Tuesday, July 18, 2017

All the Cock-eyed Towers

from:NASA APOD


Thunder Moon over Pisa
 
Image Credit & Copyright: Marco Meniero
Explanation: What's wrong with this picture? If you figure it out, you may then realize where the image was taken. The oddity lies actually in one of the buildings -- it leans. The Leaning Tower of Pisa has been an iconic legend since shortly after its construction began in the year 1173. Now part of a UNESCO World Heritage Site, folklore holds that Galileo used the leaning tower to dramatically demonstrate the gravitational principle that objects of different mass fall the same. Between the Leaning Tower of Pisa on the right and Pisa Cathedral and the Pisa Baptistery on the left, a full "Thunder" moon was visible last week when the image was taken. Using modern analyses, the tower has been successfully stabilized and, barring the unexpected, should hold its present tilt for the next 200 years.
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Will the Leaning Tower of Pisa ever fall?

The tower of Pisa has been leaning so long -- nearly 840 years -- that it's natural to assume it will defy gravity forever. But the famous structure has been in danger of collapsing almost since its first brick was laid.

It began leaning shortly after construction began in 1173. Builders had only reached the third of the tower's planned eight stories when its foundation began to settle unevenly on soft soil composed of mud, sand and clay. As a result, the structure listed slightly to the north. Laborers tried to compensate by making the columns and arches of the third story on the sinking northern side slightly taller. They then proceeded to the fourth story, only to find themselves out of work when political unrest halted construction.

The tower sat unfinished for nearly 100 years, but it wasn't done moving. Soil under the foundation continued to subside unevenly, and by the time work resumed in 1272, the tower tilted to the south -- the direction it still leans today. Engineers tried to make another adjustment, this time in the fifth story, only to have their work interrupted once again in 1278 with just seven stories completed.
Unfortunately, the building continued to settle, sometimes at an alarming rate. The rate of incline was sharpest during the early part of the 14th century, although this didn't dissuade town officials or the tower designers from moving forward with construction. Finally, between 1360 and 1370, workers finished the project, once again trying to correct the lean by angling the eighth story, with its bell chamber, northward.

By the time Galileo Galilei is said to have dropped a cannonball and a musket ball from the top of the tower in the late 16th century, it had moved about 3 degrees off vertical. Careful monitoring, however, didn't begin until 1911. These measurements revealed a startling reality: The top of the tower was moving at a rate of around 1.2 millimeters (0.05 inches) a year.

In 1935, engineers became worried that excess water under the foundation would weaken the landmark and accelerate its decline. To seal the base of the tower, workers drilled a network of angled holes into the foundation and then filled them with cement grouting mixture. They only made the problem worse. The tower began to lean even more precipitously. They also caused future preservation teams to be more cautious, although several engineers and masons studied the tower, proposed solutions and tried to stabilize the monument with various types of bracing and reinforcement.

None of these measures succeeded, and slowly, over the years, the structure reached an incline of 5.5 degrees. Then, in 1989, a similarly constructed bell tower in Pavia, northern Italy, collapsed suddenly.

Leaning on a New Plan for Pisa
 
Officials became so worried the tower of Pisa would suffer a fate similar to the collapsed tower in Pavia that they closed the monument to the public. A year later, they rallied together an international team to see if the tower could be brought back from the brink.

John Burland, a soil mechanics specialist from Imperial College London, was a key member of the team. He wondered if extracting soil from below the tower's northern foundation could pull the tower back toward vertical. To answer the question, he and other team members ran computer models and simulations to see if such a plan might work. After analyzing the data they decided that the solution was indeed feasible.

Armed with a plan, workers went to the site and wrapped steel bands around the first level to prevent the stone from fracturing. Next, they placed 750 metric tons (827 tons) of lead weights on the northern side of the tower. Then they poured a new concrete ring around the base of the tower, to which they connected a series of cables anchored far below the surface. Finally, using a drill 200 millimeters (7.9 inches) in diameter, they angled underneath the foundation. Each time they removed the drill, they took away a small portion of soil -- only 15 to 20 liters (4 to 5 gallons). As the soil was removed, the ground above it settled. This action, combined with the pressure applied by the cables, pulled the tower in the opposite direction of its lean. They repeated this in 41 different locations, over several years, constantly measuring their progress.

