Leaning Tower of Pisa
The Tower of Pisa, more popularly known as the Leaning Tower of Pisa, is the freestanding bell tower for the cathedral of Pisa, Italy. It is considered one of the most remarkable architectural structures from medieval Europe and is one of the most popular tourist attractions in Italy.
It is one of four buildings that encompass Pisa’s cathedral complex known as Campo dei Mircacoli (Field of Miracles), along with a baptistery, cemetery and the cathedral itself. Unusually for cathedrals of the period, the tower (or campanile) is some distance from the main building.
The construction of the tower began in 1173 and upon its completion around two centuries later, it was one of the tallest bell towers in Europe. Over the last 800 years there have been many attempts to correct the structural lean of the tower caused by unstable soil and foundation settlement. Extensive work was done at the end of the 20th century to stabilise the tower, and its lean was reduced to less than four-degrees, with it currently being more than 5 m (17 ft) off perpendicular.
The decision to build the tower was made with the intention of showcasing the city of Pisa as both powerful and influential. It was designed as a circular bell tower made of white marble in the medieval Romanesque style.
The architect who initially designed the tower remains a mystery, although the first construction phase is attributed to either Bonanno Pisano or Gherado di Gherado. The second phase of the construction was overseen by Giovanni Pisano and Giovanni di Simone. Tommaso Pisano managed the tower’s completion in 1399.
The tower has 8 storeys, including the chamber for the bells, and reached an original height of 60 m. The bottom storey consists of 15 marble arches, while the next six storeys each consist of 30, and the top bell chamber consists of 16. Two spiral staircases run up the inside of the tower. One consists of 294 steps while the other required two additional steps to compensate for the lean.
The lean only became apparent once construction of the third storey had been completed in 1178, as the shifting soil of clay, fine sand and shells began to destablise the foundations. A series of wars interrupted the construction for nearly a century which enabled the soil to consolidate and avoided the inevitable collapse had the tower been completed on schedule.
When construction was resumed, the architects tried to accommodate the lean by designing the additional storeys shorter on the uphill side but this failed to have any impact, instead the weight caused further sinkage. As the centuries went by it became clear that the tower was actually slowly falling over at a rate of 1-2 mm a year.
By the early 20th century, it became clear that the tower was inherently unstable and needed remedial work to try and prevent further leaning. The tower’s seven bells, which had been installed over the previous four centuries, were silenced, as it was thought that their movement while ringing could worsen the lean.
Many ideas were suggested for how to straighten the tower, including one which was to take it apart stone-by-stone and rebuild it somewhere else.
During the 1920s, the foundations were injected with cement grouting which helped to stabilise the tower in the short term, although it was clear that a more substantial solution would be needed in the future.
During the Second World War, the Allies came to believe that the tower was being used by the Nazis as an observation point. An artillery strike on the tower came close to being ordered by a US sergeant, but upon seeing the structure’s beauty he chose not to.
In 1983, the tower was apparently ‘straightened’ by a disaffected Superman in the film ‘Superman 3’.
In 1987, the cathedral site was listed as a UNESCO World Heritage Site. Despite this, Superman’s best efforts, and the tower’s popularity as a tourist attraction, there were growing concerns about its future.
 Preventing collapse of the tower
Structural analysis showed the tower to be severely stressed at the level where the cross-section suddenly reduces. In 1989, work began to strengthen the structure and apply lead weights to the high side of the foundation. Following this, a series of ground anchors were installed around the north side of the tower to replace the lead weights.
These were, however, temporary measures and in 1990 the tower was officially closed as a team of engineers began a major straightening project. The lean was decreased by 44 cm to 4.1 m (13.5 ft) as a result of siphoning soil from beneath the foundations.
However, there was cause for alarm in 1995 when the tower leant 2.5 mm in one night. Engineers decided that instead of trying to work on the structure itself, they would seek solutions for the soil. They proposed an idea that had been raised in the 1960s, which was to extract soil from the high side of the tower, a technique which had been used to correct large differential settlement of a Mexican cathedral.
The trials proved successful and work commenced on boring 12 holes beneath the structure, extracting a total of around 37 m3 of soil. As the operation progressed, the lead weights were gradually removed and the tower responded as hoped, with the north edge settling 13 mm and the south side rising by 1.5 mm.
Work was completed and the structure reopened to tourists in May 2001. Without further excavation, the tower continued to straighten until a sensory analysis in 2008 found that the settlement had come to a stop with a total improvement of 48 cm (19 in.). Engineers have made a confident assessment that the tower will remain in a stable condition for at least the next 200 years.
 Project data
- Address: Piazza del Duomo, 56126 Pisa, Italy
- Construction started: 1173
- Completed: 1399
- Height: 56.7 m (at highest side), 55.8 m (at lowest side)
- Storeys: 7 (8 including the bell chamber)
 Related articles on Designing Buildings Wiki
- 7 Engineering Wonders of the world.
- 9 of the world’s most impressive structures.
- Building of the week series.
- Cracking and building movement.
- Florence Cathedral.
- Hagia Sophia.
- Library of Celsus.
- Roman Colosseum.
- Sagrada Familia.
- The history of fabric structures.
- Unusual building design of the week
 External references
- Tower of Pisa - Official site
The Gus Astley Student Award closes on 31 July, so if you have relevant coursework submit it online to have a chance to win £500 and a place at Belfast2018.
Among the most enjoyable of the huge variety of tours at the IHBC’s Manchester 2017 Annual School was the visit to Liverpool as featured in the Liverpool Echo of 16 July.
The ‘SkillBuild’ competition sees the very best in construction talent compete against each other in local heats across the country culminating in the Final at the NEC Birmingham.
11 awards to schemes and projects across England were handed out at an event at The Leadenhall Building, London attended by community groups and industry professionals.
The fifth Inquiry from the APPG for Excellence in the Built Environment looks at Brexit’s impact on future skills needs in the construction industry and built environment professions.
The BBC reports on how the Humber Bridge has been given Grade I listed status to become one of only six structures built in England since 1961 to be afforded the honour.
BDP and CH2M have been selected for vital work to protect the heritage of the Palace of Westminster from the substantial and growing risk of failure of its essential services.
The Scottish Government’s commissioned independent analysis of responses to the ‘Places, People and Planning’ consultation has been published with a position statement on proposals it plans on taking forward.
HES offers a blog on creating a series of Digital Stories to help schools learn about blackhouse history and Gaelic, with the help of pupils from Sgoil an Taobh Siar.
The Committee has launched a report that looks at measures to protect and improve the capital’s green spaces ‘Park life: ensuring green spaces remain a hit with Londoners’.
Dame Helen Ghosh has announced she will be stepping down as director general of the National Trust in March next year to become master of Balliol College at Oxford University.