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

00:04:51
https://www.youtube.com/watch?v=H4VQul_SmCg

概要

TLDRThe video explains why buildings collapse during earthquakes and how this can be prevented. It highlights that most collapses are not directly caused by the ground shaking but rather by the displacement of the building's foundation, leading to vibrations. Key factors include the building's mass and stiffness, with both affecting its natural frequency. The concept of resonance shows that if the earthquake frequency matches a building's natural frequency, it can lead to amplified vibrations and potential collapse. Engineers today use various techniques, such as base isolation and tuned mass dampers, to absorb shocks and enhance stability, emphasizing that smart design is more crucial than sheer sturdiness in earthquake-prone areas.

収穫

  • 🏗️ Buildings collapse due to foundation displacement and vibrations.
  • 📊 Engineers use models to predict building responses during earthquakes.
  • ⚖️ A building's mass and stiffness are key to its stability.
  • 🎵 Natural frequency determines how a building vibrates during quakes.
  • 🔄 Resonance can amplify building vibrations, leading to collapse.
  • 📉 Shorter buildings aren't always safer; dynamics vary based on height.
  • 🔬 Engineers collaborate with geologists to enhance earthquake predictions.
  • ⚙️ Innovative systems like base isolation help protect buildings.
  • 🔍 Building materials and design can significantly affect earthquake resilience.
  • 🤓 Smart engineering is essential for earthquake safety, not just sturdy design.

タイムライン

  • 00:00:00 - 00:04:51

    The video discusses the increasing danger of earthquakes as urban development creates higher risks of building collapses. It explains that building collapses during earthquakes aren't solely due to violent ground shaking, but rather the seismic wave displacements impacting the foundations, causing oscillations based on the building's mass and stiffness. The video highlights that shorter buildings sway less in an earthquake, but mid-sized structures can collapse even when shorter buildings remain standing due to a phenomenon known as resonance, where the frequency of ground motion matches the building's natural frequency.

マインドマップ

ビデオQ&A

  • What causes buildings to collapse during earthquakes?

    Buildings collapse primarily due to the displacement of their foundations and lower levels, causing shock waves to vibrate through the structure.

  • What is natural frequency in the context of buildings?

    Natural frequency is the specific frequency at which a building's vibrations tend to cluster, influenced by its mass and stiffness.

  • What is resonance, and how does it affect buildings?

    Resonance occurs when the frequency of seismic waves matches a building's natural frequency, amplifying its vibrations and potentially causing collapse.

  • How do engineers prevent resonance-induced collapses?

    Engineers predict earthquake frequencies at building sites and design systems, like base isolation and tuned mass dampers, to absorb shocks and reduce vibrations.

  • Are shorter buildings always safer during an earthquake?

    Not necessarily; the 1985 Mexico City earthquake showed that midsized buildings can be more at risk due to resonance effects.

  • What factors influence a building's response to earthquakes?

    Key factors include the building's mass, stiffness, height, and the soil type at the construction site.