By 2001, the team had decreased the tower's lean by 44 centimeters (17 inches), enough to make officials confident that they could reopen the monument to the public. Even after the drilling had stopped, the tower continued to straighten until, in May 2008, sensors no longer detected any motion. By then, the tower had lost another 4 centimeters (2 inches) of its lean and seemed to be in no immediate peril.

The actions taken by Burland and his team could, theoretically, stabilize the structure permanently. The real threat now comes from the masonry itself, especially the material in the lower stories, where most of the forces caused by the centuries-long leaning have been directed. If any of this masonry crumbled, the tower could collapse. And even a minor earthquake in the region could have devastating consequences.

In spite of these potential problems, engineers expect the famous structure will remain stable for at least another 200 years. By then, another intervention may be required, but the technology available to make improvements could be far more advanced and preserve the tower for another 800 years.

Leaning tower of Suurhusen, Germany source 

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10 Leaning Towers of Italy (not just in Pisa)


While the Leaning Tower of Pisa is certainly the most famous tilting tower of Italy (if not world-wide), it is not the only Italian tower that was either intentionally or unintentionally constructed to not stand perpendicular to the ground.


Leaning towers of Italy
Leaning towers of Italy

Italy counts at least 10 leaning towers, of which two more are located in Pisa: the Campanile of San Nicola and the Campanile of San Michele degli Scalzi. Not surprisingly, Venice, with its unstable soil, also counts three leaning towers. Four other tilting towers can be found in Bologna, Caorle, Burano and Rome.

1. The Leaning Tower of Pisa, Pisa

pisa-leaning-tower

The most famous of all leaning towers is, undoubtedly, the Campanile or Bell Tower of Pisa, located in Piazza dei Miracoli.

Construction of the Bell Tower began on August 9, 1173, but the works were interrupted for about a century at about one fourth of the fourth cornice. In the mean-time the tower began to sink due to a poor foundation, set in weak, unstable subsoil. When construction resumed in 1271, engineers built the subsequent floors with one side taller than the other to compensate the tilt that had built up in the mean-time. In 1278, when the seventh cornice had been reached, work was again suspended and only resumed in 1360. Without these two century-long interruptions, which allowed the underlying soil to settle and the design to subsequently be adjusted, the tower would most probably have toppled.

The Tower measures 58.36m in height, which appears to be a random number, but which actually corresponds to exactly 100 braccia pisane (Pisan cubits). In Medieval times there was no unity in the metric system and each city-state, Florence, Pisa and Arezzo, had their own metric system with a different value for the braccio and other units.

It is from the top of the Leaning Tower that Galileo Galilei supposedly carried out his experiments to prove the Laws of Gravitation.
 
2. The Campanile of San Nicola, Pisa


The octagonal bell tower, the second most famous in the city after the Leaning Tower of Pisa is also slightly tilting. Most likely built in 1170, the tower was originally separated from the nearby buildings. The base of the tower is under the current street level.

3. The Campanile of San Michele degli Scalzi, Pisa

San-Michele-degli-Scalzi-bell-tower
The bell tower of San Michele degli Scalzi stands on the right side of the church, on the side of Viale delle Piagge. The term Scalzi refers to the barefoot monks linked to the church.

The quadrangular tower is heavily tilted ( 5% slope ) due to the weak, unstable soil. The name of the neighborhood ” Piagge ” stems from the Latin plagae, meaning “low plains, highly prone to flooding”.

San Michele degli Scalzi, Campanile, Pisa. Photo by Giovanni V.San Michele degli Scalzi, Campanile, Pisa. Photo by Giovanni V.

4. The Towers of Bologna Asinelli and Garisenda towers in Bologna

 The Asinelli and Garisenda towers of Bologna are located at the intersection of the roads that lead to the five gates of the old ring wall (mura dei torresotti).

