One of the most fundamental jobs of an engineer is to compare loading conditions to strengths. If the loads exceed the strengths, you know you’ve got a problem. Buildings and other structures face a huge variety of loads, including floods, snow, rain, ice, earthquakes, and crowds of people. One of the most interesting forces faced by civil structures is the wind. Hey I’m Grady and this is Practical Engineering. Today we’re diving into one of the classic case studies of engineering failure: the Tacoma Narrows Bridge.
A bridge is a quintessential civil structure. Humanity’s need to get from one place to another without getting wet is as old as history itself. And for so many years, there was one force with which bridge engineers had to contend: gravity. The fundamental question of bridge design was this: how can we hold up the structure itself and all the people and vehicles that may cross against the force of gravity pulling them downward. And secondary to that, how can we do it economically, for the least cost to the public, since most bridges are funded by the taxpayer. So over time, bridge designs evolved with our understanding of structural engineering and ability to produce better construction materials towards lighter and more efficient shapes, one of those shapes being the suspension bridge.
A suspension bridge is essentially just a deck, two towers, two main cables, and connector rods which suspend the deck, hence the name. The primary advantage of suspension bridges is that they can so efficiently span long distances with only two towers, reducing the amount of material required, and more importantly, the cost. This advantage of being able to span long distances while minimizing material gives suspension bridges their iconic slender and graceful appearance. But that same lack of material reduces the rigidity and stiffness of the structure. Where, before, bridges were generally stiff enough that gravity was the only load that needed to be considered, now a new force started to impact their designs: the wind.
In July 1940, the Tacoma Narrows bridge opened to traffic between Tacoma, Washington and the Kitsap Peninsula. At the time, it was the third-longest suspension bridge in the world. Financing construction of the bridge was a major obstacle, which led the state to pursue an innovative design. Rather than the originally-proposed trusses, the bridge used two narrow plate girders to stiffen the deck, giving the bridge its iconic steel ribbon appearance across the Puget Sound. Unfortunately that analogy extended beyond its appearance. Even during construction, it was apparent that the bridge was too flexible even under moderate winds. Construction workers gave it the nickname “Galloping Gertie.” Only four months after it opened, the bridge collapsed in dramatic fashion. In fact, this failure was so dramatic, that there’s a good chance you’ve seen this video before. So what’s happening here?
You’ve probably heard of resonance. This is the phenomenon where a periodic force syncs up with the natural frequency of a system. The classic 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 why some chimneys are equipped with helical vanes to create turbulence and break up the vortices. The day of its failure, the Tacoma Narrows Bridge did experience resonance from the vortex shedding. You can see this in the vertical undulations for which the bridge was famous. But this resonance isn’t why it failed. About 45 minutes before failure, a different kind of oscillation started.
You can see in the historical footage that, right before failure, the bridge isn’t oscillating vertically, but in a twisting or torsional motion. The reason for this change in oscillation is still debated, but one of the best suggestions has has to do with the aerodynamics of the bridge. Rather than a truss through which wind can flow, this shape of the Tacoma Narrows Bridge with the 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, in locations that actually amplify the twisting motion. As the bridge returned to its natural state, its momentum twisted it in the other direction where the wind could catch it and continue the twisting. This phenomenon is called aeroelastic flutter. It’s the same reason that a strap or sheet of paper vibrates in the wind. It’s a completely separate mechanism than resonance from vortex shedding, because the periodic forces are self induced from the naturally unstable aerodynamic shape of the bridge. This torsional flutter eventually created too much stress in the suspension cables, and the bridge failed.
One way that modern bridges avoid flutter is to include a gap in the center of the deck so that the pressures on either side can equalize. I cut a slot in my model, and sure enough the vibrations almost completely stopped. Another option is just to make the bridge deck more aerodynamic to avoid creating vortices that push and pull on the structure. Of course, bridges aren’t the only civil structures affected by the wind. Take a look at the very first Practical Engineering video about Tuned Mass Dampers to learn about how wind-induced motion can be mitigated in skyscrapers. For a simpler example, take a look outside at just about any high voltage power line. You might notice small devices hanging near the insulators at each pole. These are stockbridge dampers that help suppress wind-induced vibration on long cables and signs. And of course, other types of engineers contend with flutter as well. I’ve heard that airplanes are designed for wind loads, but I can’t confirm it.
These days, we have a much better understanding of the wide variety of loading conditions that can be faced by buildings and other structures. But, much of our current understanding has come from failures of the past. The case of the Tacoma Narrows bridge is a well-known cautionary tale that’s discussed in engineering and physics classrooms across the world. The main lesson isn’t necessarily that you should make sure to consider aeroelastic effect when you design a suspension bridge (even though you definitely always should), but I think more importantly it’s a reminder of how profoundly capable we are of making mistakes. When you push the envelope, you have to be vigilant because things that didn’t matter before start to become important. Unanticipated challenges are a cost of innovation and that’s something that we can all keep in mind. Thank you for watching, and let me know what you think.