The Tower of Pisa

The Tower of Pisa (La Torre di Pisa) is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa. The height of the tower is 55.86 m (183.27 ft) from the ground on the lowest side and 56.70 m (186.02 ft) on the highest side. The width of the walls at the base is 4.09 m (13.42 ft) and at the top 2.48 m (8.14 ft). Its weight is estimated at 14,500 metric tons (16,000 short tons). The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. The tower leans at an angle of 3.97 degrees. This means that the top of the tower is 3.9 metres (12 ft 10 in) from where it would stand if the tower were perfectly vertical.

The Tower of Pisa was a work of art, performed in three stages over a period of about 177 years. Construction of the first floor of the white marble campanile began on August 9, 1173, a period of military success and prosperity. This first floor is surrounded by pillars with classical capitals, leaning against blind arches.

The construction begun in 1173 and it must have been suspended at the completion of the third ring, around ten years later, since a subsidence of the soil of between 30 and 40 cm. had thrown the tower out of the perpendicular, causing an initial overhang of circa 5 cm. More than a century after the laying of the foundation stone, was once again begun (1275) by Giovanni di Simone, who added three more levels, correcting the axis of the Campanile. In 1284 the six stories of loggias were to all effects finished, bringing the height of the building to 48 m., and employing a technical expedient that was meant to diminish, at least optically, the effects of the inclination, accomplished by raising the galleries of the upper floors on that side.

At the time the inclination of the Tower was more than 90 cm. The tormented vicissitudes of the Tower did not, as one might expect, greatly worry those who were involved in the construction and completion. The long intervals between building activity were dictated, most likely, by the need of letting the Campanile 'rest', but above all by letting both the foundations and the ground on which they rested settle down.

In a certain sense it can be said that the subsidence of the soil and the consequent inclination had, on the whole, been foreseen. At the beginning of the 14th century the bells were placed at the sixth level, in the large opening still visible in the marble cylinder beyond the loggia. Between 1350 and 1372 Tommaso di Andrea Pisano (according to Vasari) terminated the installation of the belfry on the summit of the sixth order of loggias, increasing the correction of the axis, and thus diminishing the load on the side that was in inclination, which in the mean while had become fixed at 1.43 m.

Photo by leonceeo

Conceived of not only as a bell tower, but also as a belvedere for the square below - from the earliest times the loggias have served as 'grandstand' for religious events and fairs - it rises 58.36 m above the level of the foundation, just under 56 m over the level of the countryside, and its inclination, measured at the base, is over 4 m. The average subsidence of the base is 2.25 m, while the progressio of the overhang, despite all attempts so far made to bring it to a halt, is about 1.2 mm per year.

Only a few years after the completion of Tower, damage to the elevation structure became manifest, upon which the most damaged elements in stone were substituted. The first documentation of restorations involving substitutions dates from 1398, when marbles were acquired for this purpose. After this, restoration work carried on uninterruptedly, including some very extensive operations: in the XVI century alone the substitution of 50 columns is documented.

Photo by fever hat

The elements originally realised with San Giuliano marble were progressively replaced using white Carrara marble. The most significant data regards the most exposed parts: of a total of 269 column shafts 175 are in white Carrara marble, while out of 207 capitals, 195 have been substituted. Metallic supports (rings, brackets, chains etc.), some applied many centuries ago, are visible in numerous areas of the Tower.

At the beginning of the XX century chains were applied to the vaults of the arcades. Here the substitution of the stone architraves connecting the arcade columns to the drum is also frequent. Recent studies on the interaction between the atmosphere and the Tower have demonstrated that the columns and capitals are subject to the greatest damage as a result of cycles of heating and cooling caused by their dimensional structure and by their direct exposure to the sun.

In the sector underneath the inclination of the Tower, diffused instances of crushed or compressed areas can be noted through attention to cracks and fissures and the detachment of materials along their line of contact with borders. The numerous restorations attempted over time, with the numerous substitutions they involved, does not allow us to trace the evolution of the ruined areas nor to localise the areas with a greater concentration of lesions by way of reference to the stone wall facing.

The mechanical damage caused by such factors is accompanied by the deterioration of the stone, which provokes modifications to the volume of the various architectural elements, causing the detachment of parts sometimes of significant dimensions. Major damage to the surfaces of the Tower can be related to the impact of rainfall, while the area under the inclination is subject to greater particle deposits, which, being screened from the rainfall by the Tower itself, are not washed away.

In 1911 the first measurements of the tower with instruments and methods capable of accurately following the course of the Tower's inclination began. The inclination measured in 1911 was 5ƒ14'46", corresponding to a projection of 4.22m from the seventh cornice to the first. The first surveys were based on the measurement of the angle "q" between the first cornice and the seventh, using a theodolite placed at a precise point. Later (1928), four benchmarks were placed on the base of the Tower, from the levels of which the value of the inclination could be deduced. In 1934 a pendulum was introduced within the hollow cylinder and a highly accurate spirit level was placed in the instrument room at the 1st order.

Finally, in 1992, an electronic monitoring station was installed, with automatically recording inclinometers which allow the real time transmission at the frequency intervals required (even every 4 minutes if necessary) of the values north-south and east-west of the inclination. With the help of instruments it is also possible to distinguish the movement of the base upon which the tower rests through analysis of deformations in the upper structure, in order to identify the effects of single causes, potentially of brief duration, such as winds and seismic activity. The diagram below shows the inclination of the Tower over time, reconstructed according to measurements taken in the XX century. This confirms the great sensitivity of the Tower to any variation in the ground conditions and to works undertaken at the base. Leaving to one side variations caused by specific occurrences, the rotation speed of the Tower has accelerated from 4" per year in the 1930s to 6" per year at the end of the 1980s.

In 1993 a counterweight of about 600 tonnes, made from lead ingots, was placed on the north side of the Towers' base in order to arrest the southward rotation. The diagram shows the rotation towards the north of about 60" and the later stabilisation of the monument, recorded for the first time in over eight centuries of the Tower's history. A contained perturbation was recorded in September 1995: the unforeseen effect of the link between the Tower and the basin formed by cropped steel tubes installed in 1935 to facilitate the waterproofing of the base. The renewed tendency to rotate towards the south was checked and halted by augmenting the counterweight from 600 tonnes to about 870 tonnes.