Tacoma Narrows Bridge

This was now the shortest distance linking the two places and was a great convenience to those

who commuted the route. The bridge was used by over 85,000 people per day. Engineers wanted to make a new and unique bridge with new architectural design that made the bridge a pleasure to drive on. Not only was it unique, it also saved money. When the bridge first opened there was a problem with the structure. It was a suspended plate girder type that caught the wind rather than allowing it to pass through. Because of this, in high winds the bridge could be felt rolling in the wind. It was this rolling action which gave the bridge the nickname “Galloping Gertie”. Four months after it opened, the problem which caused the bridge to roll, caused the bridge to collapse during a 42m/h wind storm. Having a bridge focussed on looks and saving money overlooked the importance of safety and reliability.

The “Galloping Gertie’s shape did not allow wind to pass through therefore acting like an aircraft wing. The shape would create lift in the wind and become unstable. The bridge was designed with a relatively shallow plate girder instead of

the heavier, conventional truss adjacent to the platform. The goal was to provide the minimal stiffness necessary to prevent excessive movement of the roadway under heavy loads. The main reason for this was to minimize spending. The more wind, the more rolling and unstable the bridge became. Cables and hydraulic buffers were used to make the bridge stable in the wind, but they did not work and it was this flaw which put the bridge into Puget Sound four months after it opened.

To look at it in a simpler way, for example maybe the bridge was built to withstand winds of up to 35mph. This means there would be no movement from 0-35mph winds. When the winds get past that limit the bridge will begin to sway. This is what happened to the bridge, the wind exceeded the limit of the bridge. It is the same as looking at fishing line. If a line is rated at10 pounds, if the fish is over it will break. The wind was over the limit resulting in movement causing the bridge to collapse

The bridge was exposed to 42m/h winds which caused vibrations in a phase which built up in proportions large enough to be destructive. The name of this act is resonance. This means that the wind provided an external periodic frequency that matched the natural structural frequency.The wind produced a fluctuating resultant force in resonance with a natural frequency of the structure. This caused a steady increase in amplitude until the bridge was destroyed.

Unfortunately sometimes it takes a disaster to bring about change. The disaster of the Tacoma Narrows Bridge was a shocker for the USA along with the world,

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