[dchan]

The problem here is that you're comparing two different structure types. Not all steel building structures are the same, and they don't collapse in the same way when they fail. The one that you're using in your argument is a standard steel skeleton structure, in which steel beams and columns criss-cross across a grid in a regular pattern. Another (the WTC towers) consisted of a steel core structure tied by floor plates and steel joists to perimeter columns, a structure designed explicitly to maximize the rentable floor space.

With the standard grid-based steel skeleton, one side of the building can be heated and collapse while the other side remains relatively unaffected and can remain standing (as seen in your 3-D structural program renderings you posted). This is especially true if the steel fireproofing on the other side remains intact.

I remember reading about old wooden fighter airplane whose structure consisted of a wicker-like crisscrossing wooden frame. The advantage to this design was that if one part of the plane was hit, the wicker-like structure in the area would simply break off without affecting the rest of the airplane's structure. The rest of the plane's wicker-like structure will remain intact, and the airplane can continue to fly without too much trouble. The standard grid-based steel skeletons are sort of like this in this regard.

The tube-frame system works differently. Instead of a grid of beams and columns supporting the structure, the main supporting elements are now the outer perimeter structure and the inner core structure which are tied together with floor trusses. The structural elements behind this design are also behind why the towers collapsed as they did.

Whereas the various parts of the grid-based steel skeleton act relatively independently of each other, the the tube-frame system in the WTC towers work together to compromise for the fact that there aren't regular steel column and beam running throughout the building. As such, they also assist towards the collapse of the entire structure if some of the elements are compromised as they were on 9/11. Once too many of those elements are compromises and start to collapse, the structural elements that once worked to hold together the building are still there also to hold together the building. Only now, parts of the building are failing, and as they collapse, they pulling and push on the other elements that continue to hold together the building. As a result, the other parts of the building now start to collapse as well.

In essence, the structural elements in the WTC towers worked in harmony to resist gravity and lateral forces during normal operation. But during 9/11, they worked in harmony to collapse together as well.

From this lesson, structural engineers have learned to avoid tying together their building frames too excessively and without safeguards against progressive collapse. As such individual structural beams are now designed to fail without affecting the adjacent structural components much.