Filippo Brunelleschi’s Florentine Duomo has been an engineering marvel and mystery for more than 500 years. How did the master goldsmith and sculptor create the largest masonry dome in existence? It is an engineering feat that has not been accomplished since antiquity. Brunelleschi did not leave behind drawings or construction notes to explain how the famous Duomo, part of the Cathedral of Santa Maria del Fiore, was self-supporting and constructed without the use of shoring (a system of supports) or formwork.
Researchers at Princeton University in the United States and at the University of Bergamo in Italy collaborated on a study that revealed the engineering techniques behind self-supporting masonry domes inherent to the Italian Renaissance. Their findings were published in the July 2020 issue of Engineering Structures. Their study is the first to quantitatively prove the physics at work and to explain the forces that allowed such structures to be built without the formwork typically required for modern construction. The researchers posed two significant questions: How were these large and beautiful structures built without the use of temporary structures to hold them up during construction? What can we learn and use today?
The researchers could not study the Duomo in Florence or any others that were plastered over. To help solve the mystery, they studied the dome of Santa Maria, in Ciel d’Oro, in Montefiascone (Viterbo, Lazio) and one of the side domes of San Pietro, built by Antonio Sangallo the Younger. These domes were not plastered and therefore showed the arrangement of the bricks. The detailed computer analysis conducted by the researchers accounted for the role of each individual brick. Their analysis showed that the geometric pattern of bricks was the lynchpin in making the structure self-supporting. The herringbone pattern generated a spiral curve with perfect balance. As Attilio Pizzigoni, a professor at the University of Bergamo and one of the authors of the study, said, “We have seen that each brick affects the balance and resistance of the dome”.
The domes of these churches were constructed with a double-helix of support that distributes and equalizes weight and thrust within the structure. This herringbone system is called a double loxodrome, which was used after Brunelleschi by the Sangallo masters throughout Italy, including in St. Peter’s Basilica in the Vatican. They were an evolution on Brunelleschi’s original single loxodrome structure. While the physics of the structures is now understood, the origins of the technique remain a mystery. Some researchers attribute the technique to an influence from Arab and Byzantine domes. Others regard it as a derivation of the Roman technique called opus spicatum, in which bricks were tightly laid in a herringbone pattern without requiring much mortar. This pattern was commonly used in ancient Roman and medieval fire backs in stone hearths to prevent corrosion of lime mortar that occurs from exposure to heat and flames.
What also remains a mystery is why the loxodrome method was not used after the 16th century. But the researchers hope to resurrect these forgotten techniques. They believe their study could have practical applications for developing construction techniques deploying aerial drones and robots. Using unmanned machines for construction would increase worker safety, as well as enhance construction speed and reduce building costs. There are also environmental benefits: The construction industry is one of the most wasteful ones. Using drone techniques for building large span roofs that are self-supporting and require no shoring or formwork would create a lot less construction waste. The systems of Brunelleschi and the Sangallo Masters, concludes Pizzigoni, can be applied so that “even a robot can build a dome in this way…which is what they are already doing in America…making domes with glass bricks.”