At the time of the attack, 18 floors on the north tower and 13 floors in the south tower had been upgraded, by increasing the thickness of the fireproofing from 2cm to 4cm. But as it happened, the planes hit floors in the north tower whose fireproofing thickness had already been increased, and floors in the south that had not -- with one exception, the 78th floor.
Team members are carefully debating what role that difference may have played in the time the towers stood after impact: 56 minutes, 10 seconds for the south tower and 102 minutes, 5 seconds for the north. But there are several other possible explanations. The plane that struck the south tower was moving at least 160kph an hour faster than the other -- heightening the energy upon impact. And it hit ten floors lower, resulting in far more weight bearing down on the damaged area.
Whatever its thickness, much of the fireproofing was probably dislodged by the impact of the planes, the investigators concluded. One official knowledgeable about the fireproofing said that even with the wipe of a finger, the woolly, mineral-based material could be brushed away.
Stripped of its fireproofing, a steel column heats up much more quickly in a fire. The hotter the steel, the less it is able to support loads, as it eventually becomes soft as licorice. Investigators believe the structural steel was also greatly imperiled because the sprinklers and standpipes supplying water for firefighting were almost certainly disabled, their supply pipes cut by flying debris in the initial crash.
The sprinklers were installed by about 1990 and included tanks as high as the 110th floor of the buildings. Witnesses below the areas of impact, and therefore below the primary fires, described water cascading down the stairwells where pipes from those tanks ran. The hoses that firefighters carried on their backs up the same stairwells would therefore have been largely useless, since the standpipes used the same supplies.
"Damage caused by the aircraft impacts is believed to have disrupted the sprinkler and fire standpipe systems," the report says.
The report cites the tightly clustered exit stairways, three per tower, as a factor that may have made it easier to cause damage to all of them with one blow. These exit stairwells also had relatively lightweight gypsum board sheathing, providing little armor. Partly for these reasons, thousands of people above the floors of impact in the towers were trapped and killed.
Most of the tenants in the floor below impact, to the credit of the building and the emergency lighting in the stairwells, escaped. More than 400 firefighters, police officers and other rescue personnel and dozens of tenants who stayed behind during the evacuation were also killed when buildings finally collapsed. An estimated 2,830 people are considered dead or missing in the tower disaster.
Against the Manhattan skyline, the gleaming towers looked nearly identical, except for the television tower atop the north tower. But the inquiry has found that the forces that brought them down had distinct differences.
Their basic structures before the attacks were extremely similar, even if they were not quite perfect twins. Each tower was supported against the downward force of gravity by a tightly arranged matrix of columns at its core and another palisade of columns, spaced just 120cm, around its exterior.