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オートスクロール:
  • 00:00:06
    Earthquakes have always been a terrifying phenomenon,
  • 00:00:10
    and they've become more deadly as our cities have grown,
  • 00:00:13
    with collapsing buildings posing one of the largest risks.
  • 00:00:17
    Why do buildings collapse in an earthquake,
  • 00:00:19
    and how can it be prevented?
  • 00:00:22
    If you've watched a lot of disaster films,
  • 00:00:24
    you might have the idea
  • 00:00:25
    that building collapse is caused directly by the ground beneath them
  • 00:00:29
    shaking violently, or even splitting apart.
  • 00:00:32
    But that's not really how it works.
  • 00:00:35
    For one thing, most buildings are not located right on a fault line,
  • 00:00:39
    and the shifting tectonic plates go much deeper than building foundations.
  • 00:00:43
    So what's actually going on?
  • 00:00:46
    In fact, the reality of earthquakes and their effect on buildings
  • 00:00:49
    is a bit more complicated.
  • 00:00:51
    To make sense of it, architects and engineers use models,
  • 00:00:55
    like a two-dimensional array of lines representing columns and beams,
  • 00:00:59
    or a single line lollipop with circles representing the building's mass.
  • 00:01:05
    Even when simplified to this degree, these models can be quite useful,
  • 00:01:09
    as predicting a building's response to an earthquake
  • 00:01:12
    is primarily a matter of physics.
  • 00:01:14
    Most collapses that occur during earthquakes
  • 00:01:16
    aren't actually caused by the earthquake itself.
  • 00:01:20
    Instead, when the ground moves beneath a building,
  • 00:01:23
    it displaces the foundation and lower levels,
  • 00:01:26
    sending shock waves through the rest of the structure
  • 00:01:28
    and causing it to vibrate back and forth.
  • 00:01:32
    The strength of this oscillation depends on two main factors:
  • 00:01:36
    the building's mass, which is concentrated at the bottom,
  • 00:01:39
    and its stiffness,
  • 00:01:40
    which is the force required to cause a certain amount of displacement.
  • 00:01:44
    Along with the building's material type and the shape of its columns,
  • 00:01:48
    stiffness is largely a matter of height.
  • 00:01:51
    Shorter buildings tend to be stiffer and shift less,
  • 00:01:54
    while taller buildings are more flexible.
  • 00:01:57
    You might think that the solution is to build shorter buildlings
  • 00:02:00
    so that they shift as little as possible.
  • 00:02:02
    But the 1985 Mexico City earthquake is a good example of why that's not the case.
  • 00:02:08
    During the quake,
  • 00:02:09
    many buildings between six and fifteen stories tall collapsed.
  • 00:02:13
    What's strange is that while shorter buildings nearby did keep standing,
  • 00:02:18
    buildings taller than fifteen stories were also less damaged,
  • 00:02:22
    and the midsized buildings that collapsed
  • 00:02:24
    were observed shaking far more violently than the earthquake itself.
  • 00:02:28
    How is that possible?
  • 00:02:30
    The answer has to do with something known as natural frequency.
  • 00:02:34
    In an oscillating system,
  • 00:02:35
    the frequency is how many back and forth movement cycles occur within a second.
  • 00:02:41
    This is the inverse of the period,
  • 00:02:43
    which is how many seconds it takes to complete one cycle.
  • 00:02:47
    And a building's natural frequency, determined by its mass and stiffness,
  • 00:02:51
    is the frequency that its vibrations will tend to cluster around.
  • 00:02:55
    Increasing a building's mass slows down the rate at which it naturally vibrates,
  • 00:03:00
    while increasing stiffness makes it vibrate faster.
  • 00:03:03
    So in the equation representing their relationship,
  • 00:03:06
    stiffness and natural frequency are proportional to one another,
  • 00:03:09
    while mass and natural frequency are inversely proportional.
  • 00:03:14
    What happened in Mexico City was an effect called resonance,
  • 00:03:17
    where the frequency of the earthquake's seismic waves
  • 00:03:20
    happen to match the natural frequency of the midsized buildings.
  • 00:03:24
    Like a well-timed push on a swingset,
  • 00:03:27
    each additional seismic wave amplified the building's vibration
  • 00:03:31
    in its current direction,
  • 00:03:32
    causing it to swing even further back, and so on,
  • 00:03:36
    eventually reaching a far greater extent than the initial displacement.
  • 00:03:41
    Today, engineers work with geologists and seismologists
  • 00:03:44
    to predict the frequency of earthquake motions at building sites
  • 00:03:48
    in order to prevent resonance-induced collapses,
  • 00:03:51
    taking into account factors such as soil type and fault type,
  • 00:03:55
    as well as data from previous quakes.
  • 00:03:57
    Low frequencies of motion will cause more damage to taller
  • 00:04:01
    and more flexible buildings,
  • 00:04:02
    while high frequencies of motion pose more threat
  • 00:04:05
    to structures that are shorter and stiffer.
  • 00:04:08
    Engineers have also devised ways to abosrb shocks
  • 00:04:11
    and limit deformation using innovative systems.
  • 00:04:14
    Base isolation uses flexible layers
  • 00:04:17
    to isolate the foundation's displacement from the rest of the building,
  • 00:04:21
    while tuned mass damper systems cancel out resonance
  • 00:04:25
    by oscillating out of phase with the natural frequency
  • 00:04:28
    to reduce vibrations.
  • 00:04:29
    In the end, it's not the sturdiest buildings that will remain standing
  • 00:04:33
    but the smartest ones.
タグ
  • earthquake
  • building collapse
  • natural frequency
  • resonance
  • engineering
  • design
  • seismic waves
  • base isolation
  • tuned mass damper
  • earthquake prevention