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In the whole history of construction the building of the double shelled dome of Florence Cathedral was an outstanding and unprecedented achievement (Fig. I)' Its most significant diameter is slightly greater than that of the Pantheon. It is more than twice as tall. And it was built without supporting centring although octagonal in plan.
Had it been circular in plan there would have been little difficulty in doing without centring. Throughout construction there would have been a ring at the top acting as a common keystone to the incomplete radial arches leaning against it. The chief need would have been to ensure that newly placed masonry did not slip into the void when the inward slope of the bed joints increased, to do which numerous techniques had been devised.2 There was even a fairly recent precedent at the mausoleum of Oljeitu in Iran that approached the Florentine dome in scale.3
On an octagonal plan continued safety of the incomplete dome is less easily ensured without temporary support. There is risk of inward collapse at the middle of the sides unless they are considerably thicker than would be necessary for a circular dome. Support from centring is therefore normally required. As well as having the desired geometry this must be strong and stiff enough to limit the inevitable progressive distortions as the loads on it increase and their distribution changes. For the same reason construction should proceed quickly rather than slowly as is desirable for centreless construction. Finally it must also be possible to remove it safely, and only when it is removed can it be seen whether the structure will stand without it. To these requirements must be added the difficulties of assessing and providing the strength and stiffness needed, the costs of the necessary timber and the labour expended in fabricating and setting it in place, possible difficulties in obtaining the timber, and the virtual impossibility of later lifting construction materials from directly below.
In spite of these drawbacks and difficulties, it was thought that centring would be needed when, in 1366, an enlarged octagonal crossing crowned by a dome also octagonal in plan was decided upon for the as yet unbuilt eastern part of the new church. A design had been prepared...
