Pons Fabricius, built in 62 BCE
By Ian Dahl / 10.04.2013
Introduction and History
When I first heard about the “Oldest Bridge in Rome,” I was intrigued to say the least. While everything in Rome is old, it is rare to find anything built in the first century BC that has been preserved in its original state, while still functioning in the exact way it was designed. Although crumbling monuments like the Colosseum are certainly impressive, there is something special about the Pons Fabricius. Nestled between the Tiber Island and Sant’Angelo region in Rome, the Pons Fabricius is an incredible example of a Roman arch bridge that has endured the test of time. Walking by it for the first time, its simple design and bold arches stand out. Unlike some of the other bridges of the Tiber, only two arches are needed to connect it from one bank of the river to the other. A massive pier sits the the middle of river, while shallow water trickles by. The bridge is lined by giant leafy sycamore trees to its north, creating a picturesque scene. Walkers pass over it nonchalantly, seemingly ignorant of the great engineering feat it represents, and all it has endured in its over 2000 year history. Despite its aesthetic qualities, the lure of the Pons Fabricius truly comes in its age. It’s hard to walk by and not imagine all the history the bridge has quietly witnessed. The long lifespan of the Pons Fabricius does not come by chance. It was carefully designed and constructed by the engineers of Ancient Rome to last, which it has accomplished better than any other bridge in the Roman Empire. The Pons Fabricius serves as a model of Roman ingenuity and is an engineering marvel that has lasted more then two millenniums.
In the era of Spartucus’ slave revolt and Julius Ceasar’s betrayal, the Pons Fabricius was completed in 62 BC. It replaced an entirely wooden bridge, which were common in Ancient Rome before the use of stone bridges. It gets its name from Lucius Fabricius, the curator viarum of Rome, who was in charge of roads at that time and was attributed to originally building the bridge. According to historian Cassius Dio, a giant flood in 23 BC necessitated repairs on the bridge, which were carried out by M. Lollius. (Planter & Ashby, 1929) These repairs can be seen by the use of brick on parts of the bridge. On the arches of the bridge, two inscriptions mark the satisfactory work of both Fabricius and Lollius, guaranteeing the solidity of the work for 40 years. As seen in Figure 1, the inscriptions reads L.FABRICIUS C. F. CURATOR VIARUM FACIUNDUM COERAVIT, meaning “Lucio Fabrizio, Responsible of the Roads, supervised the execution of the job.” (“Pons Fabricius,” 2003)
Figure 1: Inscription on the Bridge
Despite these repairs, the bridge is thought to be practically in its original state and is widely considered the oldest and best preserved stone bridge in Rome, and possibly the world. At 62 meters long and 5.5 meters wide, it spans part of the Tiber river with two arches, each around 24.5 meters in diameter. (Planter & Ashby, 1929) In the giant middle pier lies another arch built for times of flooding. Although they aren’t visible today, it is believed to originally contain two more arches at its widest points as seen in Figure 2 below. These were covered once the walls along the Tiber were built to stop flooding in the city beginning in 1875.
Figure 2: Sketch of the original bridge
Pons Fabricius served as a connection between the mysterious Tiber Island and the Forum Boarium, which was a major center of commerce at the time. The island was called by Romans “Intra duos pontes,” meaning “between two bridges,” referring to the Pons Cestius which also connected the mainland to the island at that time. In the first century BC, the Pons Fabricius was used by Pilgrims, using the bridge to get to and from the island for the use of ferries for transportation. There also was a sanctuary on the island, an ancient form of hospital. Due to its expansion, more resources were needed on the island including the daily delivery of food and other supplies. The bridge served as the only way of transporting these things, as the Pons Cestius had not been built yet. In the middle ages, a tower was built on the far side of the bridge on the island, later bought by the Caetani family. They had transformed the island into a small fort, which was later renovated by the Pope, Sixus V, during the mid 16th century. Legend has it the double herms on the bridge today (Figure 3) are the heads of 4 architects who worked on this restoration, who unfortunately got into a disagreement with the Pope and were beheaded. (“Tiber Island,” 2005)
Figure 3: Herm of the four architects thought to have been beheaded by Pope Sixus V
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. In Figure 4, the bridges that surround Tiber island are shown. By late Antiquity, Rome had more bridges across the Tiber than existed in any other city in the world. (Taylor, 2002) 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. (Frunzio, Monaco & Gesualdo, 2001) 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.
