Will the Leaning Tower of Pisa Fall Over?
I remember my trip to the Leaning Tower of Pisa — I must have been about 3 or 4 and when my legs got too tired, my Dad simply picked me up and carried me the rest of the way to the top — in a ‘fireman’s lift’. My laughing and screeching no doubt annoyed a few fellow tourists wanting to soak up the atmosphere…
The Leaning Tower of Pisa is the free standing bell tower or campanile of Pisa Cathedral, construction began in 1173 and occurred in 3 stages lasting over 199 years. The Tower actually began to tilt during construction, this was caused by an inadequate foundation on soft ground.
Construction was halted for nearly a century as Pisa was engaged almost continuously in battles with Genoa, Lucca and Florence. Fortunately the underlying soil had plenty of time to settle, otherwise the Tower would have fallen over long before it was finished.
In 1272 construction began again, and in order to try and compensate for the tilt upper floors were built with one side higher than the other — because of this the Tower is curved. Construction halted again in 1284 and the Tower was finally finished in 1372. The Tower is 55.86 metres (183.27 feet) high on the low side and 56.67m (185.93 ft) on the higher side. It’s estimated to weigh a whopping 14,500 metric tons (16,000 short tons). Before restoration work between 1990–2001 the tower leaned at an angle of 5.5 degrees, but now it leans at about 3.99 degrees.
It’s not known for sure who actually designed the Leaning Tower of Pisa. For a long time Guglielmo and Bonanno Pisano were thought to be the Architects, now a 2001 study indicates that Diotislavi was the original Architect. Construction halted again in 1284 and the Tower was finally finished in 1372.
During WW2, the Allies suspected that the Germans were using the Tower as an observation post. However a US Army Sergeant sent to confirm the presence of German Troops in the Tower was impressed by it’s beauty and did not order an artillery strike.
In 1350, the Tower’s lean was reported to be 1.4°, it worsened over time until it reached an alarming threshold in 1993, passing the 5° marker. There have been 3 attempts over the years to straighten and stabilise The Leaning Tower of Pisa, the first was Alessandro Della Gherardesca in 1838. He thought that Pisa would benefit from exposing the base of the Tower which was mostly underground and a lot of the intricate artwork was not visible. The Tower’s leaning worsened…
The second attempt was made at the behest of Benito Mussolini in 1934, in his opinion the Tower was an embarrassment for all of Italy and the Tower had to be straightened immediately (which is not possible due to the Tower’s ‘banana shape’…). 361 holes were drilled into the foundations and concrete was poured in, the result — a significant increase in the inclination of the Tower…
The Tower was closed to the public in January 1990 — it’s lean was 5.5 degrees and the Civic Tower or Paviain had just collapsed. The bells were removed to relieve some weight and cables were attached to the third floor and anchored several hundred metres away. Apartments and houses in the path of the Tower were also vacated for safety.
The Tower should have toppled at 5.44 degrees, fortunately it defied computer model predictions long enough for Engineers to come up with a solution.
The restoration project was led by John Burland, Emeritus Professor at the Department of Civil & Environmental Engineering at Imperial College, London. Other members of the team included Architect Jean Barthélémy, Professor Michele Jamiolkowski and Professor Salvatore Settis.
“ if we had not stepped in, the Tower would have collapsed between 2030 and 2040 “.
Salvatore Settis
The project went through the following phases :-
1. Steel Bands were wrapped around the first floor to prevent the stone from fracturing.
2. Counterweights — 750 metric tons (827 tons) of lead weights were placed on the base of the Tower on the North side in order to slow down the tendency for the Tower to lean to the South.
3. The Tower was harnessed with enormous steel cables to prevent it from collapsing during the works.
4. A new concrete ring was poured around the base of the Tower — this was connected to cables anchored far below the surface.
4. Wells were dug under the foundations and over 60 cubic metres of clay were removed.
5. Water was drained from the wells.
6. A 200mm (7.9 inch) diameter drill was angled beneath the foundation, a small portion of soil (15–20 litres or 4–5 gallons) was removed each time, thus settling the ground above.
7. The steel cables were pulled to straighten the Tower.
By 2001 the Tower’s lean had been decreased by 44 centimetres (17 inches). By May 2008 movement finally stopped and a the Tower had moved back a further 4cm.
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.
The Tower was reopened in December 2001. Over 70 metric tons (77 short tons) of soil were removed and over 30 Million Euros were spent…
Since 2001, no other stabilisation work has taken place, but in 2013 Engineers monitoring the Tower reported that the straightening motion hadn’t stopped yet and the Tower had leaned back a further 2.5 centimetres.
Guiseppe Bentivoglio supervising Engineer said that it was to be expected and the Tower will continue to lean further north for a few more years, then it will settle and start falling south again. Bentivoglio said that more work would have to be done in the future to keep the Tower standing, but the tower will be safe for another 200 years.
An object will stand until the line drawn from the centre of gravity to the ground falls outside it’s base.
“ In theory, it would be possible to straighten it completely, but nobody really wants that! …the Tower was born leaning and leaning it shall remain “.
Guiseppe Bentivoglio
There are 3 main reasons why the Tower has stood for nearly 700 years :-
1. The Centre of Gravity
Every object has a mass and a centre of gravity (or centre of mass). The centre of gravity is the point where all weight is evenly distributed and all sides are in balance. Holding something by it’s centre of gravity would keep it in perfect equilibrium as gravity acts within the same torque in any direction around that point.
