[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.08,0:00:10.59,Default,,0000,0000,0000,,Earthquakes have always been \Na terrifying phenomenon,
Dialogue: 0,0:00:10.59,0:00:14.05,Default,,0000,0000,0000,,and they've become more deadly\Nas our cities have grown,
Dialogue: 0,0:00:14.05,0:00:17.72,Default,,0000,0000,0000,,with collapsing buildings posing\None of the largest risks.
Dialogue: 0,0:00:17.72,0:00:20.28,Default,,0000,0000,0000,,Why do buildings collapse \Nin an earthquake,
Dialogue: 0,0:00:20.28,0:00:22.76,Default,,0000,0000,0000,,and how can it be prevented?
Dialogue: 0,0:00:22.76,0:00:24.99,Default,,0000,0000,0000,,If you've watched a lot of disaster films,
Dialogue: 0,0:00:24.99,0:00:26.10,Default,,0000,0000,0000,,you might have the idea
Dialogue: 0,0:00:26.10,0:00:29.57,Default,,0000,0000,0000,,that building collapse is caused directly\Nby the ground beneath them
Dialogue: 0,0:00:29.57,0:00:32.98,Default,,0000,0000,0000,,shaking violently,\Nor even splitting apart.
Dialogue: 0,0:00:32.98,0:00:35.30,Default,,0000,0000,0000,,But that's not really how it works.
Dialogue: 0,0:00:35.30,0:00:39.37,Default,,0000,0000,0000,,For one thing, most buildings\Nare not located right on a fault line,
Dialogue: 0,0:00:39.37,0:00:43.97,Default,,0000,0000,0000,,and the shifting tectonic plates \Ngo much deeper than building foundations.
Dialogue: 0,0:00:43.97,0:00:46.32,Default,,0000,0000,0000,,So what's actually going on?
Dialogue: 0,0:00:46.32,0:00:50.08,Default,,0000,0000,0000,,In fact, the reality of earthquakes\Nand their effect on buildings
Dialogue: 0,0:00:50.08,0:00:52.06,Default,,0000,0000,0000,,is a bit more complicated.
Dialogue: 0,0:00:52.06,0:00:55.28,Default,,0000,0000,0000,,To make sense of it,\Narchitects and engineers use models,
Dialogue: 0,0:00:55.28,0:00:59.78,Default,,0000,0000,0000,,like a two-dimensional array of lines\Nrepresenting columns and beams,
Dialogue: 0,0:00:59.78,0:01:05.43,Default,,0000,0000,0000,,or a single line lollipop with circles \Nrepresenting the building's mass.
Dialogue: 0,0:01:05.43,0:01:09.28,Default,,0000,0000,0000,,Even when simplified to this degree,\Nthese models can be quite useful,
Dialogue: 0,0:01:09.28,0:01:12.01,Default,,0000,0000,0000,,as predicting a building's response\Nto an earthquake
Dialogue: 0,0:01:12.01,0:01:14.55,Default,,0000,0000,0000,,is primarily a matter of physics.
Dialogue: 0,0:01:14.55,0:01:16.87,Default,,0000,0000,0000,,Most collapses that occur \Nduring earthquakes
Dialogue: 0,0:01:16.87,0:01:20.33,Default,,0000,0000,0000,,aren't actually caused \Nby the earthquake itself.
Dialogue: 0,0:01:20.33,0:01:23.26,Default,,0000,0000,0000,,Instead, when the ground moves\Nbeneath a building,
Dialogue: 0,0:01:23.26,0:01:26.28,Default,,0000,0000,0000,,it displaces the foundation \Nand lower levels,
Dialogue: 0,0:01:26.28,0:01:28.98,Default,,0000,0000,0000,,sending shock waves through \Nthe rest of the structure
Dialogue: 0,0:01:28.98,0:01:31.83,Default,,0000,0000,0000,,and causing it to vibrate back and forth.
