“St. Peter’s, the most magnificent church in Christendom and the fruit of many talents, soars triumphantly above the Vatican Hill. For nearly 150 years, a succession of popes entertained the idea of glorifying the shrine of their patron saint.”
— John Julius Norwich, ed. Great Architecture of the World. p153.
“The medal by Caradosso (1506) and the partial plan drawn by Bramante (in the Uffizi, Florence), probably represent the earliest stage of the design, before the difficulties appeared which obliged the architect and his successors to propose, and in some cases implement, numerous changes. These changes related not only to the general conception of the plan—first a Greek cross, then a Latin one—but also to the plan of the transepts, which at one time were to have ambulatories; to the role of the Orders, first purely decorative (Bramante), then structural (Raphael, Michelangelo); and to the construction and shape of the dome, first with a single masonry shell (Bramante), then a double one (Sangallo, Michelangelo). The piers at the crossing, which were intended to support the dome, were one of the biggest problems; too slender in Bramante’s plan, they were frequently reinforced… In the 17th century further important modifications were made by Bernini when he created the great colonnade that encircles the Piazza San Pietro.”
— John Julius Norwich, ed. The World Atlas of Architecture. p276.
The brick dome 138 feet in diameter rises 452 feet above the street, and 390 feet above the floor, with four iron chains for a compression ring. Four internal piers each 60 feet square.The dome is 452 ft high (above the pavement) and is buttressed by the apses and supported internally by four massive piers more than 18 meters (60 feet) thick.
— taken from John Julius Norwich, ed. Great Architecture of the World. p153.
referenced from: greatbuildings.com/