The collapse of the Tacoma Narrow Bridge in 1940 a few months after its opening stunned everyone involved. It was constructed as a modern suspension bridge using the most advanced designed of the time. The State of Washington, insurance companies and the U.S. government appointed experts to investigate the causes of the collapse. Experts disagree on some of the aspects that led to the collapse, but the core reasons are generally accepted (Petroski, 2009).
The Collapse
The bridge collapsed on November 7th 1940, a few months after its opening. This disaster was totally unforeseen as its design was viewed to be modern and capable of withstanding all forces of nature. The greatest concern at the time for the bridge was the load it would carry in the form of vehicles transit on it. The construction of the bridge took into consideration all the factors that could result from vehicles moving on it. However, little attention was paid to the factors such as wind. The bridge was also known to move in windy conditions during its construction, which led to the nickname Galloping Gertie (Petroski, 2009).
The damage caused by the collapse was extensive. The main suspension cable were twisted and fell on the approach spans. Other wires were severely stressed and deformed. Their only value was as scrap metal. The same could be said of the suspender cables that broke as a result of the collapse. Some of these cable were also lost. The towers formed the spine of the bridge, but were also damaged. The main towers were twisted and bent. The most devastating occurrence was the twisting of the bridge that caused the towers to buckle and deform.
The deck-floor received the most damage as concrete and steel broke off and suck to the bottom of the Narrows. Most of the deck-floor remained intact, but the concrete was broken. It was structurally compromised and had to be removed. Its floor system had sections that were stressed and bent. The damage to the deck-floor led to the falling of the bridge’s side spans. It caused the stressing and deformation of the plate girders and floor beams that some were beyond repair. Both the East and West pier did not sustain any damage. The anchorages for the main cable were also not damaged (Petroski, 2009).
Investigation into the Collapse
The disaster had surprised everyone, and different entities set up commissions made up of experts to investigate the collapse. The Washington State, insurance companies, and the U.S. government were the key players in the investigation. The Federal Works Administration (FWA) also set up a board to investigate the same. There was a general consensus that the wind played the key role in the disaster, but this required an expert’s opinion on how that could occur. The board set up by the FWA announced ambiguously in March 1941 that the cause of the disaster was ‘Random action of turbulent wind’ (WSDOT, 2015).
This inspired the need to explain the complex concept of wind-induced motion. The construction of suspension bridges would take another turn with this study. It had not been deemed that wind was a major player in the construction process. The disaster had changed this perspective. Some key points were made about the disaster. One was that the main cause of the bridge’s failure was its ‘excessive flexibility’. The construction team had made it more flexible that it ought to have been. Another point was that girder and deck created excessive drag and lift. The final point was that little was understood about aerodynamic forces and that it was necessary to incorporate them in the designing process of suspension bridges (WSDOT, 2015).
A report written in the Engineering News Record cited the ‘excessive flexibility’ of the bridge to be the cause of the disaster. The report included details on what made the bridge very flexible. The deck was viewed to be too light and too shallow. In comparison with the length of the center span, the side spans were deemed to be too long; further making the bridge more flexible. The cables were also affixed as a great distance from the side spans. All this made the bridge undesirably flexible. The cause of the disaster was cited as the change to devastating twisting and torsional motion.
The findings made by the experts were basic and obvious; it is essential for the engineering community to understand the phenomena of aerodynamics when designing such structures. The use of wind tunnel tests replicating similar conditions as those that caused the disaster showed that intense resonant oscillation played a key role. The flexible bridge could not withstand the accumulation of the wind pressure (WSDOT, 2015). Wind pressure had accumulated over a very flexible structure and persistently twisted it. The concrete had broken and caused the deck-floor system to crumble.
Lessons from the Disaster
During the construction of the Narrows Bridge in 1940, designers believed that lighter and narrower suspension bridges structurally sound. There were few suspension bridge engineers at the time due to the fact that there were few such projects that were deemed to be very expensive. Work on such a scale was closely monitored by the government and the public. The opinion of the few engineers in the field was that the best way to design a suspension bridge was by making it light and narrow (Petroski, 2004). The reason for this perspective is that wind was not viewed to be a significant factor in the designing process.
Earlier failures of suspension bridges was viewed to be as a result of poor design and traffic overloading. The factor of wind was not thought of as important. Both lateral and horizontal deflections were viewed to be as a result of traffic loads and temperature changes. The solution to this was to stiffen the trusses. However, this was also the systems that has been practiced in earlier designs and had resulted in disasters. Earlier design failures resulted from light spans with very flexible decks that could not withstand aerodynamic forces. In the late 19th century, bridges had become more stiff and bulky to offset such forces. However, these lessons were not learnt during the construction of the Tacoma Narrows Bridge (Petroski, 2004).
In an article published after the disaster, a summary of suspension bridge failures was made. The article highlighted the issue of aerodynamic forces and how it had caused the failure of bridges in the past. It viewed the disaster of the collapse of the Tacoma Bridge as a result of forgotten lessons from past failures (WSDOT, 2015).
Designs since the Disaster
The collapse of the bridge presented the limitations of the deflection theory that engineers at the time utilized. It embraced the fact that the wind factor was pivotal in the designing process. The disaster also ended a generation of suspension bridge engineering theory and practice. The size and proportions of a bridge would no longer be the determining merits of the structure. Mackinac Strait Bridge and the Verazzano-Narrows Bridge were two of the subsequent structures after the disaster. They were also two of the largest suspension bridges in the world. Aerodynamics were a key consideration in their construction as with the lessons learnt from the prior disaster (Kucher, 2008).
Aerodynamics has become a key area of study since the Tacoma Bridge disaster. All the structures erected after the incident have taken consideration of the forces related to aerodynamics. Wind tunnels have become more common with the aim of better understanding the area of knowledge by replicating natural conditions that structures with withstand. Regulations by the U.S. government require that all bridges that are funded by the federal government to be tested in wind tunnels before they are erected (Kucher, 2008).
Although the deflection theory was found to be wanting in the case of the Tacoma Bridge, it still remains as a critical concept to the construction of bridges. The introduction of aerodynamics supplemented the theory, but did not replace it. The merging of the different aspects provides a more stable and informed perspective towards the construction of bridges. Advances in computer technology has facilitated the designing period. Computer graphics and complex calculations can be made by these computers with little input from engineers.
The disaster served as a learning lesson for engineers across the globe. Similar failures had been witnessed but had not received as much publicity. The designing and construction of suspension bridges has become more effective after the disaster (Kucher, 2008).
Kucher, M. P. (2008). Catastrophe to Triumph: Bridges of the Tacoma Narrows. The Pacific Northwest Quarterly, 99(3), 146.
Petroski, H. (2004). Engineering: Past and Future Failures. American Scientist, 92(6), 500-504.
Petroski, H. (2009). Engineering: Tacoma Narrows Bridges. American Scientist, 97(2), 103-107.
WSDOT. (2015). Lessons From the Failure of a Great Machine. Retrieved from Washington’s State Department of Transportation: http://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm