Answer:

No one has had the ability to prove that the towers were poorly constructed. Yet our major media has suggested, in TV programs and printed articles, that (1) the buildings were poorly constructed; (2) the structural steel used in their frames was defective; (3) the fireproofing of this steel was inadequate.

In fact, the buildings were "over-engineered."  This means that each floor could bear many times its weight, and the endo- (inside) and exo- (outside) steel skeletons actually constituted two frames, not just one.  The core (inside frame) was composed of 47 steel columns made of 4-inch-thick steel at the base (go to a construction site and ask to see an I-beam for an example of what structural steel is like).  The exterior frame was made of 236 columns, plus four corner columns.

Kevin Ryan (Google his articles) of Underwriters' Laboratories, the global testing giant that certified the WTC steel decades ago, has exposed UL's strange acceptance of the decree that the structural steel failed, even though not a single one of the test models failed.  Ryan drew attention to this while at UL, wondering why his company was willing to put its reputation in question.  He was fired immediately.

The New York Times reported that when the planes hit the towers, the impact knocked the asbestos fireproofing off the structural steel, and the fires that ensued attacked unprotected steel.  This caused certain floors to fail, and the falling floors brought down the floors beneath them, resulting in a pancaking of the entire tower.  Twice!

In fact, the towers' tremendous strength explains how they continued to stand after the airplanes plowed into them.  They would have stood for many more years, with their gaping holes, if no repairs were conducted.  Instead, they came down in approximately 10 seconds, which suggests that something other than the plane impacts was involved.  (See "Jet Fuel" FAQ for more info on the fires.)