[worldtrade2021]

Came across this reasoning for the spire installation on One WTC not including the cladding as was originally planned (spoiler: has nothing to do with inability to clean or install the cladding):

https://www.youtube.com/watch?v=ar8TkQmEALI&t=2243s

From 37:36 to 38:34 in the video:
"But then wind tunnel testing showed the design had a serious flaw.

At certain low wind speeds, the kind New York sees regularly - the spire would start to vibrate.

The effect resulted in significant motions of the spire at the tip of the spire.

They could be on the order of several feet, and that could occur on a monthly basis.

As the wind picked up, the effect would diminish, but then return, much, much stronger.

The effects of that long term would be fatigue in the material, and then ultimately failure of the material as it fatigued.

The only way to avert disaster was to was to produce the spire without its cladding.

Now just a network of beams and cables, the wind can pass through it and the problem disappears."

The information in this documentary appears to be from the PBS Documentary Super Skyscrapers EP01 One World Trade Center that aired on February 5, 2014.