Wednesday Jan 5, 1172 to Present
Pisa, ItalyThe Leaning Tower of Pisa or simply the Tower of Pisa (Torre di Pisa) is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa, known worldwide for its nearly four-degree lean, the result of an unstable foundation. The tower is situated behind the Pisa Cathedral and is the third-oldest structure in the city's Cathedral Square (Piazza del Duomo), after the cathedral and the Pisa Baptistry.
Construction of the tower occurred in three stages over 199 years. On 5 January 1172, Donna Berta di Bernardo, a widow and resident of the house of dell' Opera di Santa Maria, bequeathed sixty soldi to the Opera Campanilis petrarum Sancte Marie. The sum was then used toward the purchase of a few stones which still form the base of the bell tower.
On 9 August 1173, the foundations of the tower were laid. Work on the ground floor of the white marble campanile began on 14 August of the same year during a period of military success and prosperity. This ground floor is a blind arcade articulated by engaged columns with classical Corinthian capitals.
The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-meter foundation, set in the weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for almost a century, as the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca, and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled.
At least four strong earthquakes hit the region since 1280, but the apparently vulnerable Tower survived. The reason was not understood until a research group of 16 engineers investigated. The researchers concluded that the Tower was able to withstand the tremors because of dynamic soil-structure interaction (DSSI): the height and stiffness of the Tower, together with the softness of the foundation soil, influence the vibrational characteristics of the structure in such a way that the Tower does not resonate with earthquake ground motion. The same soft soil that caused the leaning and brought the Tower to the verge of collapse helped it survive.
Between 1589 and 1592, Galileo Galilei, who lived in Pisa at the time, is said to have dropped two cannonballs of different masses from the tower to demonstrate that their speed of descent was independent of their mass, in keeping with the law of free fall.
Numerous efforts have been made to restore the tower to a vertical orientation or at least keep it from falling over. Most of these efforts failed; some worsened the tilt. On 27 February 1964, the government of Italy requested aid in preventing the tower from toppling. It was, however, considered important to retain the current tilt, due to the role that this element played in promoting the tourism industry of Pisa.
after more than two decades of stabilization studies and spurred by the abrupt collapse of the Civic Tower of Pavia in 1989. The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred meters away. Apartments and houses in the path of a potential fall of the tower were vacated for safety. The selected method for preventing the collapse of the tower was to slightly reduce its tilt to a safer angle by soil removal 38 cubic meters (1,342 cubic feet) from underneath the raised end. The tower's tilt was reduced by 45 centimeters (17.7 inches), returning to its 1838 position.
After a phase (1990–2001) of structural strengthening, the tower has been undergoing gradual surface restoration to repair visible damage, mostly corrosion, and blackening. These are particularly pronounced due to the tower's age and its exposure to wind and rain. In May 2008, engineers announced that the tower had been stabilized such that it had stopped moving for the first time in its history. They stated that it would be stable for at least 200 years.