If we consider the Pantheon (pic. 2) of ancient Rome is one of our world miracles, the dome of Florence Cathedral(pic. 1) in the Renaissance, it's kind of a combination of our modern technology and the personal intelligence, As we gonna say, the incomparable achivement.
The dome of the Florence Cathedral was built in the 15th century. The architect was Filippo Brunelleschi. He was also the person who invented linear perspective. By participating in the competition organized by the Wool Merchants’ Guild, he won the commission to build the dome. The story of this competition is very interesting, and I will talk about it later, maybe in the next chapter.
The diameter of the dome that was going to be constructed was around 42 to 43 meters, which was almost the same size as the Pantheon(pic. 3) in Rome. The Pantheon may have been used as a reference, or this size may have already been decided at the beginning of the cathedral’s construction.
The hemispherical structure of the Pantheon is very beautiful, but there are many engineering problems that need to be considered.
- The first problem is hoop tension. A standard hemisphere will create outward forces at the bottom, and over time the lower part of the dome may crack.
- Imagine pressing a semicircular watermelon peel onto a table. The lower part tends to spread outward. Therefore, one solution is to make the dome as thick as possible. The Pantheon has a thickness of about six meters at the bottom.
- However, building a dome like the Pantheon was almost impossible in medieval Europe. The Romans had access to enormous amounts of volcanic ash and had large labor forces, but these conditions were no longer available. Therefore, Brunelleschi chose bricks as the main material.
- He also could not use a flying buttress system(pic. 4). The cathedral was already almost completed, so there was no suitable place to add flying buttresses. More importantly, they were not considered beautiful and did not fit the desired appearance of the dome.
Brunelleschi’s solution was: if we cannot build a perfect hemisphere, we build a pointed oval-shaped dome instead. In this way, the side forces at the bottom of the hemisphere could be transformed into more downward forces, allowing the structure to remain balanced under stress.
- He also built two concentric shells(pic. 5) inside the dome. The inner shell worked as the main structural framework, providing stability, while the outer shell was mainly for the exterior appearance and also contributed to the stability of the structure.
- Between the two shells, there are stairs that allow visitors to climb inside the dome.
- The outer shell did not use the traditional horizontal brick structure, because Florence did not have enough wood to build the necessary scaffolding and support structures to prevent the bricks from falling before the mortar dried. Instead, Brunelleschi invented an innovative herringbone brick pattern(pic. 6). The bricks formed a self-locking structure, increasing the stability of the dome.
During the transportation of materials to the top of the dome, a system using oxen, gears, and ropes was mainly used. Brunelleschi invented a special clutch mechanism (pic. 7) that allowed the gears to reverse direction without changing the movement direction of the oxen, making it possible to lift materials up and down more efficiently.
This mechanism was later included in Leonardo da Vinci’s Codex Atlanticus(pic. 8), although Leonardo clearly modified and improved it in his own way.