The Tacoma Narrows Bridge
The Tacoma Narrows is a strait in Puget Sound which separates Tacoma from Kitsap Peninsula, where in the 1930s there was a strong military presence, including the US Naval base at Bremerton and the Army facilities at Fort Lewis and McChord Field. The US military were strong supporters of a bridge linking the peninsula to Tacoma, and a plan was devised for a conventional suspension bridge similar to the Golden Gate Bridge. The prevailing winds in the Narrows led to the plan being modified to produce a thinner than conventional deck, which made it lighter, stiffened by plate girders. This also reduced costs of construction and the modified plan was used when the bridge was built.
The decision to use the modified plan meant that the much lighter deck was insufficiently rigid and thus less able to resist wind forces. This problem wasn’t apparent until after completion of the towers and cable system allowed for construction of the span. Construction workers felt significant vibrations on the towers, and the deck was reported to move several feet, rising and falling like waves. The movement was so prevalent that the bridge was given the nickname Galloping Gertie by the construction crews as the engineers examined ways to reduce the movement and stabilize the vibrations it caused to the other components of the bridge.
Several proposals were examined and applied to the bridge as construction continued. These included the use of tie downs to the deck which were anchored ashore. The tie downs snapped when the bridge bucked. Additional cable stays were added which secured the deck to the suspension cables, but they had little effect on reducing either motion or vibration. Hydraulic pads were then installed between the deck and the towers, to act as cushions which would suppress the motion. These were damaged by sand blasting prior to the bridge being painted, and thus rendered incapable of performing the task for which they were designed, because the seals were broken and the fluid within escaped.
The bridge, which was a toll bridge, opened to the public on July 1, 1940. From the first motorists using the bridge reported feeling the sensation of rising and falling as the traversed the span. Almost as soon as the bridge was in use studies were underway to find a means of controlling the movement, but the bridge remained open to the public, with no warnings regarding its safety. On November 7, four months after opening, the bridge collapsed when a phenomenon known as aeroelastic flutter caused it to twist and turn in the wind like a gigantic streamer, before breaking apart and falling into the Narrows. The only casualty was a cocker spaniel.
While the wind induced the failure of the bridge by creating the motion of the deck, it was the flutter which resulted that brought down the bridge by spreading to the other components of the bridge. This caused the failure of several cables, each of which caused a shifting of loads which led to further failures, until the entire center span broke apart and fell. The bridge was eventually replaced after the shortages of steel and manpower caused by the Second World War were alleviated and the Tacoma Narrows Bridge was completed in 1950. It still stands, joined by a parallel bridge which shares the traffic load, which opened in 2007.