You're welcome Mr.H. I often look on the BBC website for different takes on the news.
Like I said, I'm not an engineer, but I can say that steel framed buildings of all types really are designed to spread the weight back to main support beams that can carry the overall weight of the entire building. It seems that buildings are built with individual floors in mind, but that's not really the case. Each floor's design is built to evenly spread that floor's weight to the main beams. In other words, the beams really are the building, and the floors kind of hang on the steel frame, so to speak. It's like a Christmas tree... the "trunk" is the main steel support beams, the "branches" are the various steel cross beams that distribute weight back to the main steel beams (or, the "trunk" in the case of our tree), and the "ornaments" are the concrete floors and stairwells, etc that hang on them.
The fifth main beam that the WTC had was probably put there because at that height, it becomes neccessary to put in cross beams that would have to span probably five floors. Not practical. It would make inhabiting the building very difficult. The solution is to put the fifth main support in the center of the building. That way, any cross beams could make much shorter runs to the center main beam, and could probably do it without crossing through the floors at all.
I think that what happened to the WTC is that probably two (or more) of it's main beams were heated to the point that they could no longer bear the weight of everything above them, causing them to buckle. As soon as they begin to buckle, more and more weight is distributed unevenly, making the remaining main beams also buckle. When the top 20 floors fell onto the beams below, the weight naturally comes down very unevenly, concentrated instead of distributed, crumbling everything on the way down. That's why the building appears to crumble in the video. As the material falls down, the beams below break, one by one instead of all at once.
It's a strange analogy, but the "house of cards" is a good example, I think. It'll stand as long as every card is just where it's supposed to be. Move just one or two cards out of place and it collapses.
Anyway, that's my opinion, but again I'm no engineer. Hope I didn't confuse you too much. It's hard to explain on a BB.
BTW- I think the article was correct to say that the WTC performed amazingly well under the circumstances. Very few buildings even have a fifth main support column, and the WTC not only had one, but it was encased in concrete, then insulated with asbestos (as were it's other mains beams). I agree with the article when it said that most other modern buildings would never have lasted so long. While it does seem that this catastrophe will tragically cost thousands of lives it also bears noticing that the hour or so that the building stood gave thousands more a chance to escape. It's a credit to the excellent construction that went into it.
