Ep 71 Tacoma Narrows Bridge Collapse
Engineering News – Counterfeit Part Detection (0:40)
This week's engineering failure is the Tacoma Narrows Bridge (4:15). Designed by some of the same engineers (10:50) as the Golden Gate Bridge we talked about last episode, this bridge only stood for 3 months before collapsing (18:55). A full inquiry (21:00) discovered flaws in the lightweight design and a new, stronger bridge was built that’s still standing today (26:05).
Sources:
Engineering News
Tacoma Narrows Bridge
Episode Summary
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
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This week in engineering news, Counterfeit Part Detection.
Researchers at Texas A&M University developed a method to imprint a magnetic tag within manufactured hardware during fabrication. The tag is hidden and contains authentication information. They are also more permanent and unique than barcodes or QR codes.
With the US investing billions in manufacturing, ensuring security and reliable authentication is a concern.
Nowadays, someone can copy and fabricate cheaper alternatives to your designs, sometimes passing them off as yours, at a lower price and lower quality. These tags aim to stop that or at least make it more difficult. Competition to an extent is good, but if months after you put a product you worked hard to develop on that online store that's named after a rainforest, they come out with a knock off version, that would suck.
Previous methods for imprinting information are expensive and require specialized tools which creates a barrier for use.
This most recent study at Texas A&M is the first to use magnetic properties of the material to imprint information on a non magnetic part.
The method of reading the tags is still being optimized to be more secure, even looking at potential dual-authentication.
If you want to read more about the study, check out the link on the web page for this episode at failurology.ca
Now on to this week’s engineering failure; the Tacoma Narrows bridge, also known as Galloping Gertie.
We don’t normally like to do two similar failures back to back. Or at least we try our best not to. But today we’re making an exception. Last episode we talked about the Golden Gate Bridge. The structural engineer, Leon Moisseiff, who designed the Golden Gate’s basic structural design and used his “deflection theory” to design a thin, flexible roadway that flexed in the wind.
Moisseiff also designed the Tacoma Narrows Bridge, which opened three years later on July 1, 1940.
While the Golden Gate bridge is still standing 86 years later, the Tacoma Narrows Bridge collapsed on November 7th, 1940; yes the same year it opened. It was a little over three months old at the time.
This is a fairly popular failure, it’s one that is talked about alot. Probably significantly more than it would have been if it was still standing today. The bridge has actually been rebuilt twice, which we will get into shortly. And the design offered a lot of lessons for structural engineers and bridge builders throughout the US and around the world.
Construction
The bridge is located in Washington state in the US and connects Tacoma, south of Seattle, with the Kitsap Peninsula across the Puget Sound. The bridge spans the Tacoma Narrows, or The Narrows which is a strait connecting the south basin and central basin in the Puget Sound. A bridge had been proposed in this area since as early as 1889 as a railway trestle bridge, amongst other proposals by Joseph Strauss from the Golden Gate Bridge and David Steinman who was fired but later went on to design the MacKinac Bridge in Michigan in 1957. Steinman apparently predicted the bridge collapse two years before it opened.
Two of the major roadblocks were the cost of the bridge itself, and the ferry contract that was running services across the Narrows at the time.
Clark Eldridge, a Washington state engineer proposed a “tried and true” conventional bridge design that would cost $11 million USD, which is over $239 million USD today. This plan included a set of 7.5m deep trusses to sit under the roadway and stiffen it. Today experts believe if this was the bridge that had been built, it would still be standing today.
Leon Moisseiff claimed he could build the bridge for cheaper. Moisseiff and Frederick Lienhard used deflection theory to show that the stiffness of the main cables from suspenders, would absorb as much as half of the static wind pressure that would push the structure laterally. The forces would transfer to anchorages and towers. This only required 2.4m plate girders instead of the 7.5m deep trusses, resulting in a more elegant and sleek design and reduced construction costs.
