Brunelleschi’s aesthetic achievement is known to the whole world through countless travel posters. The dome has come to symbolise the city of Florence, an instantly recognisable landmark, rising above a sea of red terracotta roof tiles and seeming to soar as high as the surrounding mountains. To appreciate his engineering achievement it is necessary to climb up to the dome.
The master plan for the cathedral had always envisaged a central dome (cupola), but no one knew how to erect one of the required height and span without prodigious expenditure on timber for scaffolding. Filippo Brunelleschi, the future dome’s architect, travelled to Rome to study the prototype of all domed structures – the Pantheon – after which he came up with his master plan: a solution based on classical Roman technology.
Poor Brunelleschi must sometimes have hated the Florentines. Sceptical financiers first made him build a model on the bank of the Arno to prove that his dome would stand up, and then appointed the cautious, interfering and incompetent Ghiberti, Brunelleschi’s old rival in the competition for the baptistery doors, to supervise the overall construction.
A great problem-solver, Brunelleschi got rid of Ghiberti by simply walking out of the project, pretending to be ill. Without him, work ground to a halt, and he agreed to return only if he was put in sole charge.
The staircase passes between two shells. The inner one is built of brick laid-herringbone fashion, providing a virtually self-supporting structure that could be built from above without support from below. This then
provided a platform for the scaffolding to erect the outer shell.
The dome was completed in 1436, but the lantern, planned by Brunelleschi, was completed by Michelozzi Michelozzo in 1461, 15 years after the original architect’s death. The final touch was the external gallery running round the base. This was begun in 1506 by Baccio d’Agnolo, but work stopped in 1515 with only one side finished, when Michelangelo, whose word was law in Florence, described it as a “cricket’s cage”, implying that the design was rustic and childish. Few visitors will agree with his judgement, which has left the base of the dome with no facing to disguise the raw stonework on seven of its sides.
An Engineering Marvel
In practical terms, the dome’s octagonal shape is defined by eight marble ribs, matched by 16 ribs on the inside, with the structure strengthened by bands of stone and herringbone-patterned brickwork in the ancient Roman manner. Constructed without scaffolding, the cupola was built by means of a cantilevered system of bricks that could support itself as it ascended. As in imperial Roman buildings, the brickwork was placed in a framework of stone beams.
Brunelleschi’s other stroke of genius was to devise a system of an inner skeleton and outer dome in order to distribute the weight of the cupola evenly, with thick walls negating the need for further buttressing. The space between the two concentric shells made the structure supple and light, yet highly resistant.
View to the western part of the city from the top of Brunelleschi’s dome.