In previous Top Engineering Achievements blogs we have focused on modern feats of engineering, such as the Blackbird SR-71 and the International Space Station.
In this edition, we look at something a little different (and older): the Florence Cathedral in Italy, and specifically the dome of the cathedral, Il Duomo, built by Filippo Brunelleschi. The dome can not only be said to be one of the great civil engineering achievements of bygone times, but also a masterpiece of beauty.
How do you design and oversee the construction of an unsupported dome containing 37,000 tonnes of marble using the tools available to you 600 years ago? Construction jobs don’t come much harder than that. Carry on reading to find out how one man achieved what many said was impossible.
Before the dome
Construction started on Florence Cathedral (Cattedrale di Santa Maria del Fiore) in 1296, designed by Arnolfo di Cambio, and was to last nearly 125 years, slowed by the death of Cambio in 1310 and the Black Death.
Cambio did create a plan for the dome and left enough space for a huge dome 150ft wide and 180ft tall to be built on top of the existing walls. However, Cambio didn’t reveal how this huge dome could be achieved on a practical level.
The plans also didn’t include the flying buttresses that were used to support the design of structures that were favoured by the north of Italy in places such as Milan. This was the only known way to support such large structures at the time.
Not only this, but it was presumed that the sheer size of the dome would have required an enormous amount of scaffolding. With questionable amounts of timber available in the Tuscany region, the dome’s achievability was called into question once more. Even if the timber could be sourced, would the size and shape of the dome mean that the scaffolding would simply collapse in on itself?
In 1418, only the dome remained incomplete. A competition for who would construct the dome was held and Filippo Brunelleschi won, with Lorenzo Ghiberti appointed as co-superintendent, a rival of Brunelleschi. This caused numerous dramas throughout the project, including on one occasion Brunelleschi ripping Ghiberti’s work down.
Brunelleschi looked to the Pantheon’s dome for inspiration, but this was made out of a concrete material and the mixture has long been forgotten. Not only this, but the design would not work for the sheer size of the Florence Cathedral dome.
Instead, Brunelleschi proposed the construction of two domes, one inside the other: an inner dome visible from within and an outer dome, which was taller.
What perhaps surprised people the most was the fact that Brunelleschi proposed to build this without scaffolding.
Engineering ahead of his time
The usual way to build a dome was to support it with scaffolding called centring, but the open space in the cathedral was only 42 meters wide and the region lacked the timber that would have been required. Brunelleschi’s double-shell design made the structure far lighter and loftier than a solid dome would have been, and the design meant that the dome supported itself whilst the work progressed.
Brunelleschi also used herringbone brickwork in the cupola to give the whole dome structure more strength and to counteract stress caused by the weight he bound the walls with tension rings of stone, iron and wood.
He also invented an impressive three-speed hoist. At the time, no known contraption would have managed to lift the heavy sandstone required to such heights. The hoist included gears, drive-shafts and pulleys that were powered only by two oxen. Once the sandstone was pulled to the desired height, a 65ft crane moved them laterally. These lifts were so advanced for the time period that no lifts of similar complexity would be seen again until the industrial revolution.
The cathedral was consecrated in 1436 by a ceremony led by the Pope at the time and other bishops. A bronze ball was placed on top of the lantern on the top of the dome decades after Brunelleschi’s death in 1446. To achieve this, they used the same machines invented by Brunelleschi.
Not only is the dome an engineering marvel, but it also had an outstanding safety record (for its time at least). Brunelleschi built parapets on platforms and ordered the workers’ wine to be cut with water (jobs in construction were different back then!) and as a result only 3 work-related fatalities throughout its construction.
It can also be argued that Brunelleschi gave a serious voice to both civil engineers and architects, the latter of which were considered the lowest of the arts at the time. Brunelleschi became famous and was even buried in a crypt in the cathedral – a high honour for a mere ‘craftsman’.
Today the dome stills stands without any supporting structures, and you can see that each architectural component of the dome contributes to its stability. Brunelleschi’s designs and machines were admired by both Michelangelo and Leonardo da Vinci, and inspired the design of St. Peter’s Basilica, a true testament to his work.