The old Tappan Zee was basically a perfect encapsulation of "dumb midcentury infrastructure decisions."
1. Built to last only 50 years to save on materials (as the other commenter noted).
2. Built over literally the widest possible part of the Hudson because the governor got in a pissing contest with the Port Authority and wanted all the tolls to go to the state, which wouldn't have been the case had it been built like 2 miles south where the river is narrower.
3. Designed with zero redundancy, such that a "critical fracture could make the bridge fail completely because its supports couldn’t transfer the structure’s load to other supports." [0]
So yeah if we're being real, 50 years was quite optimistic.
The new Tappan Zee is apparently supposed to last 100 years, though given the incidents with substandard materials being used, as well as ever-increasing traffic, who knows.
That said, driving over a bridge 10 years past its planned EOL and being able to look down directly to the water through gaps in the concrete was always a nice feature though -- who needs coffee when you've got that to get your heart rate up!
Keeping in mind that mid-century Japan and Germany were in ruins and the possibility of nuclear annihilation of urban centers very much at the top of many people minds. 50 years may have seemed an optimistic survival rate of built structures at the time.