Dialogue: 0,0:01:31.83,0:01:36.14,Default,,0000,0000,0000,,The strength of this oscillation\Ndepends on two main factors:
Dialogue: 0,0:01:36.14,0:01:39.20,Default,,0000,0000,0000,,the building's mass, \Nwhich is concentrated at the bottom,
Dialogue: 0,0:01:39.20,0:01:40.57,Default,,0000,0000,0000,,and its stiffness,
Dialogue: 0,0:01:40.57,0:01:44.60,Default,,0000,0000,0000,,which is the force required \Nto cause a certain amount of displacement.
Dialogue: 0,0:01:44.60,0:01:48.16,Default,,0000,0000,0000,,Along with the building's material type\Nand the shape of its columns,
Dialogue: 0,0:01:48.16,0:01:51.21,Default,,0000,0000,0000,,stiffness is largely a matter of height.
Dialogue: 0,0:01:51.21,0:01:54.10,Default,,0000,0000,0000,,Shorter buildings tend to be stiffer\Nand shift less,
Dialogue: 0,0:01:54.10,0:01:57.35,Default,,0000,0000,0000,,while taller buildings are more flexible.
Dialogue: 0,0:01:57.35,0:02:00.56,Default,,0000,0000,0000,,You might think that the solution\Nis to build shorter buildlings
Dialogue: 0,0:02:00.56,0:02:02.84,Default,,0000,0000,0000,,so that they shift as little as possible.\N
Dialogue: 0,0:02:02.84,0:02:08.70,Default,,0000,0000,0000,,But the 1985 Mexico City earthquake is\Na good example of why that's not the case.
Dialogue: 0,0:02:08.70,0:02:09.81,Default,,0000,0000,0000,,Durng the quake,
Dialogue: 0,0:02:09.81,0:02:14.02,Default,,0000,0000,0000,,many buildings between six \Nand fifteen stories tall collapsed.
Dialogue: 0,0:02:14.02,0:02:17.100,Default,,0000,0000,0000,,What's strange is that while shorter\Nbuildings nearby did keep standing,
Dialogue: 0,0:02:17.100,0:02:22.40,Default,,0000,0000,0000,,buildings taller than fifteen stories\Nwere also less damaged,
Dialogue: 0,0:02:22.40,0:02:24.56,Default,,0000,0000,0000,,and the midsized buildings that collapsed
Dialogue: 0,0:02:24.56,0:02:28.73,Default,,0000,0000,0000,,were observed shaking far more violently\Nthan the earthquake itself.
Dialogue: 0,0:02:28.73,0:02:30.59,Default,,0000,0000,0000,,How is that possible?
Dialogue: 0,0:02:30.59,0:02:34.32,Default,,0000,0000,0000,,The answer has to do with something\Nknown as natural frequency.
Dialogue: 0,0:02:34.32,0:02:35.99,Default,,0000,0000,0000,,In an oscillating system,
Dialogue: 0,0:02:35.99,0:02:41.58,Default,,0000,0000,0000,,the frequency is how many back and forth\Nmovement cycles occur within a second.
Dialogue: 0,0:02:41.58,0:02:43.73,Default,,0000,0000,0000,,This is the inverse of the period,
Dialogue: 0,0:02:43.73,0:02:47.52,Default,,0000,0000,0000,,which is how many seconds it takes\Nto complete one cycle.
Dialogue: 0,0:02:47.52,0:02:51.76,Default,,0000,0000,0000,,And a building's natural frequency,\Ndetermined by its mass and stiffness,
Dialogue: 0,0:02:51.76,0:02:55.33,Default,,0000,0000,0000,,is the frequency that its vibrations \Nwill tend to cluster around.
Dialogue: 0,0:02:55.33,0:03:00.28,Default,,0000,0000,0000,,Increasing a building's mass slows down\Nthe rate at which it naturally vibrates,
Dialogue: 0,0:03:00.28,0:03:03.84,Default,,0000,0000,0000,,while increasing stiffness\Nmakes it vibrate faster.
