One of the deepest secrets of the two buildings was that their structural steel—webbed together in a novel, lightweight design—had never been fireproofed to the satisfaction of the trade center’s engineers or architects. No one had ever tested the fireproofing of the steel in two of the tallest buildings in the world. In fact, it was crumbling off. Not long after the bombing, the Port Authority began to replace the fireproofing, and by the morning of September 11, had completed about 30 floors of the 220 in the towers.
