Placeholder Image

Subtitles section Play video

  • Earthquakes have always been a terrifying phenomenon,

  • and they've become more deadly as our cities have grown,

  • with collapsing buildings posing one of the largest risks.

  • Why do buildings collapse in an earthquake,

  • and how can it be prevented?

  • If you've watched a lot of disaster films,

  • you might have the idea

  • that building collapse is caused directly by the ground beneath them

  • shaking violently, or even splitting apart.

  • But that's not really how it works.

  • For one thing, most buildings are not located right on a fault line,

  • and the shifting tectonic plates go much deeper than building foundations.

  • So what's actually going on?

  • In fact, the reality of earthquakes and their effect on buildings

  • is a bit more complicated.

  • To make sense of it, architects and engineers use models,

  • like a two-dimensional array of lines representing columns and beams,

  • or a single line lollipop with circles representing the building's mass.

  • Even when simplified to this degree, these models can be quite useful,

  • as predicting a building's response to an earthquake

  • is primarily a matter of physics.

  • Most collapses that occur during earthquakes

  • aren't actually caused by the earthquake itself.

  • Instead, when the ground moves beneath a building,

  • it displaces the foundation and lower levels,

  • sending shock waves through the rest of the structure

  • and causing it to vibrate back and forth.

  • The strength of this oscillation depends on two main factors:

  • the building's mass, which is concentrated at the bottom,

  • and its stiffness,

  • which is the force required to cause a certain amount of displacement.

  • Along with the building's material type and the shape of its columns,

  • stiffness is largely a matter of height.

  • Shorter buildings tend to be stiffer and shift less,

  • while taller buildings are more flexible.

  • You might think that the solution is to build shorter buildlings

  • so that they shift as little as possible.

  • But the 1985 Mexico City earthquake is a good example of why that's not the case.

  • During the quake,

  • many buildings between six and fifteen stories tall collapsed.

  • What's strange is that while shorter buildings nearby did keep standing,

  • buildings taller than fifteen stories were also less damaged,

  • and the midsized buildings that collapsed

  • were observed shaking far more violently than the earthquake itself.

  • How is that possible?

  • The answer has to do with something known as natural frequency.

  • In an oscillating system,

  • the frequency is how many back and forth movement cycles occur within a second.

  • This is the inverse of the period,

  • which is how many seconds it takes to complete one cycle.

  • And a building's natural frequency, determined by its mass and stiffness,

  • is the frequency that its vibrations will tend to cluster around.

  • Increasing a building's mass slows down the rate at which it naturally vibrates,

  • while increasing stiffness makes it vibrate faster.

  • So in the equation representing their relationship,

  • stiffness and natural frequency are proportional to one another,

  • while mass and natural frequency are inversely proportional.

  • What happened in Mexico City was an effect called resonance,

  • where the frequency of the earthquake's seismic waves

  • happen to match the natural frequency of the midsized buildings.

  • Like a well-timed push on a swingset,

  • each additional seismic wave amplified the building's vibration

  • in its current direction,

  • causing it to swing even further back, and so on,

  • eventually reaching a far greater extent than the initial displacement.

  • Today, engineers work with geologists and seismologists

  • to predict the frequency of earthquake motions at building sites

  • in order to prevent resonance-induced collapses,

  • taking into account factors such as soil type and fault type,

  • as well as data from previous quakes.

  • Low frequencies of motion will cause more damage to taller

  • and more flexible buildings,

  • while high frequencies of motion pose more threat

  • to structures that are shorter and stiffer.

  • Engineers have also devised ways to abosrb shocks

  • and limit deformation using innovative systems.

  • Base isolation uses flexible layers

  • to isolate the foundation's displacement from the rest of the building,

  • while tuned mass damper systems cancel out resonance

  • by oscillating out of phase with the natural frequency

  • to reduce vibrations.

  • In the end, it's not the sturdiest buildings that will remain standing

  • but the smartest ones.

Earthquakes have always been a terrifying phenomenon,

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B2 US TED-Ed frequency earthquake building stiffness resonance

Why do buildings fall in earthquakes? - Vicki V. May

  • 2346 198
    Coco Hsu posted on 2016/05/12
Video vocabulary