Name: Why doesn’t the Leaning Tower of Pisa fall over? - Alex Gendler
Uploaded: 2019-12-04T06:30:54.000Z
Duration: 5 min 6 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

In 1990, the Italian government  enlisted top engineers

to stabilize Pisa's famous Leaning Tower.

There'd been many attempts to right the tower during its 800 year history,

but this team's computer models revealed  the urgency of their situation.

They projected the tower would topple if  it reached an angle of 5.44 degrees—

No one knew how the tower was still  standing, but the crisis was clear:

they had to solve a problem that  stumped centuries of engineers,

it's helpful to understand why the  tower tilted in the first place.

In the 12th century, the wealthy  maritime republic of Pisa

set about turning its cathedral square  into a magnificent landmark.

Workers embellished and enlarged  the existing church,

and added a massive domed  baptistry to the plaza.

In 1173, construction began on a  free-standing campanile, or bell tower.

The engineers and architects of the  time were masters of their craft.

they knew far less about the  ground they stood on.

Pisa's name comes from a  Greek word for “marshy land,"

which perfectly describes the clay, mud,  and wet sand below the city's surface.

Ancient Romans counteracted similar  conditions with massive stone pillars

called piles which rest on  Earth's stable bedrock.

However, the tower's architects believed  a three-meter foundation would suffice

Unfortunately for them,  less than five years later,

the tower's southern side  was already underground.

Such a shifting foundation would  normally have been a fatal flaw.

the pressure from upper stories  would sink the structure

But construction halted at the  fourth story for nearly a century

as Pisa descended into prolonged warfare.

This long pause allowed  the soil to settle,

and when construction  began again in 1272,

the foundation was on  slightly more stable footing.

Under the direction of  architect Giovanni di Simone,

workers compensated for  the tower's minor tilt

by making the next few floors taller  on the southern side.

But the weight of the extra masonry  made that side sink even deeper.

By the time they completed the seventh  floor and bell chamber,

For centuries, engineers tried numerous  strategies to address the lean.

In 1838, they dug a walkway around the  base to examine the sunken foundation.

But removing the supporting sand only  worsened the tilt.

In 1935, the Italian Corps of Engineers  injected mortar to strengthen the base.

However, the mortar wasn't evenly  distributed throughout the foundation,

All these failed attempts, along with  the ever-sinking foundation,

moved the tower closer to  its tipping point.

And without definitive knowledge  of the soil composition,

engineers couldn't pinpoint  the tower's fatal angle

researchers developed tests to  identify those missing variables.

And in the 1970's, engineers calculated  the curved tower's center of gravity.

With this data and new  computing technology,

engineers could model how stiff  the soil was, the tower's trajectory,

and the exact amount of excavation  needed for the tower to remain standing.

In 1992, the team drilled diagonal tunnels

to remove 38 cubic meters of soil  from under the tower's north end.

Then, they temporarily counterbalanced  the structure with 600 tons of lead ingots

before anchoring the base  with steel cables.

More than six centuries  after its construction,

the tower was finally straightened…  to a tilt of about four degrees.

but they also didn't want to lose the  landmark's most famous feature.

Today the tower stands at  55– or 56– meters tall,

and it should remain stable  for at least 300 years

as a monument to the  beauty of imperfection.

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Why doesn’t the Leaning Tower of Pisa fall over? - Alex Gendler

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