Asinelli is the taller one while the smaller but more leaning tower is called the Garisenda. It is believed that they owe their names to the families who commisioned their construction between 1109 and 1119. However, the lack of documents before 1185 make this theory hard to prove.

Initially, the two towers were approximately the same height, but the Asinelli tower was later raised to the current 97.2m, while the Garisenda tower was lowered in the 14th century when its structure became unstable due to a yielding of the ground.

Inside the Asinelli tower of Bologna

5. The Campanile of San Martino church on the island of Burano, Venice

Leaning tower of San Martino, Burano
The Campanile, like the Church, are the work of Andrea Tirali. The current church, built in the 16th century, has no facade as its west side borders onto houses. The Bell Tower was built in 1703-1704.

6. The Campanile of San Giorgio dei Greci in Venice

Venice counts three tilting towers, the Campanile of San Giorgio dei Greci, described here, as well as the Campanile of Santo Stefano and the Campanile of San Pietro di Castello, described below.


The Campanile of San Giorgio dei Greci was built by Bernardo Ongarin between 1587 and 1592, following a project by Simone Sorella. The Bell Tower started tilting from the beginning of its construction. Its inclination can best be seen from the bridge over the rio dei Greci, close to the Ponte dei Sospiri (Bridge of Sighs).

7. The Campanile of Santo Stefano in Venice

leaning-tower-santo-stefano-venice-b
The leaning tower of Santo Stefano (1544) is located in the sestiere San Marco, not far from the Ponte dell’Accademia. It is one of the highest bell towers of Venice, with a height of 66m. Its inclination is similar to that of the Tower of Pisa, about 2 meters.

Leaning-tower-santo-stefano-venice-2

8. Campanile of the Basilica di San Pietro di Castello, Venice

Campanile of the Basilica of San Pietro Castello, Venice. Photo via http://john-pat-italian-lakes.blogspot.ch/
Campanile of the Basilica of San Pietro Castello, Venice. Photo via john-pat-italian-lakes.blogspot.ch

9. The Campanile of the Duomo di Santo Stefano in Caorle

Cathedral-and-bell-tower-Caorle-b
The Campanile of the Cathedral of Santo Stefano in Caorle, close to Venice, is unique in the world, both for its shape and its age.

Caorle-bell-tower

Completed in 1070 AD, the 42 m high bell tower is the oldest surviving example of a cylinder-shaped bell tower with a conic cusp in the world.

For unknown reasons the bell tower began leaning sometime after 1920. Today it is leaning by 1.4 degrees in East-South-East direction (about 1/3 of the inclination of the Tower of Pisa). Its base is made of Istria stone, while the shaft of the tower is made of brick-faced rubble core masonry.

10. Torre delle Milizie, Rome
torre-delle-milizie-rome
The Torre delle Milizie (“Tower of the Militia”) is a fortified tower in Rome, also known as Nero’s tower as, according to the legend, it is from here that emperor Nero watched the Great Fire of Rome. However, the actual construction of the tower probably dates to the end of the 1100s and beginning of the 1200s, more precisely from 1198 to 1216.

Photo credits (top to bottom): Pisa Leaning Tower © fisfra/Istockphoto; Campanile of San Nicola by Geobia;  San Michele degli Scalzi Abside e torre campanaria by Samuele Manfrin; San Michele degli Scalzi by Giovanni V.; Andrea Johnston; two towers by Patrick Clenet; inside the Asinelli tower by Tango7174; San Martino by Amaya & Laurent; San Martino by Didier Descouens; Campanile of San Giorgio dei Greci by Jakub Halun; Santo Stefano by Keith Ewing; Campanile of the Basilica of San Pietro di Castello via john-pat-italian-lakes.blogspot.ch; Torre delle Milizie by Riccardo Cuppini; Cathedral and Bell Tower of Caorle by Piave.

1 comment:

Unknown said...

I hear there's also a listing apartment building in San Francisco, but I don't think it's cant is extreme enough to be readily noticeable yet. Give it time...apparently a member of a long distinguished tradition.