Figure 4: The bridges surrounding Tiber Island. Pons Fabricius is the farthest north
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 to 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.
Figure 5: A Roman Cofferdam
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 (Figure 5). 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. (Brown, 2005) 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 as seen in Figure 6. 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.
Figure 6: 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. (Brown, 2005)
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, shown in Figure 7, revealed Travertine and Peperino Tuff to be the strongest stones available to the Romans in compression, both wet and dry. (Jackson & Marra, 2006) 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.
Figure 7: Compressive strengths of stones readily available to the Romans
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 (Figure 8). 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 discused 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.
Figure 8: 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 (Figure 9). 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.
Figure 9: A view of the Travertine, Tuff, Brick, and concrete that now make up the Pons Fabricius
Throughout the history of bridges, arches have continuously been used for their strength and stability. Even today, engineers all over the world employ techniques pioneered by the Ancient Romans. The magic of the masonry arch bridge is in its design. Blocks are cut and pieced together in a way in which the arch is in total compression. Being strong in compression and poor in tension, the blocks (or concrete fill that we see in some bridges over the tiber) are used for their strengths instead of its weaknesses. The way the arch improves on the simple beam, is by dissipating some of the vertical forces coming down on it in the horizontal direction. When a load is applied on top of the arch, the base of the arch (footers) try to push out. This makes abutments to stop this outward push necessary as seen in Figure 10. In the case of the Pons Fabricius and other stone bridges seen throughout the Roman Empire, the massive self-weight of the bridge causes a giant thrust, requiring very large and massive abutments to keep the arch in place. Although it is hard to tell what type of abutments the Pons Fabricius had when it was originally built, the ends of the bridge today are kept in place by two gigantic walls with an immeasurable amount of force. Its almost as if the bridge would need to move the entire city in order for it to push out and expand. Because of this, we can assume that if it did fail, it would be very unlikely to do so at the abutments.
To analyze and perform calculations on the masonry arch, we must make certain assumptions to simplify the situation:
1. The masonry units in the bridge (Tuff and Travertine) are infinitely rigid and strong.
2. There will be no sliding at the joints of the bridge.
3. We assume the bridge is a 3 pinned arch. (explained in greater detail below)
4. No tensile strength will be transferred from block to block.
5. The load from the self-weight of the bridge will be treated as a point load instead of a distributed load.
Probably the most important of these assumptions is the 3-pinned arch assumption. We can assume this is the case based on the fact that the material is unreinforced and the applied load on the top of the arch will likely create a hinge at the top, or a crack in the case of concrete. The hinge at the top of the arch will not support a moment, however it can support loads in the X and Y directions. This allows the structure to be statically determinant. If the arch was treated as a two pinned structure, it would be statically indeterminate, and finding the support reactions would be more complicated although possible. Although this may sound like a bad thing, it is perfectly ok, and will not allow the structure to fail.
Failure, however, can occur when extra pins appear in an arch. When analyzing a single arch, 4 pins or more can cause failure when the arch simply collapses due to asymmetrical loading (Figure 11). In the case of a multi-span arch bridge such as the Pons Fabricius which has two spans, 7 or more pins can cause failure (Figure 12). (Gilbert, 2007) To make sure these extra pins aren’t present, the engineers must make sure the arch is in complete compression and that it supports and abutments are sufficiently strong.
LEFT: Figure 11: Single span Failure
RIGHT: Figure 12: Multi-span failure
To start and analyze the bridge, a few measurements were needed. I obtained most of the dimensions of the bridge through online sources, and then others by approximation and estimation. Next I found the average weight of Tuff per cubic meter as 4 kip/m^3 . Although these dimensions and weight are very general and not nearly exact, they can give a good estimation of the thrust caused by the self weight of the bridge. Using the knowledge of statics, I drew a simplified version of the bridge with dimensions, as well as free body diagrams of the arches. Finding the volume of the stone above each of the arches and multiplying this by the weight per m^3 I had obtained earlier, I was able to approximate a point load on each arch. As it turned out, the small arch in the middle of the bridge had almost the same load on it as the giant arches, due to the giant mass of stone over it. After applying the static equilibrium equations, I had found the horizontal thrust at the bottom of the arches. Both of the big arches were exactly the same, and both had a thrust outward of around 1018 kip. The detailed work done can be seen in Figure 13 below.
