"The tower floors were all based on raised floors, demountable partitions and system furniture," Mr. Craig said.
And in the tower floors of the most desirable Midtown buildings, the rate is closer to 4 percent, said Peter G. Riguardi, the president of New York operations at Jones Lang LaSalle.
"This is the first time we will be doing prebuilts for the tower floors," said Ted Koltis, a senior vice president at Paramount.
Along the impact zone of the south tower, floors 78 to 84, however, the stairs had to divert around heavy elevator machinery.
There, a $537,000 mock-up was built to represent a corner of three typical tower floors, with laminated glass panes one and a half inches thick.
The museum has more than 4,000 artifacts, from a wedding band to a 15-ton composite of several tower floors that collapsed into a stack, like pancakes, and then fused together.
Two other images from that next hour: staring up as men and women leapt to their deaths to escape the flames engulfing the upper tower floors and, a while later, as I walked up West Broadway, turning to see the dazzling, terrible sight of the second tower collapsing its windows exploding spectacularly against the deep blue sky.
Titled "A person loved me" and molded from gray clay, it towers floor to ceiling, like some monstrous fungus bristling with clublike growths.
"One of the fears," Roberto Fernandez, coordinator of the labor rights NGO Yachaj Mosoj, told a reporter in 2010, "is that Cerro Rico is going to crumble like the Twin Towers, floor by floor".
