History of Dome
St. Peter’s Dome turned out to be Michelangelo’s final great work. The dome forms the centrepiece of St. Peter’s Basilica in the unique position in The Vatican city in
Rome and the earliest representations of the design were “The medal” by Caradosso
(1506) and the partial plan drawn by Bramante. However difficulties soon appeared which obliged the architects to both propose and implement numerous changes.
The building itself is truly impressive, with a 218m long nave and an interior which comprises of 45 alters. It covers an area of 23,000 m² and has a capacity of over
60,000 and it remains one of the holiest sites in Christendom.
Construction work began in 1506 under Pope Julius II and was completed in 1615 under Pope Paul V. The chief architect on the project was Donato Pramante. The dome comprises of a dome of internal diameter 43m which in turn rests on 4 great arches that stand on 4 structural piers, each with a perimeter of 71 metres.
Michelangelo took charge of the building site at which 4 enormous piers were rising behind the remaining nave of the old basilica. He designed a dome which would dwarf the nearby Pantheon and which would be constructed of heavy masonry A dome was “called for”, similar to that engineered by Bramekesc, which dominated the skyline of Florence. It is unique in that it holds the title of the tallest dome in the world.
Michelangelo died on February 18th, 1564. At this time the dome was complete as far as the drum, the base on which the dome sits. At this stage Giacomo della Porta took on the undertaking of completing the work. A noticeable aspect of his work was that the dome was by raised by 10m and so now rises 136m above floor level.
Despite designing the majority of St. Peter's Basilica, the 71-year-old Michelangelo refused to be paid for his work on the church.

Design
By the end of the 15 th
century, the old basilica was badly in need of repairs. The architect Bramante was selected to design the new building, which incorporated the dome, which drew heavily from that of the Pantheon. The main difference existing between the two designs is that the Pantheon’s dome is supported by one continuous wall, whereas
St. Peter’s is supported by only four large piers.
Bramante was subsequently replaced by other designers such as Sangallo, Fra Giocondo and
Raphael, the only notable changes to the dome being the strengthening of the four piers which had begun to crack. The final dome design was by Michelangelo, rising to a height of 41.47 metres.
Michelangelo’s dome is constructed of two shells of brick, with the outer having 16 stone ribs, and still raised on piers on a drum. The encircling peristyle originally designed by Bramante was reduced to 16 pairs of Corinthian columns, each 15 metres high, connected by an arch. On visual inspection, they appear as buttresses to each of the ribs, but are structurally quite redundant. This is because the dome is of an ovoid shape, therefore exerting less outward thrust than a heavy hemispherical dome, like that of the pantheon. Michelangelo had only left a few drawings after his death, but had also left a large wooden model. This model remained as the definitive design, and suffered only mild cosmetic changes during the course of construction. Although several subsequent engravings show a hemispherical dome to be present,
Michelangelo in his model had clearly designed it with the ovoid dome in mind.
However, design was found structurally lacking and in the early 18 th
Century, significant cracking was noticed to be propagating up the side of the dome, so much so that in 1748 an engineer by the name of Polemi, was commissioned to analyse the dome, and take steps to rectify the problem.
Failure of Dome
The cracking in the dome was found to be due to the outward thrust of the dome that there was not enough strength in the buttressing. Polemi, the engineer that was assigned to fix the problem, noticed that the cracks all ran vertically and made a series of ‘arches’ all running through the centre key stone of the dome.

He tested the stability of the dome by testing the dome as a combination of
50 arches, testing each arch individually.
He tested the arches by dividing each the arch into 32 sections and then worked out the weight of each section of the arch. He then hung a flexible string that spanned the base of the arch and tied the corresponding weights for each section of the arch to the string. He took note of the path it followed, inverted it and this gave the line of thrust in the dome.
It was found that this line, even though it was a little off centre, was in the parameters of the arch, and hence using Hooke’s Law it was found the dome was stable.
Despite this, as a precaution
Polemi installed four iron rings around the dome binding the dome to help cope with further outwards thrusting of the dome and prevent any more cracking.
The dome has stood soundly since, and has had a few more rings added for peace of mind.
The reason for the designers failing to predict his problem was because there weren’t ways of analyzing elaborate structures such as this one in that time. Trial and Error was the main method of analysis. Giacomo della Porta who did the later building of the dome after
Michelangelo died presumed that the parallels in the dome would be enough to support the outward thrust of the dome but underestimated the forces. If such a project was undertaken today there it would be much simpler to analyse and construct.