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

為甚麼地震時建築物會倒塌？- 薇薇V.梅

譯者: 家平 林 審譯者: 瑞文Eleven 林Lim

Earthquakes have always been a terrifying phenomenon,

地震一直以來都是可怕的現象

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

因為我們的城市變大 同時也變得更加危險

with collapsing buildings posing one of the largest risks.

其中最危險的是倒塌的建築物

Why do buildings collapse in an earthquake,

為甚麼建築物會在地震時倒塌呢

and how can it be prevented?

又該如何預防呢

If you've watched a lot of disaster films,

如果你看過很多災難片

you might have the idea

你可能會有個想法

that building collapse is caused directly by the ground beneath them

建築物倒塌 是底下的地面所造成的

shaking violently, or even splitting apart.

地面劇烈地顫動甚至裂開

But that's not really how it works.

但是這不全然是地面所造成的

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

一方面是大多數建築物 並不會座落在斷層線上

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

而且板塊的移動處 比建築物的地基還要更深得多

So what's actually going on?

那麼甚麼才是真正的原因呢

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

實際上地震對建築物的影響

is a bit more complicated.

是更複雜的

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

建築師和工程師利用模型來理解 地震對建築物的影響

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

像是二維陣列 代表立柱和橫梁

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

或是以單根棒棒糖狀的圓球 來代表建築的質量

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

即使把模型簡化成這種程度 這些模型還是相當有用

as predicting a building's response to an earthquake

這些模型還是可用來預測建築物 對地震的反應

is primarily a matter of physics.

主要是物理的問題

Most collapses that occur during earthquakes

大多數發生坍塌的地震

aren't actually caused by the earthquake itself.

實際上並不是由地震本身所引起

Instead, when the ground moves beneath a building,

當地殼在建築下方移動時

it displaces the foundation and lower levels,

它取代了地基與較低的樓層

sending shock waves through the rest of the structure

該結構的其餘部分通過餘波

and causing it to vibrate back and forth.

並使其來回振動

The strength of this oscillation depends on two main factors:

振盪的強度 取決於兩個重要因素

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

建築物集中在底部的質量

and its stiffness,

還有其剛性

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

後者是造成某種程度移位的力量

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

加上建材和柱子的形狀

stiffness is largely a matter of height.

剛性與建物高度關係極大

Shorter buildings tend to be stiffer and shift less,

較低的建築物比較堅硬 移動程度較小

while taller buildings are more flexible.

較高的建築物移動性就比較高

You might think that the solution is to build shorter buildlings

你可能想那解決的方法就是 蓋低一點的建築物

so that they shift as little as possible.

它們的搖動會比較小

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

但是在1985年墨西哥市地震 證明並非如此

During the quake,

在那次地震中

many buildings between six and fifteen stories tall collapsed.

許多6到15樓的建築物倒塌

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

奇怪的是，附近較低的建築物沒倒

buildings taller than fifteen stories were also less damaged,

15樓以上的建築物也較少受損

and the midsized buildings that collapsed

那些倒塌的中型建築物

were observed shaking far more violently than the earthquake itself.

搖擺的程度比地震的振幅還大

How is that possible?

為什麼呢

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

答案是［自然頻率］

In an oscillating system,

在一個擺動的系統裡面

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

頻率是一秒內來回擺動的次數

This is the inverse of the period,

這是週期(擺動一次的秒數)的倒數

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

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

建築物的自然頻率 由它的質量和剛性決定

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

是它振動集中的頻率

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

當建築物的質量增加 自然頻率會降低

while increasing stiffness makes it vibrate faster.

當剛性增加時，振動也會加快

So in the equation representing their relationship,

以一個公式來表現這些之間的關係

stiffness and natural frequency are proportional to one another,

剛性和自然頻率成正比

while mass and natural frequency are inversely proportional.

質量和自然頻率則成反比

What happened in Mexico City was an effect called resonance,

墨西哥市地震是共振的结果

where the frequency of the earthquake's seismic waves

地震波的頻率

happen to match the natural frequency of the midsized buildings.

正好與中型建築物的 自然頻率相同

Like a well-timed push on a swingset,

正如在鞦韆的最適合點推一下

each additional seismic wave amplified the building's vibration

接下來的每一個地震波擴大了

in its current direction,

建築物往同一方向的震動

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

造成它更加往那個方向擺動

eventually reaching a far greater extent than the initial displacement.

最後達到比原來更大的移位

Today, engineers work with geologists and seismologists

現今，工程師、地質學家 和地震學家共同合作

to predict the frequency of earthquake motions at building sites

在建築物現場預測地震振動的頻率

in order to prevent resonance-induced collapses,

以預防共振引起的倒塌

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

他們考量土壤與斷層的類型

as well as data from previous quakes.

並參考以往地震的數據

Low frequencies of motion will cause more damage to taller

低頻的振動對較高及 較有彈性的建築物

and more flexible buildings,

傷害性較強

while high frequencies of motion pose more threat

反之，高頻率的振動威脅到的是

to structures that are shorter and stiffer.

較低與高剛性的建築物

Engineers have also devised ways to abosrb shocks

工程師採用創新的系統

and limit deformation using innovative systems.

既能吸收振動 又可防止建築物變形

Base isolation uses flexible layers

建物與地基之間

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

與用彈性層加以隔離

while tuned mass damper systems cancel out resonance

讓阻尼器抵銷共振

by oscillating out of phase with the natural frequency

以自然頻率消除振盪

to reduce vibrations.

來降低振動

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

結論：不是最堅固 而是最聰明的建築物

but the smartest ones.

可以屹立不搖