The mass of the Leaning Tower of Pisa is about 14,500 metric tons and given it’s geometry the centre of mass (or gravity) will sit around the 4th floor. The images above clearly show that the Tower would have to be leaning at quite an angle to collapse.
2. The Soil
Pisa is between the Arno and Serchio rivers and sits on alluvial soil made up of sand, clay and shells. This soil is very soft and not suited to bear heavy loads. However it’s ironic that we can admire the Tower today because of the unstable soil on which it sits.
A team of Engineers led by Roma Tre University analysed the physical, mechanical and chemical properties of the Tower’s construction materials and the rock and soil beneath it and the available structural and seismic data records.
They reported that the Tower’s ability to withstand earthquakes is due to dynamic soil structure interaction.
Due to the interplay between the rigid marble that the tower is built out of, it’s height (55 metres or 183 feet) and the soft soil it sits on, the Tower has vibrational characteristics that prevent it from resonating or shaking when seismic waves pass through the ground.
“Ironically, the very same soil that caused the leaning instability and brought the Tower to the verge of collapse, can be credited for helping it survive these seismic events,”
George Mylonakis, Professor of Civil Engineering, Bristol University
3. The Sun
The Sun shines on the Tower’s south side every day, as the sunlight is pretty intense, the Southern surfaces increase — especially in mid Summer when temperatures over 40°C (104 Fahrenheit) are not uncommon. The resulting thermal expansion of the Southern side causes the height of the Tower to increase, whereas the Northern side (which is never in direct sunlight) does not change size.
The Tower leans in the SSW (South South West) direction, when the sun is shining on the Tower, the building bends back to the North, decreasing the overall angle of the tilt. If the Tower was leaning in a Northerly direction, this phenomenon would have increased it’s tilt, almost certainly ending in a catastrophic collapse.
The bells in the Tower (the largest weighs over 8,000 pounds or 3,628kg) are no longer rung and wells in the area have been disused, all in an attempt to keep any vibration at a minimum.
About 6 Million people visit the Miracle’s Square in Pisa every year, and over 3 Million climb the Tower.
In 1987 UNESCO added The Leaning Tower of Pisa and the Square of Miracles to the World Heritage List.
There are in fact several leaning towers in Pisa. The bell tower of the Church of St Nicola is probably the most famous, it was built in 1170.
The Bell Tower at the Church of St Michele dei Scalzi is on the Viale delle Piagge (Piagge is Latin for “low plains prone to flooding).
Galileo Galilei was said to have dropped a cannonball and a musket ball from the top of The Leaning Tower of Pisa in the late 16th Century, by then it had moved about 3 degrees off vertical, monitoring didn’t start until 1911 and it was revealed that the Tower was moving at about a rate of 1.2 millimetres (0.05 inches) a year.
The Leaning Tower of Suurhusen in Germany now holds the record as the tower with the biggest lean — 5.19 degrees.
According to the Engineers, the current soil conditions are good enough to guarantee that The Leaning Tower of Pisa will stand for at least another 200 years. That is unless a really big earthquake hits Tuscany…
The Leaning Tower presents many opportunities for photo ops….
Mystery Solved!
Call Claire — +27 82 399 0180
bluedesigns5@gmail.com
www.bluedesigns.org
I love Architecture. I think it’s vital to talk about all aspects of Architecture — whether it be planning, construction, design or green building. I have written 3 E-Books & over 110 articles. Please feel free to let me know if you have any queries regarding architecture, planning & construction & I will assist you.
I am originally from the UK and moved to South Africa in 1999. I started Blue Designs in 2004 after working as a driver for Avalon Construction on a luxury home in the Featherbrook Estate. In my spare time I am an artist and writer.
Other Articles I have written include :-
Building Green? Here are some tips.
Common Mistakes People make when Designing a House
The Advantages of Sustainable Building
Considering Buying or Renovating a Heritage Home? Pros and Cons
Renovation vs New Construction — which is Greener and Better for the Environment?
Bizarre Buildings Part Two — Space Age Fantasy
Weird and Wonderful Buildings Part Three — Three Buildings that make Music
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Sources & Further Reading :-
https://en.wikipedia.org/wiki/Leaning_Tower_of_Pisa
https://inhabitat.com/the-secret-behind-the-leaning-tower-of-pisas-resilience-is-revealed/
http://www.iflscience.com/physics/mystery-of-the-leaning-tower-of-pisa-has-finally-been-solved/
https://leaningtowerpisa.com/facts/why/why-pisa-leaning-tower-does-not-fall
https://leaningtowerpisa.com/facts/how/how-pisa-leaning-tower-was-stabilized
https://leaningtowerpisa.com/facts/why/why-is-pisa-tower-leaning
https://www.walksofitaly.com/blog/all-around-italy/leaning-tower-of-pisa-facts
https://www.popsci.com/leaning-tower-pisa-soil
http://mentalfloss.com/article/70395/13-straight-facts-about-leaning-tower-pisa
https://www.livescience.com/33379-leaning-tower-pisa-fall-over.html
https://science.howstuffworks.com/engineering/structural/will-leaning-tower-of-pisa-fall.htm
https://leaningtowerpisa.com/facts/who/who-built-pisa-leaning-tower
Originally published at https://www.bebee.com.
