Planning, Materials and Building Techniques
Dating back to its earliest origins, Rome has always been a riverside community. As time went on and the city grew, the bridges that popped up on the Tiber represented the strength of the empire, as well as the advanced knowledge it had about materials, construction and engineering. By late Antiquity, Rome had more bridges across the Tiber than existed in any other city in the world. The Pons Fabricius represents the first one of these to be made entirely out of stone, an incredible feat in and of itself. In modern 21st Century bridges, advanced knowledge of structural mechanics and the development of the elastic theory have allowed engineers to design bridges close to their expected capacity, with bending of materials in mind. They use materials such as steel and reinforced concrete, which has made stone masonry lose its principle role in building structures. Back in 62 BC, the masonry to build an arch bridge had to be flawless. By using strong, solid materials such as tuff and travertine and using the basic engineering principles they were aware of, they could build a bridge they were confident would last. As it turns out, their techniques have created a bridge that has lasted much longer than they could have ever imagined.
When Lucius Fabricius designed his bridge, he undoubtedly kept the purpose of the bridge in mind. Pons Fabricius was being built for the public for essential urban functions including transportation by foot, as well as the transportation of goods and supplies. Although the bridge was not a main highway, it would serve as an important route for city dwellers, and therefore needed to be strong, sturdy, but not massive. The bridge today is 5.5 meters wide, which is perfect for pedestrian travel, but too small for cars. More than 2000 years later, its function in modern times is almost identical to the first century BC.
Next, Fabricius would need look at the first and most difficult step in the creation of the Pons Fabricius- the construction of the foundation and piers of the bridge. This would be where the load of the bridge sat on, and would also be where the bridge was most exposed to damage caused by water. There were many factors to consider before constructing, and the first and foremost of these would be flooding. In Ancient Rome, floods of the Tiber were frequent and severe, and had to be planned for. Because the bridge only had to span roughly 62 meters of water, only one pier would be needed in the river to connect two arches which would make up the bridge. Having only one pier put damage from trees and other debris during a flood at a minimum. It also allowed the builders to focus on the foundation of one pier alone during the construction- which was not an easy process.
While some rivers in the Mediterranean had dry seasons, the Tiber was likely continuously running year round. To build the foundation, the Romans utilized the coffer-dam. This was essentially a watertight barrel made of rows of sticks that was placed into the river, which blocked off the flow over water around the area that was being used for the foundation. If the riverbed was determined to be too soft they would drive wooden stakes down into the ground. By mixing water, lime and sand with a fine powder of volcanic ash or tuff , the Romans had a waterproof concrete mixture that was perfect for bridge foundations. Mixing this with large aggregates such as rocks and debris, the concrete was placed in the coffer-dam, displacing any water that remained, and was left until it cured. After the concrete had dried, a crucial design feature of the pier had to be implemented. In the upstream direction, a V-shaped cut-water pier was added, which made of tuff. This would be instrumental in diverting water directly from the foundation, and putting too much force on it. On the downstream side, a semi circular pier was created to counteract erosion by turbulence. This structure built around the foundation of the bridge not only lessened the forces on it and helped prevent erosion, but also served as reinforcement to strengthen the base.
|Foundation and cut-water piers
Next, the arches had to be constructed. Because of the perfect symmetry of the bridge, and the fact that the bridge lied on only one pier, the construction was greatly simplified. This is because they could build the arches out one at a time instead of struggling to put up the entire bridge at once. Building out from the shorelines, each arch would meet in the middle. The arches were designed to be perfectly semicircular, as the Romans had discovered that this shape would allow for the thrust to go almost all out horizontally at the base of the arch, making it much stronger and less likely to collapse. This required extremely strong abutments at either side of the bridge, which was not an issue because they lay on the banks of the river as opposed to in it. To build the arches, a wooden framework, called the “centering” was built from the piers and was braced to them. The profile of the centering was shaped exactly as the shape of the future bridge. Parallel arcs of stone blocks were then placed on the centering behind each other to create the arch. To accomplish this, all the stones had to be cut identically, making mortar to hold the blocks together unnecessary. Once the keystone was places at the top of the arch, the compressive forces together with the cut of the stone ensured stability.
To build the structures they did, such as the Pons Fabricius, the Romans used the volcanic rock they had readily available as their primary building material. Specifically for the Pons Fabricius, they needed rocks that would be sturdy, strong and durable. They found these in the local Peperino Tuff, and Travertine. These stones were not chosen by chance. A 2005 study on Roman Masonry Stones revealed Travertine and Peperino Tuff to be the strongest stones available to the Romans in compression, both wet and dry. The Travertine had 106 MPa of uniaxial compressive strength dry, while the Peperino Tuff had 44 MPa of strength. Although the Romans obviously did not know these exact strengths, years of trial and error had showed them these were the best candidates. The Peperino Tuff was found in the Acque Albule basion, 30 km east of Rome. This stone would take up the majority of the volume of the bridge. The Travertine was found north and west of the city, near where the vatican sits today. Travertine was used in only select spots on the bridge, such as the outer parts of the arch, the small interior arch, and made up the original deck of the bridge. These stones gave the bridge superior strength and stability, which it would need to carry and load that was put on it, and well as survive the floods of the tiber.
Another feature in the design of Pons Fabricius is the small arch featured right in the middle of the bridge. This arch serves two functions. First, it plays a crucial role in times of flooding, by letting rising water flow though the arch instead of crashing into the giant pier. The reduced surface area of the upstream face of the bridge makes a huge impact when the Tiber has a high volume and velocity, which will be disused in greater detail in the next section. Secondly, the arch reduces some of the weight of the bridge, which puts less of a load on the middle pier.
|The bridge utilizes the middle arch during this Tiber River flood
Today, the Pons Fabricius remains practically in its original state. Some small changes have been made over the years, the first and most apparent of which is the brick that faces the bridge today. The brick was an addition which is believed to have been put on after the flood of 23 BC, when this and other small renovations were made. The deck and parapets have also been replaced, but the tuff core remains. If the deck is looked at carefully, the old travertine can still be seen. However due to the rising street level on the east bank, the new deck sits on rubble concrete fill above the old surface. Despite these minor changes, the structural integrity that the bridge was built with in 62 BC remains today. The craftsmanship and precision of the original stone masonry is easily apparent, a main reason why the bridge has lasted as long as it has.
|A view of the Travertine, Tuff, Brick, and concrete that now make up the Pons Fabricius
Continue to Part III: Engineering Analysis