fallen the way they did, instead of just pulling it from your ass that they couldn't have.