The 60-foot elevator tower is three-quarters complete, he reports, and the exhibit about half finished.
The scaffolding above the 20th floor dropped about eight feet, wrenching the elevator tower, which snapped off and fell.
A 60-foot-high elevator tower was built beside the building so visitors can reach the museum without going through the building.
An elevator tower was added to it in 1999.
Workers often remove the beams holding the elevator tower while they apply the facade.
Now, the grandstand project stands abandoned, with only the stark concrete of its elevator towers to show for the plans.
An 8-ton, 30-foot section of the elevator tower and the scaffolding were also removed.
He said that a 10-foot section of the 700-foot scaffolding collapsed, bending and then breaking the attached elevator tower at about the 23d floor.
The Mayor said it would take three to five days to secure the scaffolding and dismantle what was left of the elevator tower.
Heinsohn Hall underwent a fairly large renovation in 1985 when an elevator tower was added to the building.