News from Italy
The leaning tower of Pisa: a monument of lace
by Alessandro Romano
Work began on the construction of the bell tower of the cathedral of S. Maria, which would become the most famous in the world due to its inclined structure, on 9 August 1173.
It was 1172 when Donna Berta di Bernardo gifted sixty silver coins to purchase some of the stones for the foundation of a tower to be built nearby the newly constructed Cathedral in Pisa. Excavation works for the foundation started immediately, and the first stones were laid on Thursday, the 9th of August, 1173.
Bona da Pisa, a saint who lived in the Middle Ages is that she was also a great traveller.
The so-called leaning tower of Pisa (called the leaning building or Tower of Pisa, in Pisa la Torre) is the bell tower of the cathedral of Santa Maria Assunta, in the famous Piazza del Duomo of which it is the most famous monument due to its characteristic slope, a symbol of the city and one of the symbols of Italy. The tower of Pisa, 57 meters high, is inclined 3.9° towards the south, concerning the vertical axis. Over the years, the inclination has not always been the same. It has been nearly a century since the foundations of the Leaning Tower of Pisa were laid, but its lean is still controversial.
To understand why the Leaning Tower of Pisa, among the world’s iconic structures, is leaning, let’s rewind the film and go back 850 or 849 years.
The documents show 1173 as the construction start date, while the inscription at the entrance to the bell tower reads as follows:
“AN DNI (M)CLXXIIII CAPANILE HOC FVIT FVDATVS MSE AVG”
(This bell tower was founded in the year 1174 in August)
When the bell tower was built, the Pisan calendar was still in force in Pisa, according to which the beginning of the year was set on 25 March. Therefore, for the Pisan Calendar, it was already 1174, while for the Gregorian one, it was still 1173.
The tower was supposed to be a perfectly straight freestanding bell tower, at least in the original plan of Bonanno Pisano (or whoever the actual Architect was, since it still is a mystery).
Five years into construction, things started to go sideways. When builders reached the top of the 3rd floor in 1178, the foundation began to sink on one side, causing the massive tower to lean.
The instability of the soil was OK related to the location of the Tower. A few other towers in Pisa are leaning. The whole city of Pisa is built on the same unstable soil. However, in 1173 there was no precedent case, and the builders had to stop the works while looking for a solution.
Meanwhile, Pisa got involved in a long war with the nearby city-state of Genova and the works for the Tower were halted for about 100 years.
The instability of the soil was OK related to the location of the Tower. A few other towers in Pisa are leaning. The whole city of Pisa is built on the same unstable soil. However, in 1173 there was no precedent case, and the builders had to stop the works while looking for a solution.
Meanwhile, Pisa got involved in a long war with the nearby city-state of Genova and the works for the Tower were halted for about 100 years.
On August 9, 1173, work began on the construction of the Tower of Pisa; the structure constituted the bell tower of the nearby cathedral of Santa Maria Assunta, but in fact, it was an isolated structure concerning the cathedral, whose construction was completed between 1063 and 1092. There are various hypotheses regarding the paternity of the project of the Tower of Pisa; according to some, the project is attributable to the Pisan architect Diotisalvi. Also, the author of the Baptistery of San Giovanni was built in 1152 in the immediate vicinity, while more excellent credit has been given over time to Vasari’s thesis, which attributed the project to Bonanno Pisano. The name of the engineer who designed the monument is shrouded in mystery. The only certainty lies in that, so far, no signature has been found to resolve the dilemma.
The tower must have been a perfectly straight free-standing bell tower, at least in the original plan by Bonanno Pisano (or whoever the original architect was, as it is still a mystery…). Five years into construction, things started to go sideways. When the builders reached the top of the 3rd floor in 1178, the foundations began to sag on one side, causing the massive tower to lean because the Leaning Tower of Pisa leans).
The instability of the ground was fine related to the location of the Tower. Indeed, some other towers of Pisa are leaning. The entire city of Pisa is built on the same unstable ground. However, in 1173 there was no precedent, and the builders had to stop work to find a solution.
Meanwhile, Pisa was embroiled in a long-running dispute with the neighbouring city-state of Genoa and work on the tower stalled for about 100 years.
The construction site resumed in 1275 under the guidance of Giovanni di Simone and Giovanni Pisano, who added three more floors to the three rings trying to straighten the tower. After many years of work, the building was completed in 1372 with the belfry construction by Tommaso Pisano.
In 1835 the architect Alessandro Gherardesca performed the first restoration by eliminating the marshy ground and replacing it with a marble base. The result was not exceptional enough to have increased the inclination to 5.1 m measured with a lead line.
This slope continued to increase until the last years of the 1900s when the tower leaned by as much as 4.5 degrees. Over the centuries, the inclination of the Leaning Tower of Pisa has continued to increase, even undergoing sudden accelerations on the occasion of interventions carried out nearby; the situation was tackled in a scientific and coordinated way for the first time in 1990 thanks to the establishment of the International Committee for the Safeguarding of the Tower of Pisa, chaired by the geotechnical engineer Michele Jamiolkowski. The analysis conducted by the scientific committee revealed a double risk of collapse, linked on the one hand to exceeding the critical inclination that guaranteed the balance of the tower concerning overturning and on the other to the onset of stress concentrations in the masonry structures of the building, indirectly linked to the rotation of the ground floor. Consolidation interventions of the system above ground and geotechnical interventions aimed at reversing the tower’s inclination were implemented, bringing the situation under control and guaranteeing the reduction of the initial inclination towards the south.
Why is the tower leaning?
This is the big question that tourists and others have asked themselves at least once when looking at the tower. The cause of the inclination can be attributed to the subsidence of the soil composed of sandy silt. In the 19th century, an attempt was made to drain the underlying water, obtaining the opposite effect because the vertical lowering of the surface caused the slope to accentuate. In the 1990s, action was taken to create a series of counterweights and underground micro-tunnels capable of slowing down the inclination. Currently, the monument is equipped with sensors capable of capturing every slightest movement.
The construction of the Tower of Pisa took two centuries arriving at the inauguration only in 1373 and facing various difficulties, mainly related to its main peculiarity or the strong vertical inclination that characterises the entire structure. The monument has a particular system composed of two concentric cylinders joined by a spiral staircase positioned inside the frame. The tower’s design has a circular plan with an external diameter of about 15.5 m. It extends for a height varying from about 56 to 57 m concerning the ground level, depending on the rotation it has undergone. The work presents an architectural configuration of the Romanesque style. It is characterised by a base with blind arches surrounding the entire perimeter and above by six floors of small loggias, also blind. The structure is closed by a belfry, always built with a circular plan, which serves as a lookout position. Outside, the six loggia floors are visible. Each floor is marked by columns that support round arches. The tower, about 56 meters high with a weight of 14,700 tons, is located in the centre of the city of Pisa and is located in Piazza dei Miracoli, a name used by G. D’Annunzio and definitively assumed in the twenty years of the 1900s, near the Duomo. In 1987, the tower and the whole monumental complex of the square were also slightly inclined and were proclaimed a “World Heritage Site” by UNESCO.
