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Tacome Bridge Dissaster

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Dragana Stanojevic, Example of my reflection

Tacoma Narrows Bridge disaster

While working in the state of Washington, USA back in 2008 I got familiar with the Washington area, and traveled around a lot. Since Seattle airport is very close to Tacoma, in so called "SeaTac" area, I had a chance to visit Tacoma as well. I've heard stories about old Tacoma Narrows Bridge ( the "Galloping Gertie" as the residents call it), and did a little investigation what actually happened, and I didn't actually understand the cause of the bridge collapse, except the fact that it caused no casualties and that only one dog lost its life.

I forgot about the whole bridge story until I've heard the prof. Kielb mentioned it in the introduction lecture for the Advanced Vibration course on 27th August, and I got interested in the subject again, since I believed that in time and the knowledge that I would get, I couldn get to the actual scientific explanation and understand it this time.

The construction of the bridge started in 1938, and at that time financing was a big problem, so the construction engineers decided to use shallow plate girders (as shown on a figure 1) instead of the deep stiffening trusses of railway bridges [2]. The wind can pass through trusses and plate girders and plate girders are an obstacle to the wind [3].

Figure 1. To reduce costs, sides constructed of solid 8-foot

tall I-beams rather than typical open trusses [2]

Tom Irvine in his work "The Tacoma Narrows Bridge Failure" actually beats first two theories for the cause of the collapse of the bridge. First one is random turbulence, and Irvine says that random turbulence is in the direct correlation with occurrence of resonance, which is very precise phenomena and it demands that the driving force frequency value to be near natural frequency of the bridge structure in order to produce large oscillations. Turbulent wind pressure varies with time and therefor this couldn't be the cause of the failure. Also, the Federal Works Administration concluded that:" It is very improbable that the resonance with alternating vortices plays an important role in the oscillations of suspension bridges. First, it was found that there is no sharp correlation between wind velocity and oscillation frequency such as is required in case of resonance with vortices whose frequency depends on the wind velocity." [4]

Dragana Stanojevic, Example of my reflection

Nevertheless, resonance is introduced as a main cause for the disaster in many publications, but, as I found out by going through Dowell's book, it is wrong conclusion.

Irvine also doubted in the Theodore von Karman's theory that said that: "vortex shedding drove the bridge oscillations". In fact, calculated vortex



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