Tacoma Narrows Bridge

What materials and systems were used in the construction of the structure?

The Tacoma Narrows Bridge was constructed of reinforced steel structural steel, wire cables, concrete, treated timber and untreated timber. The suspension bridge was the longest standing at the time, nearly 6,000 feet long and 26 feet wide. (Washington)

Figure 1. Sketch of Tacoma Narrows Bridge

What were the causes of the structural failure?

The ratio for length to width was incorrect, but this could not have been determined at the time of its construction as no suspension bridge of this size had been made yet, so the engineers were going off of ratios and information that would be obsolete in this construction. What ended up happening was that the road itself was too light, as it was too narrow. Also, the way the roadway was designed worked as an air foil does on a plane, creating lift. These two problems combined is what made the bridge receive its nickname, “Galloping Gertie”. Although the bridge was designed to be able to withstand 140 mph winds, one day when the wind was 40 mph, the lift created proved to be too much for the structure, causing it to twist. The twisting motion soon overcame the tensile strength of the concrete and began to crack and fall into the water below in sections, with the whole roadway eventually collapsing. (Washington)

Figure 2. Twisting of bridge before collapse

What were the consequences of the failure?

Three cars and a dog trapped in a car fell into the river along with the roadway. The dog was the only death. The bridge collapsed and a new bridge was built to replace it. (Washington)

Figure 3. Collapsing of Tacoma Narrows Bridge

What technical lessons were learned from the failure?

The failure of this bridge made engineers reconstruct their idea of the proper length-width ratio, as this bridge was too narrow and weak. It also made engineers take air flow into effect, placing holes in the side of the roadways so that the air can flow freely through it, instead of possibly causing lift like it did with the Tacoma Narrows bridge. (Washington)

Figure 4. Typical shaking pattern of bridge with twisting pattern leading to collapse beneath

What ethical lessons were learned?

Ever since completion, the bridge would sway and twist, although not as severe as when it collapsed. Still, it had enough movement in areas where there should not have been movement to make the engineers reconsider the construction. The bridge should never have opened, or it should have been closed not long after opening. Instead of doing this, the engineers simply attached cables that tied down the roadway by attaching the cable to the ground and the edge of the roadway. (Washington)

What were the legal ramifications of the failure?

Since there were many different, new methods used to build this bridge, and not enough testing had been done on the new designs, investigator Othmar Amman came to the conclusion that “It has shown [that] every new structure [that] projects into new fields of magnitude involves new problems for the solution of which neither theory nor practical experience furnish an adequate guide. It is then that we must rely largely on judgment and if, as a result, errors, or failures occur, we must accept them as a price for human progress.^” (Amman). He claimed that the methods used were so new that no one could have known the bridge would have failed, and the failure in this design was unavoidable.

What are your personal take-aways from the failure?

I was appalled to find out that even during construction the bridge displayed weakness and unplanned movement, but they still continued to build the bridge anyway. While it would have been expensive to redesign and replace the bridge, I feel like it would have been quite obvious that that is what they would need to do. However, they went on with the bridge as is, shaking and all. The engineers displayed very feeble attempts to suppress the motion of the bridge, such as trying to anchor the sides with cables. All while trying to find a solution, the bridge was open and people were driving on it. To allow people to drive on a bridge that is clearly dysfunctional is something I cannot believe happened. We were lucky that the only casualty was a dog.

Amman, Othmar H (1941). “The failure of the Tacoma Narrows Bridge”. Washington, D.C.

Washington State Department of Transportation (2005). “Tacoma Narrows Bridge”. Tacoma, Washington