The First Tacoma Narrows Bridge

TACOMA BRIDGE COLLAPSE

Adeola Adegboye

Alejandra Chavez

Cansin Etli

Abhishek Kumar

Xinyin Wang

Monday, October 9th, 2017  

Project Description

The first Tacoma Narrows Bridge was a suspension bridge in the Washington and connected Mainland to Olympic Peninsula, built in 1940 (Exhibit 1). During 1940, it was one of the longest suspension bridges in the world. The physical properties of bridges were 5,000 feet total length, 2,800 span lengths, 29 feet width and 8 feet height of side girders. This very long bridge costs $8M. It is a very popular bridge not only because it was one of the top 3 longest bridges but also it tragically collapsed in 4 months after it was built (Exhibit 2).  

               Before the bridge was built, there was a ferry system that people used for crossing to the other side. In 1928, the research inspired the construction of a bridge to replace the less efficient ferry system. After pulling researches from various sources, Tacoma Chamber of Commerce found that building a bridge is feasible and they started to work on the project in 1938. The project team got lower budget than they calculated and requested. Then, they had to change some part of the project to stay within budget.

Clark Eldridge was the first project designer. Eldridge's design followed well-established practices and was quite conservative. The federal funding agency dictated that Leon Moisseiff should be hired to review the design and developed a less expensive model (Exhibit 3). After budget restriction and federal agency’s dictation, Leon Moisseiff revised the project. However, both Eldridge and Moisseiff stayed in the project management team. Just before starting the construction, the discussion started again and they turned in to the original plan in some parts, which was design by Clark Eldridge. During construction, some problems were discovered by the team; yet no action was taken. Furthermore, workers were working under dangerous conditions and one person died during the construction period.  In conclusion, the project was not healthy from design to construction. The construction took two years and they opened the bridge anyway.

               After the bridge was opened to traffic, vibrations started to be observed even during low winds. The bridge was so flexible but nobody thought it was dangerous until it collapsed. Luckily, nobody died in the collapse, but it was obvious something went wrong with project. Several investigations took place after the collapse to find out what went wrong.

Reasons for Project Failure  

The Tacoma Narrows Bridge collapsed due to high winds on November 7th, 1940. The main reason for the collapse was wind-induced motion in suspension bridges. The bridge design did not take into account the aerodynamic forces that took place in Gig Harbor with strong winds. The carbon steel structure was vulnerable to vibrations generated by the wind. The frequency oscillations increased to an extent that the collapse was inevitable.

The following events preceded the collapse,

• Engineer Leon Moisseiff countered with an $8 million offer in comparison to Engineer Clark Eldridge at $11 million by replacing the trusses with 8-foot-high plate girders
• Under construction, the bridge had bouncing problems with the deck
• Attempts to reduce the bouncing were put in place such as “tie-down cables anchoring the plate girders to 50-ton concrete blocks, the addition of inclined cable stays connecting the main cables to the middle of the deck; and hydraulic buffers to dampen the main span’s longitudinal motion” (APS News: 2016)
• Frederick Farquharson conducted wind tunnel studies to find a solution. He noticed that sometimes the model will show a twisting motion. “We watched it and said that if that sort of motion ever occurred on the real bridge, it would be the end of the bridge.” (APS News)

The reasons for the collapse were the following,

• A summary article published in Engineering News Record said that the bridge collapse was due to its "great flexibility, vertically and in torsion.” According to the Washington State Department of Transportation (WSDOT), the factors that contributed to its flexibility are,

• The deck was too light
• The deck was too shallow at 8 feet (1/350 ratio with the center span)
• The side spans were too long, compared to the center span length
• The cables were anchored at too great a distance from the side spans
• The width of the deck was extremely narrow compared with its center span length, a ratio of 1 to 72

• The pivotal point was when the change of the waves for vertical to torsional motion, which caused twisting. Once twisting began, vortex forces started. The bridge itself was twisting under what it is called self-induced energy. The twisting increased beyond the bridge structure to resist and it failed (Exhibits 4 & 5).

The following events happened the day of the collapse – Nov. 7th, 1940,

• 11 am: high winds made the bridge swayed and concrete dropped from the surface
• At one time, the elevation of the sidewalk on one side was 28 feet above that of the sidewalk of the other side
• The main suspension cables were thrown violently side to side, twisted, and tossed 100 feet into the air
• The main towers (West Tower, #4; and East Tower, #5), including the bracing struts, were twisted and bent

The lesson learned from the Tacoma Bridge collapse was that engineers should better understand aerodynamics in designing long suspension bridges. At the time, there was no notion that wind could create vertical movement, causing the bridge to collapse.

Project Failure Evaluation  

         The team failed to create a risk management plan based on identifying external risks, which in this case was how wind-induced movement would have an effect on bridges. The engineers in charge were not familiar with the forces of aerodynamic. Other reason why the project failed was due to the availability bias the engineers and designers had, as well as not identifying their blind spots. They believed that past failures of bridges were caused by other factors such as poor labor, heavy traffic, and temperature changes and failed to learn from the past suspension bridge failures. In addition, they were very enthusiastic about the new design that was being created by the reputable Leon Moisseiff. Leon was a consultant for the Golden Gate Bridge, which made his crew reluctant to contradict him.