I would like to point out that the "how much did each floor weigh" error is a basic one that is continually made in the conspiracy theories. They are thinking about this as Mass vs. Mass. The actual way to understand the fall of the towers is Load vs. Structure.
The mass of the upper sections normally dealt loads to the structure below in a static way. They weren't in motion. As designed, the structure below could redistribute those loads without falling.
When that upper mass gained momentum and began exerting dynamic loads, the structure below failed. It had to fail. The dynamic loads coming from the upper mass in a gravitational acceleration were much, much larger than the structure below could redistribute. The calculations by many scientists have shown that these dynamic loads were quickly an order of magnitude greater than the structure below could redistribute.
My growing favorite illustration of this is the picture to the left of this post. When you tap a nail into wood, it stands. It is a simple structure with a single joint - the connection of the nail to the wood (which is friction, correct?).
You can rest the hammer head onto the nail, and it will not destroy the structure. This is a static load. The nail-wood structure can easily redistribute this static load and support the hammer.
When you then raise the hammer and swing it down onto the nail squarely, the dynamic load delivered to the nail destroys this structure. It does not destroy the nail. It does not destroy the hammer. It does not destroy the wood. The dynamic load overwhelms this simple nail-wood structure, and since each of these elements are designed to work the way they do, the hammer drives the nail deeper into the surface of the wood.
The WTC towers were very complex structures, but the result is the same. When a structure's ability to redistribute a load is overwhelmed, the structure will - must -did fail. The dynamic forces in the WTC towers (all three of them) were so great that you can eliminate all the force leaving the system with mass falling away, you can eliminate all the force necessary to shatter the concrete into every particulate size imaginable, and still the dynamic force left is enough to overwhelm any of the structure below.
It is not Mass vs. Mass. It is Load vs. Structure.