Moisseiffs bridge was estimated to cost $8 million USD (almost $174 million USD today); of which $6 million would be paid by the federal Public Works Administration and the remainder would be collected from tolls. Construction took 19 months and came in under budget at $6.4 million USD.
The bridge was the third longest in the world at the time after the Golden Gate Bridge and George Washington Bridge in New York. The total length was just over 1,800m and the longest span was just over 850m. The bridge had almost 60m clearance below. But it was fairly narrow compared to the Golden Gate Bridge and was 12m wide, providing only two lanes for traffic.
The narrow plate girders proved to be the bridge's undoing. The thin girders didn’t offer much rigidity and the bridge moved easily in wind speeds over 56 kph; causing alternate halves of the center span to rise and fall a couple meters over 4-5 second intervals. The movement was felt through construction and continued after it opened, which is how the bridge got the nickname Galloping Gertie.
The state and others tried several things to stabilize the bridge, including.
Adding tie down cables to the plate girders, which were anchored to 50 ton concrete blocks on the shore. The cables snapped shortly after they were installed.
Adding a pair of inclined cables that ran from the main cables to the bridge deck in the middle of the span. They were ineffective but were still in place when the bridge collapsed.
Hydraulic buffers were installed between the towers and floor system to dampen longitudinal motion. The seals of the unit buffers were damaged when the bridge was sandblasted before paint, making the dampers effectiveness nullified.
Professor Frederick Burt Farquharson created a 1:200 scale model of the bridge and a 1:20 scale model of the deck to study methods to reduce oscillations. His study was completed on November 2, 1940, two days before the bridge collapsed, and he recommended two solutions.
Drill holes in the lateral girders along the deck so air could pass through, reducing the lift forces. This option was not chosen because it was irreversible.
Add fairings or deflectors vanes along the deck to make it more aerodynamic. This option was chosen, but not implemented before the collapse.
Collapse
The main span of the bridge collapsed on the morning of November 7, 1940 in 64 kph winds. The only casualty was a car and a Cocker Spaniel who bit rescuers trying to save him and refused to leave the car even after it fell into the water. The bridge deck had oscillated in a twisting motion that gradually increased until it tore the deck apart.
The bridge remnants were found a few weeks later at a depth of 55m. They have never been removed, and neither has the car, and remain an artificial reef. Through a few insurance policies, 80% of the bridge value was collected without incident, but one of the insurance agents pocketed the premiums and was charged with grand larceny.
The collapse was filmed by at least four people and is still shown to many engineering, architecture, and physics students today. We’ve included one of many links to the collapse on the web page for this episode at failurology.ca.
Inquiry
A commission was formed by the Federal Works Agency to explore three possible causes.
Aerodynamic instability by self-induced vibrations in the structure.
Eddy formations that might be periodic in nature
Random effects of turbulence, that is the random fluctuations in velocity of the wind.
The commission report noted three key points that led to the failure.
The principal cause of failure was “excessive flexibility” vertically and in torsion
The solid plate girder acted like an aerofoil, creating drag and lift.
Aerodynamic forces were little understood and the bridge should have been tested in a wind tunnel.
Several factors contributed the the flexibility of the bridge, including the deck was too light, the deck was too shallow (a 1:350 ratio from the center span), the side spans were too long compared to the length of the span, the cables were anchored too far from the side spans, and the width of the deck was narrows compared to the center span length (a 1:72 ratio).
The earlier design, with 7.5m deep trusses, were open webs that would allow wind to pass through. But the plate girders were solid, forcing air to go around them. The force of the wind on the solid girders caused the bridge to sway and buckle in even mild winds. To make matters worse, the vibration was transverse, so one half would rise while the other lowered. Even though these oscillations were there before the bridge even opened, it was thought that the mass of the bridge was enough to keep it together; boy were they wrong.
On the day of the failure, the wind speed created a never-before-seen twisting motion, with the two halves twisting in opposite directions with the centerline staying relatively motionless. This was associated with slippage of the cable band on the north cable at mid-span. When the band slipped, the north cable was split into two segments of uneven length. The imbalance transferred to the thing plate girders, which twisted easily, and the failure was imminent.