Dialogue: 0,0:03:03.84,0:03:06.19,Default,,0000,0000,0000,,So in the equation representing\Ntheir relationship,
Dialogue: 0,0:03:06.19,0:03:09.91,Default,,0000,0000,0000,,stiffness and natural frequency\Nare proportional to one another,
Dialogue: 0,0:03:09.91,0:03:14.18,Default,,0000,0000,0000,,while mass and natural frequency\Nare inversely proportional.
Dialogue: 0,0:03:14.18,0:03:17.66,Default,,0000,0000,0000,,What happened in Mexico City\Nwas an effect called resonance,
Dialogue: 0,0:03:17.66,0:03:20.20,Default,,0000,0000,0000,,where the frequency \Nof the earthquake's seismic waves
Dialogue: 0,0:03:20.20,0:03:24.54,Default,,0000,0000,0000,,happen to match the natural frequency\Nof the midsized buildings.
Dialogue: 0,0:03:24.54,0:03:27.46,Default,,0000,0000,0000,,Like a well-timed push on a swingset,
Dialogue: 0,0:03:27.46,0:03:31.21,Default,,0000,0000,0000,,each additional seismic wave\Namplified the building's vibration
Dialogue: 0,0:03:31.21,0:03:33.05,Default,,0000,0000,0000,,in its current direction,
Dialogue: 0,0:03:33.05,0:03:36.62,Default,,0000,0000,0000,,causing it to swing even further back,\Nand so on,
Dialogue: 0,0:03:36.62,0:03:41.30,Default,,0000,0000,0000,,eventually reaching a far greater extent\Nthan the initial displacement.
Dialogue: 0,0:03:41.30,0:03:44.68,Default,,0000,0000,0000,,Today, engineers work \Nwith geologists and seismologists
Dialogue: 0,0:03:44.68,0:03:48.70,Default,,0000,0000,0000,,to predict the frequency \Nof earthquake motions at building sites
Dialogue: 0,0:03:48.70,0:03:51.63,Default,,0000,0000,0000,,in order to prevent \Nresonance-induced collapses,
Dialogue: 0,0:03:51.63,0:03:55.02,Default,,0000,0000,0000,,taking into account factors\Nsuch as soil type and fault type,
Dialogue: 0,0:03:55.02,0:03:57.95,Default,,0000,0000,0000,,as well as data from previous quakes.
Dialogue: 0,0:03:57.95,0:04:00.100,Default,,0000,0000,0000,,Low frequencies of motion \Nwill cause more damage to taller
Dialogue: 0,0:04:00.100,0:04:02.86,Default,,0000,0000,0000,,and more flexible buildings,
Dialogue: 0,0:04:02.86,0:04:05.79,Default,,0000,0000,0000,,while high frequencies of motion\Npose more threat
Dialogue: 0,0:04:05.79,0:04:08.55,Default,,0000,0000,0000,,to structures that \Nare shorter and stiffer.
Dialogue: 0,0:04:08.55,0:04:11.32,Default,,0000,0000,0000,,Engineers have also devised ways\Nto abosrb shocks
Dialogue: 0,0:04:11.32,0:04:14.95,Default,,0000,0000,0000,,and limit deformation\Nusing innovative systems.
Dialogue: 0,0:04:14.95,0:04:17.42,Default,,0000,0000,0000,,Base isolation uses flexible layers
Dialogue: 0,0:04:17.42,0:04:21.35,Default,,0000,0000,0000,,to isolate the foundation's displacement\Nfrom the rest of the building,
Dialogue: 0,0:04:21.35,0:04:25.22,Default,,0000,0000,0000,,while tuned mass damper systems\Ncancel out resonance
Dialogue: 0,0:04:25.22,0:04:28.54,Default,,0000,0000,0000,,by oscillating out of phase\Nwith the natural frequency
Dialogue: 0,0:04:28.54,0:04:30.30,Default,,0000,0000,0000,,to reduce vibrations.
Dialogue: 0,0:04:30.30,0:04:33.84,Default,,0000,0000,0000,,In the end, it's not the sturdiest \Nbuildings that will remain standing
Dialogue: 0,0:04:33.84,0:04:35.48,Default,,0000,0000,0000,,but the smartest ones.