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The Leaning Tower of Pisa

By Yvonne Green,2014-08-13 13:13
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The Leaning Tower of Pisa ...

Exploring Structures

Ever since human beings have been building structures, we have been pushing the limits of the materials and techniques available to us. We are constantly trying to build bigger and better structures than those of the past. At times, this has resulted in some pretty spectacular structures such as buildings and bridges that are admired all over the world for their size, beauty and design.

However, sometimes when we are too eager to build the biggest and most elaborate structure, we can run into problems… Cracked walls, collapsed

bridges and fallen towers. No matter how beautiful or important a structure is intended to be, we must always remember that it must first be structurally sound. One of the most famous examples of a structure that has encountered serious problems ever since the very first stages of its construction is the Tower of Pisa.

The Tower

In Italy in 1173, construction began on La Torre di

Pisa (the Tower of Pisa). The tower was originally

designed to house the bells of its neighbouring

cathedral. It took almost 200 years to build the tower!

However, during this time there were long periods

when no construction was done at all. In 1360, the

tower was finally finished. It has eight stories or

floors in total. The height of the tower is

approximately 55 meters and it weighs 14, 453

tonnes. [1]

[1]

Try it:

Can you imagine something that weighs 14, 453 tonnes? That’s VERY heavy! To help you to better understand the weight of the tower, let’s compare it to something with which we are familiar. A regular school bus without any people in it weighs about 10 tonnes. How many school buses would you have to stack up to equal the weight of the tower?

The Problem

Very soon after construction began, the tower

started to lean to the south. Over the years, it

continued to lean and eventually the top of the

tower leaned almost 4 meters to the south of where

it would be if it stood up perfectly straight.

Try it:

Can you think of any reasons that might explain

why the tower leans? Draw a picture to represent

the reasons you think that the problem exists.

[2]

The main reason that the Leaning Tower of Pisa leans is that is was constructed on a type of soil that could not support the weight of the structure. [1] The tower has actually sunken into the soft ground more on the south side than it has on the north side. Buildings as tall as the Leaning Tower also usually have foundations (a support structure for the building) that extend deep into the ground. The job of the foundations is to find a place under the ground where the soil is strong enough to support the weight of the building. However, in the case of the Leaning Tower, the foundations only extend 3 meters below the surface. This was not deep enough to get past the soft soil. [2]

Try it:

In this experiment you will test how different types of soil support heavy loads. Fill three containers with different types of material. Fill one with dry sand, one with gravel and one with hard packed dirt. You can also try another type of material or try mixing several together. Use a heavy brick to represent the tower. Before beginning, measure the length of the brick. Take note of the measurement in your observation journal. Place a brick in each of the pails so that it is standing on its tall side. Place identical weights on top of each of the bricks. You will have to leave the bricks overnight to allow the soil time to respond to the weight that has been placed upon it. When you come back to your experiment, you will have to use a ruler to measure the height of the building that remains above the surface of the ‘ground’. At this time, it would be helpful to draw a picture of the experiment in your observation journal. Make sure that you label your drawing and include the type of material in each pail as well as the measurement of the brick that remained above the surface. Calculate how much of the brick has sunken

below the surface for each pail. What did you notice? What type of material best supports the weight of the brick? What kind of soil do you think is below the Leaning Tower? Why do you think that the tower leans to one side more than to the other?

The Solution

Even before the tower was completed, builders recognized that it was leaning and attempted to solve the problem. Look carefully at the picture of the tower on the previous page. Do you notice anything about the top level? The builders constructed this story so that it would lean slightly back towards the north in an attempt to balance the building. Since the tower was completed, there have been many attempts to stop the building from leaning and to straighten it to a safe point

Try it:

With a group of your classmates, brainstorm some possible solutions to help to fix the Leaning Tower of Pisa. Remember the tower is famous for its lean and many people travel to see the tower every year. Your goal is not to straighten the tower completely but to prevent it from leaning further south and to straighten it slightly so that it will not collapse. Draw detailed designs to represent your possible solutions.

Engineers from around the world have tried to fix the lean on the Leaning Tower of Pisa. They have tried a variety of techniques or ‘interventions’, some of which have been more successful than others. The following are just a few examples.

Beginning in 1993, workers began to

pour a concrete ring around the base of

the tower. The ring was designed to

support heavy lead weights that would

be added to the north side of the tower.

This intervention was successful in that

it reduced the lean of the tower and

slowed the speed at which the tower’s

lean was increasing.

[3]

Another step that was taken to prevent the tower from collapsing was to secure it with metal cables to another structure built in the same area. Eventually, the tension of these cables could be adjusted in case the tower suddenly began to lean to one side.

Although both of these interventions did help to support the tower and reduce or at least slow the lean to a certain extent, they were not ideal solutions. Can you think of a reason that this these types of intervention may not have been the best solutions in the case of the Tower of Pisa? Here’s a hint… Remember that the Leaning Tower is an important tourist attraction, renowned not only for its lean but also for its beauty.

In 1999, after many years of testing, engineers began to drill under the north side of the tower. Their goal was to remove small quantities of soil at a very slow rate in order to get the tower to lean back towards the north. This intervention turned out to be the most successful up to this point. When some of the soil under the north side of

the tower was removed, it left small

underground holes. Because of the

weight of the tower, the ground from

above the holes slowly sunk down to fill

the gaps. As a result of this intervention,

the tower has been straightened slightly

and has stopped moving further to the

[3] south.

[3]

Thanks to the many people who have dedicated a lot of

time and effort to saving the Leaning Tower of Pisa,

people all over the world should be able to enjoy it for

many more years to come.

[4]

Sources

[1] Retrieved July 15, 2008 from http://torre.duomo.pisa.it/index_eng.html

[2] Retrieved July 15, 2008 from http://en.wikipedia.org/wiki/Leaning_Tower_of_Pisa

[3] Retrieved July 15, 2008 from http://www.pbs.org/wgbh/nova/pisa/

[4] Logan, Aaron. Retrieved July 15, 2008, from

http://en.wikipedia.org/wiki/Leaning_Tower_of_Pisa

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