4. By the 1940s, the Manhattan Bridge started to show serious signs of strain

Photo via Dave Frieder.

Engineers Othniel Foster Nichols and Leon Moisseiff developed the “deflection theory” that would be implemented on the Manhattan Bridge. This theory stated that the following three opposing forces act on the deck and suspension cables:

• one downward force caused by the load of the roadway
• one force in one part of the cable, pulling up and to the left
• on force in the other part of the cable, pulling up and to the right

The deflection theory accounted for the economies of material, cost, and time. But in 1909, it did not account for the development of subway traffic on the outer parts of the lower deck.