Construction of the tower occurred in three stages across 177 years. Work on the first floor of the white marble campanile began on August 8, 1173, during a period of military success and prosperity.
Due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning, the tower began to sink after construction had progressed to the third floor in 1178. The tower would almost certainly have toppled if the construction was not halted for almost a century, because of continuous battles. This allowed time for the underlying soil to settle.
In 1272 construction resumed under Giovanni di Simone, architect of the Camposanto. In an effort to compensate for the tilt, the engineers built upper floors with one side taller than the other. Because of this, the tower is actually curved. Construction was halted again in 1284, due to the defeat of Pisans in the Battle of Meloria. The seventh floor was completed in 1319. It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the bell-chamber with the Romanesque style of the tower. There are seven bells, one for each note of the musical major scale. The largest one was installed in 1655. The bell-chamber was finally added in 1372. In 1987 the tower was declared as part of the Piazza del Duomo UNESCO World Heritage Site along with the neighbouring cathedral, baptistery and cemetery.
A multinational task force of engineers, mathematicians and historians gathered on the Azores islands to discuss stabilization methods. It was found that the tilt was increasing in combination with the softer foundations on the lower side. Many methods were proposed to stabilize the tower, including the addition of 800 metric tonnes of lead counterweights to the raised end of the base.
On January 7, 1990, after over two decades of stabilization studies, the tower was closed to the public. The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred metres away. Apartments and houses in the path of the tower were vacated for safety. The final solution to prevent the collapse of the tower was to slightly straighten the tower to a safer angle, by removing 38 cubic metres (50 cu yd) of soil from underneath the raised end. The tower was straightened by 18 inches (45 centimetres), returning to its 1838 position. After a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public on December 15, 2001, and was declared stable for at least another 300 years.
Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees. This means that the top of the tower is displaced horizontally 3.9 metres (12 ft 10 in) from where it would be if the structure were perfectly vertical.
In May 2008, after the removal of another 70 metric tons of earth, engineers announced that the Tower had been stabilized such that it had stopped moving for the first time in its history. They stated it would be stable for at least 200 years. After a phase (1990–2001) of structural strengthening, the tower is currently undergoing gradual surface restoration, in order to repair visual damage, mostly corrosion and blackening. These are particularly pronounced due to the tower's age and its exposure to wind and rain.
Two German churches have challenged the tower's status as the world's most lop-sided building: the 15th-century square Leaning Tower of Suurhusen and the 14th century bell tower in the town of Bad Frankenhausen. Guinness World Records measured the Pisa and Suurhusen towers, finding the former's tilt to be 3.97 degrees.
In June 2010, the Guinness Book of World Records certified Capital Gate as the “World’s furthest leaning man-made tower.” The new record shows that the Capital Gate tower has been built to lean 18 degrees westwards; more than four times that of the world famous Leaning Tower of Pisa. Investigation and evaluation, which was made by a Guinness appointed awards committee, started in January 2010, when the exterior was completed.
Capital Gate’s construction commenced in September 2007; the tower’s completion is scheduled for late 2010. From its foundations right through its pinnacle, Capital Gate is a unique building and among the most technically challenging engineering projects in the world. Some key features stand out amongst others are (i) It’s gravity-defying 18 degree lean, widely believed to be the most inclined in the world (ii) The continuous twist of its form which ensures that the tower looks different from every angle (iii) The unique nature of the floor plate, each floor is unique
The foundation contains an incredibly dense mesh of reinforced steel that sits above 490 piles, drilled 30 meters underground to accommodate gravitational, wind and seismic pressures.
The core of the building is a pre-cambered, ‘slanting’ core that pulls in the opposite direction to the lean. It straightens as the building grows, pulled into a vertical position by the change in the centre of gravity of the building as concrete was poured onto subsequent floors.
The floor plates up to the 12th level are stacked vertically over one another. Between levels 12th and 29th the floor plates stagger over each other, in relation to the lean and twist of the shell, by between 800 to 1400mm and then back to 900mm. Between the 29th storey and the top storey, the range is between 900 and 300mm in relation to the line of the façade.
Capital Gate’s shell comprises a super-strong exo-skeleton called the diagrid, that provides a clear, unobstructed floor plate, using much less steel than a conventional structural frame.
Guinness appointed awards committee found the Capital Gate surpassed the Pisa tower in its slant. However that tower has been deliberately engineered to slant is not able to shadow the glory of the “real” leaning tower.
Vaisakh G.
