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. This video is sponsored by the Great Courses
Plus. More on that later. 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 create better construction
materials into 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 which
is 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
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