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

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


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. Durng 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.

100 thoughts on “Why do buildings fall in earthquakes? – Vicki V. May

  1. such a great way to explain. The worst motion through for any building is upward motion, the slighted bit of upward motion when already shaking side to side lifts columns right off their foundations.

  2. Earthquakes are the most terrifying phenomenon I've ever experienced. I felt like I wasn't going to make it every time they happened.

  3. Why do buildings fail in Earthquakes? Seen the collapse of the second tower of 9/11 ? One floor of the building was compromised, the upper floors accelerated ONE floors worth & the structures couldn't support the rest of the building. Notice how demolition of buildings is typically done? They compromise one floors worth so that the whole of the upper structure moves to "replace" the missing bottom floor…Concrete is perhaps the worst material to use in construction…rebar is used to "reinforce" the utterly poor performance of bending forces…check the stress forces of concrete compared to virtually everything else…it is used because it is vastly cheaper to build than with anything else. Build your three-story home with 300mm stainless-steel tubing with Aluminum panels in & out, cross-braced inside…you would die of internal injuries before the building would fail.

  4. Umbrellas are a very bad metaphor for protection in this video. It took some effort to focus on the main points rather than on those silly umbrellas. You can't see what's falling on you and run away when an umbrella covers more than half of your view field, and covers the part where the falling objects come from.

    Please be more careful about choosing your metaphors.

  5. "Dlaczego podczas trzesien ziemi wala sie budynki". Hehe, dobre. To rownie dobrze mozna postawic pytanie "Dlaczego podczas rzucenia butelka o sciane ona sie zbije"

  6. So the world trade center was actually brought down by a government targeted earthquake test, my oh my how the brainwashed sheep will cling to the lies they were taught and dismiss the hard facts that oppose them

  7. No wonder they got so many earthquakes, everybody is carrying an opened umbrella on a sunny day, that's so much bad luck

  8. No, the frequency is not inversly proportional to the mass… it's inversly proportional with the square root of mass. Learn physics before you make a video about physics.

  9. I do not get why people leaving their comments at this video are so annoyed by umbrellas?
    Drawing faces is not so important to this film.
    Well, nor are umbrellas.
    It’s just funny for me.

  10. The umbrella head thing is very clever! I definitely want to use it in my projects. I was looking for a way to avoid drawing heads – faces but couldn't find a better solution than this one lol. And contrary to what most people think, it's not because we cannot draw heads or faces. Many people avoid drawing them for various reasons. This is a genius way to do it imo.

  11. Considering that, engineers could really design buildings that can withstand intensity 10 and beyond. But thats only the for substructure and superstructure. That's only half of the story. Another critical thing is the earth beneath a building's foundation which we only have limited engineering technology to make it resilient to quakes (piles and compaction). No matter how adoptive the building is against quakes, when the earth it is resting on is severly disturbed by quakes, damages occur.

  12. why did they use the Japanese buildings for the Mexico city scene, also whats with the umbrellas make a fac for them, these guys are very very very lazy 1.7 stars out of 5

  13. Nobody can stop earthquakes nobody can save live of the people.
    Best thing is simple living and high thinking

  14. I do appreciate the lesson, BUT … Who thought that was a good idea? Really!? I cannot show this to my architecture students (in Japan) with such over-stylised characters.

  15. The guys who think its people covering faces with umbrella need to think again.

    Its edna from incredibles poking her nose with a cane😂

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