One of the most fundamental job descriptions of structural engineers is to describe the loads faced by the strength of the structure. Problems arise if the load it encounters is greater than the strength of the structure. The buildings, bridges, and towers we encounter in the built environment face many loads during usage. These can be exemplified by wind, rain, snow, flood, earthquake, and the human and vehicle loads they contain. Some mistakes in structural projects can lead to iconic destruction. Tacoma Narrows Bridge is a classic engineering failure case study.
Suspension Bridges | Tacoma Narrows Bridge
Bridges have long been one of the main challenges of engineers. The need for people to work pass from one place to another without getting wet goes back a long way in history. Therefore, studies on bridges. In bridge construction, the first problem that came to mind and needed to be solved was only gravity. The second and, in many ways, the decisive problem was to make it economical. In this case, the questions to be answered in bridge construction are: first, how does the bridge, together with the weight of the bridge exerted by people and vehicles, create a force against gravity? And second, how is this accomplished at an affordable price? Suspension bridges have been the answer achieved by developing engineering studies and building materials.
A suspension bridge is a suspension of the deck with two main cables and two towers, and connecting rods, as its name suggests. The most significant advantage of suspension bridges is that they are built with only two towers, and the cost is low since wide spans can be crossed using less material. Using less material in suspension bridges to cross these wide spans creates this slender and graceful image from the outside and impresses people. However, the use of less material reduces the rigidity of the structure, which requires more attention to strength against loads. For suspension bridges, which have a lighter appearance and structure than the old rigid bridges, a different parameter emerges; the wind.
Tacoma Narrows Bridge
Tacoma Narrows Bridge, the bridge connecting the mainland of Washington state and the Olympic Peninsula, was completed and opened in July 1940. The bridge, the third largest suspension bridge in the world when it was opened, later left this title to landmark failure in engineering history. Unlike other suspension bridges, plate steel was preferred instead of truss in the deck carrier part. This provided the iconic ribbon steel look from the outside, but this approach had non-image consequences. Even during the construction period, the flexibility of the bridge was remarkable in normal winds, and it was given the name “Galloping Gertie” by the workers.
The bridge, where the vertical shaking was felt intensely in strong winds, was closed to traffic. Four months after its opening, that iconic and dramatic collapse occurred on a day with strong winds. Fortunately, it was closed to traffic when the collapse occurred. This time, the image was quite remarkable on the day when the lateral shaking was also effective; it is possible to access the video and photo records of this dramatic event. (Link for the video: Tacoma Bridge Collapse: The Wobbliest Bridge in the World? (1940) | British Pathé)
What is Behind this Collapse?
There are different arguments for the cause of the Tacoma Narrows Bridge destruction, one of which is the resonance issue. Resonance is a periodic force that creates synchronisation with the system’s natural frequency. In wind and bridge, this resonance can occur from a vortex. While this situation is also seen in many long chimney-type structures, various precautions are taken. One of them is to prevent the power of the wind from accumulating and to make it flow. While providing this flow, the energy accumulated continues by creating vibration. It is a joint decision that this oscillation was also seen on the Tacoma Narrow Bridge on the day of the collapse. However, experts say that there is another reason for the spiral-shaped rotation in the images.
There are different debates about the twisting movement of the bridge just before its collapse. One of them is that it has to do with the aerodynamics of the bridge. The bridge, which has plate steel on both sides, established various interactions with the wind. Apart from the ideal default interaction, the phenomenon called aeroelastic flutter is thought to occur on the day of the collapse. The wind current in the lower or upper corner of the deck, which has plate steel on both sides, causes wind flow in the opposite section and corner and twists the bridge. The bridge, which is restored by the passing of the accumulated wind, twists in the opposite direction with the effect of its momentum, and the same wind cycle is experienced here. This situation enters a vicious circle with the energy not being lost, and eventually, the bridge that cannot stand collapses. This situation is the same as the vibration response of a sheet of paper in the wind.
What Has This Collapse Changed? | Tacoma Narrows Bridge
As with many things, current solutions are created based on past mistakes. Although the Tacoma Narrows Bridge collapse is referred to as a significant failure in the case of suspension bridges, it has led to many solutions in bridge engineering today. It is still being studied as a case study in engineering schools. This incident, where it was understood that wind is a significant parameter to be considered for the structural strength of bridges, led to the consideration of aerodynamics in the bridge structure. Various methods have been developed to prevent events such as collapse. These can be listed as; Leaving a gap in the middle between the roundtrip lanes so that the airflow is balanced and does not accumulate and cause twists, and lateral faces are designed to ensure the flow of the wind, not as a plate, to prevent point wind accumulation. Confronting this incident triggered consideration not only in bridge engineering but also in the statics of structures heavily affected by wind, such as skyscrapers.
References:
- Tacoma Narrows Bridge | bridge, Washington, United States | Britannica. (2020). In: Encyclopædia Britannica. [online] Available at: https://www.britannica.com/topic/Tacoma-Narrows-Bridge.
- History.com Editors (2018). Tacoma Narrows Bridge collapses. [online] HISTORY. Available at: https://www.history.com/this-day-in-history/tacoma-narrows-bridge-collapses.
- Harish, A. (2018). Why the Tacoma Narrows Bridge Collapsed. [online] SimScale. Available at: https://www.simscale.com/blog/tacoma-narrows-bridge-collapse/.
- digitalcollections.lib.washington.edu. (n.d.). Tacoma Narrows Bridge midsection collapsing into the waters of the Tacoma Narrows, November 7, 1940. [online] Available at: https://digitalcollections.lib.washington.edu/digital/collection/farquharson/id/19/rec/2 [Accessed 30 Nov. 2022].
- The Seattle Times. (2021). Documentary about Tacoma Narrows bridge collapse gets more footage, wider release; here’s how to stream it. [online] Available at: https://www.seattletimes.com/seattle-news/documentary-about-tacoma-narrows-bridge-collapse-gets-more-footage-wider-release-heres-how-to-stream-it/ [Accessed 30 Nov. 2022].
- www.youtube.com. (n.d.). Practical Engineering – YouTube. [online] Available at: https://www.youtube.com/@PracticalEngineeringChannel [Accessed 30 Nov. 2022].
- British Pathé (2013). Tacoma Bridge Collapse: The Wobbliest Bridge in the World? (1940) | British Pathé. YouTube. Available at: https://www.youtube.com/watch?v=XggxeuFDaDU.
- demircan (2022). Turkey counts down to opening of landmark 1915 Çanakkale Bridge. [online] Daily Sabah. Available at: https://www.dailysabah.com/business/transportation/turkey-counts-down-to-opening-of-landmark-1915-canakkale-bridge [Accessed 30 Nov. 2022].
