One of the Most Terrifying Structural Failures in the History: The Tacoma Narrows Bridge
The year 1940 was a time where our world had learnt a lesson on developing sturdy bridges. The incident of the Tacoma Narrows Bridge had shocked many, especially the architects and engineers. They were almost sure that the bridge wouldn’t collapse or get harmed in any form. Engineers are mainly taught about over-engineering though this bridge had a huge lack of engineering skills. Though luckily no one died nor was anyone seriously injured.
The Tacoma Narrows Bridge was one of the first of its type, a Suspension bridge. A Suspension bridge has a very unique way of functioning. They are one of the most popular bridge designs. It is mainly used in tourist places as they look attractive and don’t cost as much. These bridges are extremely difficult to build without it failing/collapsing mid-way of the build as the bridge is mainly carried by the cables but they can only be added at the end. Though, fortunately the Tacoma Narrows Bridge completed without any difficulties or issues.
Unlike many other bridges, Suspension bridges carry their weight using tension instead of getting pushed on. It is designed in a way so the beam is held by the piers and the main load is being held by the cables. The main supporting wires that are connected with the beam cause a great force of tension as it is being pulled on when holding the load. The smaller cables (known as suspenders) span from the bridge deck upwards to the main supporting cables. Their job is to transfer the deck’s weight to the towers through the main supporting cables. The tension forces in the cables are converted to compression forces in the piers that then extends all the way to the ground. This bridge uses both compression and tension to completely function.
About the Tacoma Narrows Bridge
Where was the bridge located?
The Tacoma Narrows Bridge which is known for the unfortunate fail was located in Washington, US. It spanned the whole Puget Sound from Gig Harbor to Tacoma which is close to around 40 miles away from Seattle, Washington. The channel is about a mile wide where the bridge crossed the sound. It was the third longest suspension bridge in the world at the time, covering 5,959 feet.
After redesigning and developing a sturdier bridge, engineers had rebuilt the structure at the same exact spot of Washington. It opened in 1950, around 10 years after the fail and is still in good condition.
When was the bridge built?
Engineers/Architects first start the build in 1930 and took around 8 years to complete. In 1938 the Tacoma Narrows Bridge had its structure finalized but opened to traffic on July 1, 1940. Between the 2 year gap of the completion of the construction and the opening of the bridge, the military had main control over it for their own uses.
What was the purpose of the build?
The Tacoma Narrows Bridge had multiple uses but was primarily created as a military necessity. This bridge linked both Me Chord Air Field which is on the south of Tacoma and the Puget Sound Navy Shipyard in Bremerton. After the use of military, it was finally open to the public for daily use.
What caused it to fail?
Engineers of the time claimed that the design was absolutely safe, even though it surpassed the ratios of length, depth and width that were previously normal. Following the failure, it was revealed that the engineers had not properly considered the aerodynamic forces that were in play at the location during a period of strong winds. Suspension bridges are extremely weak against wind storms as it ruins the complete process of the cables lowering its functionable ability. Other material such as the beams and piers aren’t able to carry the whole load itself and hence, it fails.
Talking about the Tacoma Narrows Bridge specifically, it had a huge shortage of thought put into it. The bridge didn’t have support against any event that could harm it. Structural engineers are required to take dangerous events into consideration when designing the structure.
The term called resonance played a big role in the failure. This is the phenomenon where a periodic force syncs up with the natural frequency of a system. A good example is a swing. With resonance, small periodic driving forces, like pushing someone in a swing, can add up to large oscillations over time because the energy is stored. In the case of wind-induced motion, the periodic driving force comes from an effect called vortex shedding. This is where a fluid flowing past a blunt object oscillates as vortices are formed on the backside. When these alternating zones of low pressure occur at a frequency near the natural frequency of the structure, even small amounts of wind can lead to major oscillations. This is exactly what had occurred with the bridge.
Rather then creating a truss through which wind can smoothly flow, this shape of the Tacoma Narrows Bridge with large steel plates on either side created some strange interactions with the wind. Any amount of twist in the bridge created vortices, or areas of low pressure. As the bridge returned to its normal stage, the momentum forced it to twist to the other direction. This kept going until the bridge couldn’t handle it anymore and collapsed.
When did the structure fail?
Just following a few month of perfect condition, a huge wind storm caused the bridge to collapse at around 11 am on November 7, 1940.
After decades of research, Engineers and Architects developed new ways of creating the structure more stable against natural events that can occur at anytime. As time went on and we received more materials to work with, Engineers used metal wire as the cables making it safer to work with on a daily basis (At the first build they used dry grass rope). Engineers learnt many lessons and finally in 2002 the structure was rebuilt. It was completed on July 15, 2007 with many improvements. It was built in a lower time frame and is still in full condition after 13 years.
Engineering Lessons
The Tacoma Narrows Bridge had a huge lack of structural engineering skills. Every engineer is required to take any possible event into consideration when making a structure. The bridge wasn’t able to handle heavy wind conditions causing it to fail. This bridge was made as if the environment and community around it is perfect. The engineers need to work on making the bridge so it could handle any type of weather.