Of all the engineering projects humanity has ever built, the one that means the most to people in non-engineering language is the bridge. Despite being relatively simple concepts, the fact that bridges connect people and places that are otherwise unreachable is some powerful symbolism; their very existence stands as a message of connection and coming together. But what happens when they don’t work?
Bridges can fail in a number of ways, from structural deficiencies to natural disasters like earthquakes and tornadoes. But today, we have something very different for you – an example of a bridge that failed not because of an earthquake, a tornado, or any kind of great disaster. Instead, this bridge was felled by something far more simple: the wind. This is the story of the original Tacoma Narrows Bridge, the bridge that just blew over.
The Puget Sound
The Puget Sound is one of the most interesting waterways on Earth. Its shape was carved out by glaciers in the last ice age, forming an almost labyrinthine system of waterways that slide between the various stretches of land. But don’t be fooled by how small they look in satellite photos: these waterways are huge. Though we could fascinate you with hours of details about everything from settlement to logging to Boeing to Starbucks to Boeing again to Amazon, instead what we’ll have you focus on for now is the fact that there’s a bit of land jutting into the Sound called the Kitsap Peninsula, so named after Chief Kitsap of the indigenous Suquamish people. Side note here: copying names from American Indians is kind of a thing for this area. Seattle is named after Chief Si’ahl, a leader of both the Suquamish and Duwamish peoples, and Tacoma is named after the original Indian name of nearby Mt. Rainier. But I digress.
The Kitsap Peninsula is rather sparsely populated, at least relative to the eastern side of the Puget Sound. Nonetheless, it’s an important part of the area, being the location of both the city of Bremerton and a nearby United States navy yard. It was here that the geography of the Puget Sound worked against the Washington state government. If someone from, to pick a place completely at random, Tacoma, wanted to get to Bremerton by land, they had to go all the way around the Puget Sound to the south, then swing back up to the north where the Kitsap Peninsula connects with the mainland.
This was a pain for local officials in Tacoma. The state government had boats to and from the areas (and indeed still does, in the form of the ferry service), but those take quite a bit of time to load and unload, especially for what’s otherwise such a short distance. It’s here, in 1923, that the first serious proposals for a bridge to the Kitsap Peninsula are found.
I’ve Got a Bridge to Sell You
Several bridge architects were consulted, including Joseph B. Strauss, the designer of the Golden Gate Bridge, but there was one, tiny, pesky little problem perniciously referred to as “lack of funds”. Proposals by city officials to pay for the bridge using tolls on roads and bridges weren’t going to be nearly enough for the construction. On top of that, a private ferry company had a contract to utilize The Narrows, the waterway that the bridge was going to cross, and the government would have to buy that contract out. In short, a lot of money. This made the const-benefit analysis rather unclear, but the Army and the Navy were all in on the project, managing both the aforementioned naval base in Bremerton and a couple of nearby forts. Note that apparently being “all in” in this instance doesn’t mean “helping to pay for it”. So this bridge was going to happen, money troubles or not.
An engineer named Clark Eldridge presented a conventional suspension bridge design, and the Washington Toll Bridge Authority created for the project requested the requisite $11 million ($185 million in today’s money) from the federal Public Works Administration, seemingly resigned to the fact that this bridge was going to hang off the budget like an albatross. But then came their savior: an engineer by the name of Leon Solomon Moisseiff, a bridge builder from New York who also worked on the Golden Gate Bridge. He and another man, Frederick Lienhard, had published a paper that argued for a different engineering approach to bridge-building, one that would require less resources than a conventional design. The details of these differences are complex and difficult to understand on a cursory reading, which is probably why when Moisseiff petitioned the PWA to build the bridge at a discount of almost half the original amount, they accepted.
Moisseiff got to work, with construction starting in late September of 1938. Nineteen months and $6.4 million ($116.2 million today) later, the bridge was done, described by Moisseiff as “the most beautiful bridge in the world”, because of course he’d say that, but whatever. Everyone was happy – Moisseiff got a boost to his reputation, Tacoma residents got a bridge to Kitsap, Tacoma city officials didn’t have to bankrupt themselves to pay for it, and the armed forces got their pet project. Except that this new bridge, as it turned out, really didn’t want to exist.
The day the bridge opened on July 1, 1940, local residents noticed something when they crossed it for the first time: whenever a mild wind hit the bridge, it would twist. Not in a vertigo-sense where it’s all in your head; this movement was very noticeable, to the point where when the wind was strong enough it would cause the alternate halves of the bridge to rise and fall up to four or five feet. That’s not something bridges are supposed to do, last I checked. One resident said that the bridge would oscillate so much that “the car in front of you would disappear”. Which is… terrifying.
