To understand domes it helps to first understand the principles behind the arch.While most people would agree that the wheel has been the greatest invention in our history, the arch is not far behind.It revolutionized architecture and our capacity to control our environments.
Our earliest buildings consisted of mud and timber, but eventually stone became widely used.Stone was durable and fire-resistant: a popular trait since inside fires were needed to heat and cook.However, as good as stone was for building walls, it was equally bad for building roof structures…its weight and size restraints limited the area you could enclose.If you could find a stone massive enough to span the distance between two walls or columns you then had to figure out how to lift it into place.
The arch solved this problem by allowing smaller, easy-to-lift, pieces of stone to be used to span the distance between two walls.The stones were bound together using friction which diverted the gravitational forces piece-by-piece to the outer walls.But to make an arch work there were two important things that needed to happen: the stones needed to be cut so that the joints aligned with the arch center, and a temporary form was needed to hold the stones in place until the top piece, or keystone, was placed. The development of the arch allowed buildings to soar in comparison to their predecessors.
The use of domes in architecture dates back over two thousand years.Basically a dome consists of numerous two-dimensional arches rotated around their midpoint creating a three-dimensional structure.However, when these arches are rotated something remarkable happens: the keystone, that crucial element at the top of the arch, becomes unnecessary when they transform into a dome.The very top of the dome can be left open without compromising its structural integrity.This hole is also known as an oculus, and it allows light to enter the space.
The primary challenge to building a dome is the outward pressure placed on the structure surrounding it.This pressure is calledhoop stress and it can be easily understood by performing a simple experiment.Take an orange, slice it in half, and remove the inside fruit.Place the two peels on a board so that they resemble small domes.Around one peel trace the outline of its circumference on the board with a pencil.Then take the second peel and apply a continuous bead of glue adhering its entire base to the board.After the glue hardens take your hand and press down on the first peel (in a way that simulates how gravity puts force on a dome).You will see that the flattened peel has pushed out beyond the outline you drew around its base.When you place the same pressure on the second peel you will see there is much more resistance to the pressure you apply.The glue has countered the hoop stress forces on your small dome.
The Pantheon in Rome is perhaps the purest example of a dome to be found.It was built out of concrete by the Roman emperor Hadrian some 1200 years before work began on the Santa Maria del Fiore.Inside this dome one can see progressively smaller recesses, or coffers, as the dome extends upward toward the oculus.These coffers not only reduced the weight of the massive structure, but divided the dome into a series of structural ribs and rings.The hoop stress forces previously mentioned were dealt with by the use of 23 foot thick solid brick and concrete walls around the perimeter of the dome, but even walls that thick could not adequately resist the hoop stress forces and cracks appeared.
The Pantheon’s dome spanned 142 feet across, about the width intended for Florence’s dome, but there were major differences and complications…the first one being that the art of using concrete had been completely lost in the Medieval Ages.The Romans had developed this product that could be poured in fluid form onto temporary wooden forms and allowed to harden.After reaching its desired hardness the wooden forms were removed exposing the finished underside of the dome.Numerous examples of this construction technology could be seen in the ruins of Rome at the time Florence was planning their dome, and it must have frustrated architects that the technology was no longer available.
Another reason the architects for Santa Maria del Fiore could not use the Pantheon’s dome as a prototype was its height.The base of the dome at the Pantheon was about 70 feet above the floor.This height enabled the Roman builders to erect wood scaffolding to support the forms the concrete was poured into, and to hold this concrete in place while it dried and hardened.Wood scaffolding had always been an essential element for building masonry arches and domes because it could hold the stones or bricks in place while the mortar binding them together dried and hardened, a process that could take a year or more.Because wood warps and shrinks as it ages it was very challenging to build these temporary structures. There were limitations as to how high you could reliably build wood scaffolding, and the base of the proposed dome in Florence was to start at about 170 feet above the floor and top off at about 300 feet, more than twice as high as the Pantheon.Knowledgeable people understood that wood scaffolding could not be built that high.Again the leaders planning the cathedral in Florence casually disregarded this fact, and continued building the walls of the church while having no idea how this problem could be overcome.
There were still more differences: the base of the Pantheon dome was circular while the Florentine dome was to have an octagonal configuration, and the dome itself at the Pantheon was spherical, while the dome in Florence was to have a “pointed fifth” or cone-like shape.