Assignments

Tacoma Narrows Bridge Collapsed: Engineering Analysis

Introduction:

The Tacoma Narrows Bridge was built in 1940. Obviously the engineers behind this design overlooked something and this was especially apparent when the bridge collapsed on November 7th 1940  (Harish, 2020). The disaster was undoubtedly caused by a serious design flaw. The engineers behind the design had taken into account gravity, however they neglected to take into account another force which happened to be unusually strong in the Tacoma straight. Wind proved to be the ultimate cause of failure because their poor design did not account for its impact on the bridge. Fortunately, no lives were lost, and some could argue this learning experience has probably saved lives as now engineers around the world are still learning from this failure.

 

Engineering Perspective:

The main reason for this bridge’s failure was indisputably a poor aerodynamic design. Originally an $11 million proposal was set forth, but Engineer Leon Moisseif proposed a more aesthetically pleasing design that was only $8 million(Gaal, 2016). This design provided much less resistant to bending and twisting. The thinking was that the cables would be able to prevent this twisting affect; however this was based on the cables being in tension. The problem arose when the wind shot up under the bridge, relieving the tension on the cables and causing vortex shedding pictured below:

 

The above FBD shows how the poor aerodynamics of the bridge would contribute to its ultimate failure. As the bridge rocked back and forth an amazing amount of stress was induced, and the repetitive motion, known as cyclic loading led to the ultimate failure when eventually the cables snapped. The bridge had very little torsional resistance.

All bridge specifications sourced from (The Bridge Machine, 2020).

 Expanded view here

As shown above I was able to calculate the normal stress in the cables since the bridge was in a static state, and then again, the max stress as the bridge rocked back and forth in the wind. Comparing these values I found that the % differences was 6.35%. This shows how the bridge was not designed with an adequate factor of safety. This small change in stress in the cables was enough to overcome the yield stress of the cables ultimately causing failure.

Lessons Learned:

Through this failure engineers have learned many things. One of the biggest takeaways is not putting cost or aesthetics above safety.

Personally, this example shows me how important it is to analyze everything put everything and through rigorous testing especially when the thing that I am designing is literally holding people’s lives up.

Engineers are now taught to implement adequate safety factors into their designs making them strong enough to withstand unpredicted forces.

References:

The Bridge Machine Since 1950. (n.d.). Retrieved October 16, 2020, from https://www.wsdot.wa.gov/tnbhistory/machine/machine5.htm

Harish, A. (2020, January 27). Why the Tacoma Narrows Bridge Collapsed: An Engineering Analysis. Retrieved October 16, 2020, from https://www.simscale.com/blog/2018/07/tacoma-narrows-bridge-collapse/

Gaal, R. (2016). November 7, 1940: Collapse of the Tacoma Narrows Bridge. Retrieved 2016, from https://www.aps.org/publications/apsnews/201611/physicshistory.cfm