Name: 【地震知識】什麼樣的房子比較容易在地震中倒塌？ (Why do buildings fall in earthquakes? - Vicki V. May)
Uploaded: 2016-05-12T05:22:53.000Z
Duration: 4 min 52 s

Earthquakes have always been a terrifying phenomenon,

地震一直是令人懼怕的自然現象

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

且隨著都市的發展而越來越致命

with collapsing buildings posing one of the largest risks.

Why do buildings collapse in an earthquake,

為什麼建築物在經歷地震時會倒塌？

If you've watched a lot of disaster films,

that building collapse is caused directly by the ground beneath them

建築物倒塌是因為直接接受地表

shaking violently, or even splitting apart.

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

首先，大部分的建築物都沒有蓋在斷層上

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

況且，板塊運動是在比建築物地基還要深很多的地底發生的

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

事實上，地震對於建築物的影響

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

為了搞懂這些，建築師和工程師利用模型

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

像是二維的線狀陣列來表示柱子和橫樑

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

或是一根根棒棒糖狀的東西來表示建築的質量

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

雖然是非常簡化的模型，但還是很有幫助的

as predicting a building's response to an earthquake

Most collapses that occur during earthquakes

aren't actually caused by the earthquake itself.

Instead, when the ground moves beneath a building,

而是當建築物下方的土地搖晃時

it displaces the foundation and lower levels,

讓地基和低樓層的部分跟著移動

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:

這種震盪的大小跟兩種主因有關：

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

第一個是建築物的質量，主要集中在建築物下方

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

也就是需要多少力量才能造成相當的位移

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

伴隨著使用的建材和柱子的形狀

剛性跟建築物的高度也有很大的關聯

Shorter buildings tend to be stiffer and shift less,

較矮的建築通常剛性較大且不容易位移

while taller buildings are more flexible.

You might think that the solution is to build shorter buildlings

你也許會認為解決辦法就是盡量蓋比較矮的建築

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.

但1985年的墨西哥城大地震已經證明了那是錯的

many buildings between six and fifteen stories tall collapsed.

很多介於六到十五層樓間的大樓倒塌了

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

奇怪的是，除了那些比它們矮的房子沒有倒之外

buildings taller than fifteen stories were also less damaged,

那些超過十五層樓高的大樓也沒有倒

and the midsized buildings that collapsed

事後發現那些倒塌的中等高度建築物

were observed shaking far more violently than the earthquake itself.

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

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

頻率代表著一秒鐘前後移動循環的次數

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

週期是一次循環需要花幾秒的時間

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

而一棟建築物的自然頻率（由質量與剛性決定）

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,

增加一棟建築的質量會讓自然震動的速率變慢

while increasing stiffness makes it vibrate faster.

如果增加剛性，則會讓震動變快

So in the equation representing their relationship,

所以在表示它們之間關聯性的等式中可以看到

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,

在那次墨西哥城大地震中，發生了所謂的共振

where the frequency of the earthquake's seismic waves

happen to match the natural frequency of the midsized buildings.

剛好與中等建築物的自然頻率吻合

就像是盪鞦韆時有人在最佳的時機推你一把讓你盪更高

each additional seismic wave amplified the building's vibration

每一次的震波都會讓建築物在當時搖晃的方向下

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

現今，工程師跟地質學家與地震學家合作

to predict the frequency of earthquake motions at building sites

想要預測地震在建築工地上搖晃的頻率

in order to prevent resonance-induced collapses,

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

還有過去發生的地震資料來計算

Low frequencies of motion will cause more damage to taller

低運動頻率會對較高和剛性低的建築

while high frequencies of motion pose more threat

相反的，高運動頻率則會對較矮且剛性高的建築

to structures that are shorter and stiffer.

Engineers have also devised ways to abosrb shocks

工程師們也運用最新科技發展出吸收震盪

and limit deformation using innovative systems.

「基礎隔震」利用有彈性的樓層

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

來阻隔地基的位移造成整動建築物的搖晃

while tuned mass damper systems cancel out resonance

「調諧質塊阻尼器」則是減緩建築本身的擺動

by oscillating out of phase with the natural frequency

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

結論是，最堅固的建築並不會撐最久

字幕列表影片播放

【地震知識】什麼樣的房子比較容易在地震中倒塌？ (Why do buildings fall in earthquakes? - Vicki V. May)

increase

represent

build

mass