Florence Cathedral - Table of Contents
Florence Cathedral: Dome
An excerpt from
Why Buildings Stand Up
by Mario Salvadori
New York: W.W. Norton & Co., 1980
The Pantheon was unsurpassed in span for 1,300 years, until the octagonal dome of the Cathedral of Florence outdid it by only seven feet in its maximum span.
In 1417 the "Commune" of Florence, finally in possession of a cathedral fully representative of its culture and glory, decided to erect a dome over its crossing. Two basic limitations had to be accepted in the conception of this work:
Both structurally and aesthetically it was inconceivable that the great dome should not have an octagonal shape or present a different profile. The all-powerful construction guilds of Florence under the direction of the Opera del Duomo ( the board supervising the erection of the church) had sworn to adhere to the 1367 model, although nobody had yet made serious proposals concerning the execution of the project. In fact, it was known that as far back as 1394 the experts of the Opera del Duomo had expressed the opinion that the construction of the dome was "so big and in such a state that it could not be completed" and that "it had been naive of the earlier masters and whoever else had deliberated on the matter to have believed it could be done."
- First, the drum on which it was going to be supported had an octagonal shape and was surrounded on three sides by octagonal half domes.
- Secondly, the brick model of the complete church, erected in 1367, showed the corner ribs of the proposed dome to have a particular profile, called "a quinto acuto," consisting of pointed circular arches with a radius equal to four-fifths of the crossing's span. The octagonal half-domes around the drum also had this particular profile.
The idea of a scaffold or centering capable of supporting such a monumental structure during its construction had always appeared outrageous to the experts of the time, indeed to all who had studied the problem since the Cathedral was started in 1294 by Arnolfo di Cambio and continued by Francesco Talenti in 1357. Even after the completion in 1413 of the octagonal drum on which the dome was to be supported - although the drum had been built fourteen feet thick as a matter of wise precaution -the cost of the centering alone had been a deterrent to the project.
Notwithstanding these inauspicious beginnings, a competition for the construction of the dome was called in 1418. Some of the greatest architects of the time participated in it, but no winner was chosen. In 1419 Filippo Brunelleschi, a member of the Silk Guild trained as a goldsmith, painter, and sculptor, made a revolutionary proposal to the board: the dome or "cupola" could be built without a wooden centering. He submitted a brick model to prove it. The board took him seriously enough to pay for the model, even though no large dome had ever been built without an interior temporary support.
The art of building was in the hands of specialized guilds opposed both to new ideas and to the leadership of a single man. But Filippo Brunelleschi was not easily beaten. Although only twenty-four years old, he was already well known in his town as a superb sculptor who had placed second in the competition for the doors of the Baptistery (the so-called Doors to Paradise) and had proudly refused to collaborate with the winner, the great Ghiberti, in their execution.
Son of an illustrious magistrate, Brunelleschi was a humanist at heart; a great mathematician particularly enamored of geometry, a mechanic and inventor of tools and clocks, a scholar who had dedicated years to the study of the "Divine Comedy" in order to grasp the many meanings of the great poem. Over a period of only twenty-seven years Brunelleschi created a new architecture, crowned by the completion of Santa Maria del Fiore transformed Florence from a medieval town into the capital of the Italian Renaissance.
Filippo meant business. He took residence in a house at the foot of the dome. He chose the clay and the dimensions of the large bricks for the masonry and supervised their burning. He determined from which quarries the stone and the marble for the dome would come, established kitchens inside the church so that the workers would not waste time going out for food, and erected scaffolds and banisters to make the masons' work less dangerous and psychologically more comfortable. All in all he spent the next sixteen years of his life bringing to fruition, day by day, his revolutionary idea.
The "incredible invention of Pippo" had many components, the most important of which was its double-masonry dome. This double-dome consisted of a thick inner octagonal shell connected by meridional arched ribs to a thinner outer shell. The inner dome was thus protected from the weather, and the exterior of the cupola given a more majestic shape. Two domes increased the resistance of the entire structure and permitted the inspection of both domes from stairs and galleries meandering through the space between them. The most essential structural component of the domes consisted of its hoops - six horizontal rings of sandstone reinforced on their outer surface by iron chains, which would prevent the bursting of the domes under the enormous tensile forces in their parallels. While the Pantheon owed its strength to the brute weight of its thick masonry, the cupola consisted of two relatively thin, light domes, which relied on the strength of their stone-and-iron hoops to prevent collapse at any stage of construction.
The skeleton of the cupola consists of the eight corner ribs in the shape "a quinto acuto" and of two additional smaller ribs located between the corner ribs, on each side of the octagon. The corner ribs are fourteen feet wide, the intermediate ribs eight feet wide. Both are deep enough to connect the inner to the outer dome but decrease in depth, although not in width, as they grow from the base octagon to the top octagon. The top octagon acts as the keystone for all the twenty. four ribs and supports the magnificent lantern of hollow marble, then a conical roof that in turn holds the golden ball and its cross.
The inner dome is seven feet thick at the bottom and five at the top; the outer is two and one-half feet at the bottom and one and one-fourth feet at the top. Both domes consist of eight cylindrical faces that curve inward toward the axis of the dome but have straight horizontal sections throughout their height. The iron-reinforced hoops of sandstone, embedded in the brick masonry of both domes at equal vertical intervals. have fulfilled their role as Brunelleschi conceived it. They prevent the bursting of the cupola and the appearance of those cracks that have required later remedies in all the large domes of the past, including Hagia Sophia and Saint Peter's.. One must not think of the cupola as of a skeleton of meridional ribs and horizontal hoops supporting the thin surfaces of the inner and outer domes. Although most of the enormous weight of the cupola is carried to its octagonal base by the wide ribs, one must realize that the twenty-four ribs and the two domes reached the same height simultaneously during construction, and this is the key to the last component of "Pippo's incredible invention," the erection of the largest dome ever built without the help of a centering.