The twisting motion was caused by aeroelastic fluttering that did not self limit, meaning that it continued to grow with each swing. On top of that, after some time, the forces of the wind were outweighed by the forces of the bridge swinging back and forth. Eventually, the swing was more than the suspender cables could hold and several cables failed. When the weight of the deck was transferred to the remaining cables, they were overloaded and broke, causing most of the bridge deck to fall into the water.
Moisseiff visited the bridge a week after the collapse but was “completely at a loss to explain the collapse”. Even though his design pushed the boundaries, it was still within accepted theory at the time. Moisseiff’s deflection theory has now been supplemented by aerodynamic analysis to prevent a repeat of the Tacoma Narrows Bridge.
At the time of the collapse, there was almost no recognition amongst bridge engineers at the time that the wind created vertical movement at all. They added stiffening trusses for sideways movement of the cables and roadway from traffic loads and temperature changes, but had no means to accommodate wind or vertical movement. Even though earlier bridges had failed due to wind, they believed those occurred due to heavy traffic loading and poor workmanship. As well, bridges such as the 1883 Brooklyn Bridge were designed to stabilize against wind stresses. But the perspective at the time the Tacoma Narrows Bridge was built was more on visual preference over structural engineering.
The bridge has been studied by many people smarter than us over the years. We are not going to get into all the different theories. But if you want to, please check out the links on our website at failurology.ca for more info.
New Bridge
The cable anchors, tower pedestals and most of the substructure that survived the collapse were reused to create a replacement bridge that opened in 1950. The towers themselves deflected 3.7m towards the shore as a result of the collapse and they were dismantled. The replacement bridge exceeded its traffic capacity in the late 90s, early 2000s and a second parallel bridge was added to carry eastbound traffic while the 1950s bridge carried westbound traffic. All three bridges are called the Tacoma Narrows Bridge, just to keep it interesting.
For future bridges, engineers incorporated aerodynamics into the design, and wind tunnel testing became mandatory. In the US, bridges receiving federal funding are required to have their design subjected to a 3D wind tunnel analysis model.
The Bronx-Whitestone bridge, which is a similar design that spans the East River, was reinforced in 1943 with 4.3m steel trusses on both sides. This was intended to weigh down and stiffen the bridge. In 2013, the trusses were replaced with aerodynamic fiberglass fairings on both sides of the bridge deck.
A December 1, 1951 windstorm revealed instabilities on the Golden Gate Bridge similar to the Tacoma Narrows Bridge. In 1953 and 1954 lateral and diagonal bracing was installed to connect the lower chords and the two side truss to stiffen the bridge deck in torsion.
Because of the Tacoma Narrows Bridge collapse, suspension bridge designs reverted to the deeper and heavier truss design until box girder bridges were developed in the 1960s. Although those had their own issues, check out Ep 22 of this podcast about the West Gate Bridge in Melbourne Australia for more.
Moisseiff’s deflection theory has now been supplemented by aerodynamic analysis to prevent a repeat of the Tacoma Narrows Bridge. And today there are modeling software programs that assist engineers with complex calculations. I would guess most engineers use some type of software to assist them, be it spreadsheets, online calculators, or modeling programs. I use a few different ones depending on what I need to calculate. It’s much faster than by hand, easier to tweak if you want to test a few options, and less likelihood for error.
So there you have it, the Tacoma Narrows Bridge collapse. Even though the bridge collapsed three months after opening, it has been studied by engineers, physicists, and mathematicians for decades.
For photos, sources and an episode summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to Failurology, so more people can find us. If you want to chat with us, our Twitter handle is @failurology, you can email us thefailurologypodcast@gmail.com, you can connect with us on Linked In or you can message us on our Patreon page. Check out the show notes for links to all of these. Thanks, everyone for listening. And tune in to the next episode where we’ll talk about the Mount Polley Tailings Dam failure near Williams Lake, British Columbia.
Bye everyone, talk soon!