My short article on Brunelleschi’s dome in Florence - François Raulier

In an effort to elaborate upon and continue our theme of engineering as art, or perhaps the art in engineering, it occurred to me to reach back into the past for an apt example. The example I would like to discuss here is that of the magnificent structure that sits atop Santa Maria del Fiore in Florence - Brunelleschi’s famous dome. By the beginning of the 15th century, after a hundred years of construction, the structure was still missing the cathedral's dome. Work started on the dome in 1420 and was completed in 1436. It was the first 'octagonal' dome in history to be built without a temporary wooden supporting frame. The construction of such a masonry dome posed many technical problems. Brunelleschi looked to the great dome of the Pantheon 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 of the size envisioned for the cathedral and, in any case, this would have put the church out of use for a considerable time, which would have been unacceptable. For the height and breadth of the dome designed by Neri (one of the long succession of architects involved in the design of the cathedral) - starting 52 metres above the floor and spanning 44 meters - there was not enough timber in Tuscany to build the scaffolding and forms. Brunelleschi chose to employ a double shell, but to build the dome out of brick, due to its light weight compared to stone and being easier to form, with nothing under it during construction. His 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 point I am attempting to make here is that, at the time of the construction of the dome of Santa Maria del Fiore, no distinction existed between architects and engineers. The construction of the cathedral, however, took over 140 years, excluding the external decorations, and the dome itself 16 years. Programmes of that length have long since become obsolete and the need to build more quickly and more effectively has led to a fragmentation of the intellectual processes behind construction projects and to the acute specialisation of the dozens of disciplines that contribute to the creation of the modern masterpieces of our built environment. That is generally for the good and our highly developed, and developing, understanding of materials and systems have allowed us create buildings we would have struggled to imagine just 30 years ago, let alone 600 years ago. However, the need for all participants to invest in the aesthetic outcome of a project remains as strong as in Brunelleschi’s time, if we are to continue to create edifices that will thrill and enthral over the centuries to come. This brief synopsis of the technical solutions developed by Brunelleschi for the dome has been prepared using Wikipedia information. For those who would like the find out more about the history and background to this remarkable aesthetic and technical achievement, I would refer them to the full article at https://en.wikipedia.org/wiki/Florence_Cathedral and to Ross King’s wonderful book “Brunelleschi’s Dome” for further reading