The dome that covers the Florence Cathedral (Santa Maria del Fiore or Cathedral of the Saint Mary of the Flower) is known as Brunelleschis Dome. Duomo di Firenze, as it is ordinarily called, was begun in 1296 in the gothic style with the design of Arnofio de Cambio and completed structurally in 1436 with the dome engineered by Filippo Bruneleschi. The exterior of the basilica is faced with polychrome marble panels in various shades of green and pink bordered by white and has an elaborate 19th-century gothic revival by Emilio de Fabris
When it was designed, it was the largest dome in the world. This immediately created problems as its size prevented the traditional method of construction. Its structure is a double shell supported by sturdy pillars.
The usual way to build an arch or dome was to support it with scaffolding called “centring.” However, the open space in the cathedral was 42 metres wide, and the Florentines wanted a tall, soaring dome.All the timber in Tuscany would not have sufficed to make the centring.Brunelleschi ended up building the dome without scaffolding in such a way that it supported itself as the work progressed.Each of Brunelleschi’s stone chains was built like an octagonal railroad track with parallel rails and cross ties, all made of sandstone beams 43 centimetres (17 in) in diameter and no more than 2.3 metres (7.5 ft) long.
The first problem to be solved was purely technical: no known lifting mechanisms at the time were capable of raising and manoeuvring the enormously heavy materials he had to work with, including sandstone beams, so far off the ground. Here Brunelleschi outdid himself. A modern understanding of physical laws and the mathematical tools for calculating stresses were centuries in the future. Brunelleschi, like all cathedral builders, had to rely on intuition and whatever he could learn from the large scale models he built. To lift 37,000 tons of material, including over 4 million bricks, he invented hoisting machines and lewisons for hoisting large stones.
He invented a three speed hoist an intricate system of gears, pulleys, screws, and driveshafts powered by a single yoke of oxen turning a wooden tiller and the castello, a 65-foot-tall crane with a series of counterweights and hand screws to move loads laterally once they’d been raised to the right height.
The building of such a masonry dome posed many technical problems. Brunelleschi looked to the great dome of the Patheon in Rome for solutions. The dome of the Pantheon is a single shell of concrete, the formula for which had long since been forgotten. Soil filled with silver coins had held the Pantheon dome aloft while its concrete set. This could not be the solution in the case of a dome this size and would put the church out of use. For the height and breadth of the dome designed by Neri, starting 52 metres (171 ft) above the floor and spanning 44 meters (144 ft), there was not enough timber in Tuscany to build the scaffolding and form.Brunelleschi chose to follow such design and employed a double shell, made of sandstone and marble. Brunelleschi would have to build the dome out of brick, due to its light weight compared to stone and being easier to form, and with nothing under it during construction. To illustrate his proposed structural plan, he constructed a wooden and brick model.
Brunelleschi’s solutions were ingenious, such as his use of the catenary arch for support. The spreading problem was solved by a set of four internal horizontal stone and iron chains, serving as barrel hoops, embedded within the inner dome: one at the top, one at the bottom, with the remaining two evenly spaced between them. A fifth chain, made of wood, was placed between the first and second of the stone chains. Since the dome was octagonal rather than round, a simple chain, squeezing the dome like a barrel hoop, would have put all its pressure on the eight corners of the dome. The chains needed to be rigid octagons, stiff enough to hold their shape, so as not to deform the dome as they held it together.
The dome is a masterpiece of beauty and engineering, a pioneering construction for its time, and in many ways remains unmatched.