According to Moisseiff’s design philosophy, this was completely normal, and was in keeping with the blueprint of the bridge; it was supposed to be flexible in the wind, it would break otherwise. The movement of the bridge had been known to its construction workers, who nicknamed it “Galloping Gertie”, and multiple modifications were made to the bridge to ensure that it could properly withstand any high winds. Nothing to worry about, really.
But worry those residents did, and the Toll Bridge Authority hired Professor Frederick Burt Farquharson, engineering professor at the University of Washington, to help them out. Farquharson performed some wind-tunnel tests and eventually came up with a couple of solutions after the test concluded on November 2. The details of these tests and proposed solutions became a rather moot point, however, when just five days later the bridge stopped oscillating. Because it was gone.
On the morning of November 7, 1940, a particularly strong wind began to blow in the Narrows, up to 40 miles per hour (64 km/hr). The bridge proceeded to start its usual wavy motions, but it was clear this time that something was seriously wrong. A local journalist by the name of Leonard Coatsworth described the scene: “Around me I could hear concrete cracking. I started back to the car to get the dog, but was thrown before I could reach it. The car itself began to slide from side to side on the roadway. I decided the bridge was breaking up and my only hope was to get back to shore. On hands and knees most of the time, I crawled 500 yards or more to the towers.”
That dog in Mr. Coatsworth’s car, a cocker spaniel named Tubby, would be the only fatality of the bridge collapse. Professor Farquharson, who just happened to be near the bridge that day, and a news photographer attempted to rescue him, but the dog was understandably terrified and bit one of them, forcing them to leave him behind. Tubby and the car he was in were never recovered.
Questions started being asked almost immediately. How did this bridge, which was rated to withstand 120mph winds, completely collapse after just 40? Moisseiff had no answer to give; he was as dumbfounded as anyone else. Even today, there’s some conflicting information as to what actually happened with explanations like “vortices” and “resonance” and the like. The answer is a complex one, but here’s the officially accepted explanation.
At the time of the Tacoma Narrows Bridge being built, suspension bridges were still a relatively new engineering concept. Moisseiff came up with the idea that, instead of using trusses on the sides of the deck (the “bridge” part of the bridge), he would use two solid metal plates running the entire length of the bridge. In short, the sides of the bridge had no holes in them to let wind through, which made the bridge catch the wind instead.
This made the bridge vulnerable to a phenomenon known as “aeroelastic flutter”. To demonstrate this, imagine holding a piece of paper up to your face, and then blow on it lengthwise. The edge will proceed to vibrate up and down. Scale that up a billion times – don’t quote me on that number, I’m not a bridge builder – and you have basically what happened with the Tacoma Narrows Bridge. But whereas a piece of paper is light and thin and can sufficiently handle those vibrations, a bridge is a bridge, and it’s going to break.
So, what happened was that the wind was striking the bridge, causing one side to go up and the other to go down. But the bridge was so heavy and the wind so strong that when gravity pulled it back down, it went further than before, and then the wind would push it in the opposite direction, which would make the resulting twist back upward even more violent than the first. Picture that same piece of paper from before, now imagine that when you blow on it, the edge vibrates faster and faster until it tears itself apart. That’s basically what happened here.
Eventually, after all that twisting around, something broke. Most likely, the suspension cables. The breaking of some cables likely increased the load on the remaining cables that hadn’t broken, which caused them to snap as well. And then, the bridge proceeded to fall 195 feet (~60m) into the Puget Sound below.
Although the collapse of the bridge was a very obvious embarrassment for everyone involved in its construction, this may be one of the few instances where the silver linings outweigh the losses. In the aftermath of the bridge, extensive research was done on the causes and the physics involved. The results of that research would go on to influence future long-span bridge designs, making them more or less wind-proof. It also served to broaden our understanding of aeroelastic flutter, which was good because that’s a thing that used to affect the wings of passenger aircraft, also a terrifying prospect. Imagine you’re in a plane. Then, the wing shakes itself apart. (Our apologies to people about to board a flight. It hasn’t happened for decades, don’t be worried.)
Even for Leon Moisseiff, whose reputation would be tarnished by the collapse, the disaster proved a chance for him to get something out of it. He would dedicate the last few years of his life to an exhaustive study of everything that went wrong, before dying of a heart attack in 1943. He would not live to see that bridge rebuilt, which still stands today, but even with this failure under his belt, the Moisseiff award would be created later that decade to recognize supreme contributions to the field of civil engineering. It just goes to show that failure is but a stepping stone to success.