Figure 13: My static analysis
A crucial spot where these thrusts meet is the center pier of the bridge. Assuming almost all the thrust is pushed outwards horizontally at the base of the arches, the thrusts from the two bigger arches will cancel out in the middle pier. In other words, the net force in the horizontal direction on the pier in the river should be close to zero. On the outer parts of the bridge, it is up to the abutments to hold the arch together, which is not an issue due to the giant walls on either side of the bridge. Upon being constructed however, Fabricius undoubtedly constructed sturdy foundations at the outer banks of the river to deal with the thrust.
Another way to analyze the bridge is a capacity over demand ratio, also known as the Factor of Safety. To do this, the equation Vn=2(√Fc)Av will calculate the capacity of the material, given the compressive strength and the shear area. At each end of the bridge the shear area is essentially infinite, which means it will not fail in these locations. A better place to find the shear area is the middle pier of the bridge, which for one arch alone is 25,575 in^2. Using the compressive strength of peperino tuff stone found earlier of 44 MPa, or 6382 psi, the calculated capacity is 4086 kip. This yields an incredible factor of safety of 4. The detailed work for these calculations can be seen below in Figure 14. While exact dimensions and weights might make the factor of safety more accurate, the point is the same either way. This bridge is built way beyond its necessary capacity- and the result is a structure that lasts over 2000 years without failing.
Figure 14: Capacity of Material and Factor of Safety Analysis
Another interesting factor is the thrust from the smaller arch designed for flooding. It creates a thrust that I calculated as almost 330 kip. This force is not being applied at the footing of the bigger arches, it is acting midway up them. This means that at that point in the bridge, the net force does not cancel out to zero, and a stress is being applied on the arch. Although this seems like a problem, the bridge has the capacity to counteract that force, keeping everything in equilibrium. This flood gate is necessary when water rises up around 5 meters. At this point, it flows through the arch instead of pushing on the bridge. The arch reduces the surface area of the bridge that the water hits greatly, which makes up for any stress that this arch puts on the bridge due to its design.
Implications, Legacy and Conclusion
On the surface, the Pons Fabricius looks fairly average. It’s definitely not the longest or the tallest bridge in Rome and certainly doesn’t have ornate decorations and marble statues such as the Ponte San’Angelo further north. Being the first bridge constructed on the Tiber in Rome, Pons Fabricius came at a time when Rome was starting a revolution of technological advancements. The creation of a waterproof cement by mixing sand, lime and pozzolana opened up myriads of new opportunity for building in water. Naturally occurring alumni silicates allowed the concrete to set without shedding water, allowing bridge foundations, like the one in the Pons Fabricius, to be placed in water. (Wilson, 2006)
The rise of concrete and brick building techniques also marked a major switch from the use of a very skilled and specialized labor force for construction projects, to a more unskilled and lower socio-economic class. In early Roman times, most structures were built with with stone blocks. This required a tremendous amount of labor and time. Stones had to be cut at a quarry, and hauled great distances to get to the site. A standard size of a stone block was 2x2x4 Roman feet (11.75 in). A travertine block this size weighted 2,705 pounds, too big to even put on the average wagon. (Moore, 1995) Despite the struggle of transportation, structures like the Pont du Gard aqueduct stand at an incredible 155 feet, built entirely out of stone blocks. The use of concrete and brick created a faster, cheaper and more efficient way of building. Where as masonry required a single source, building with brick and concrete allowed the use of multiple sources of materials and manufactures. What this did economically was de-skil the labor force and employ the lower class of Rome. As the population of Rome boomed, building practices such as using Opus Reticulatum became a common practice (Figure 15).
Figure 15: Opus Reticulatum
The bridge shows a unique mix of the old craftsmanship of masonry as well as the new techniques using brick and concrete. The original structure is built almost entirely using stone masonry, while repairs carried out in 23 BC shows the new style of brick and concrete. The bridge looks to have been repaired using concrete with tuff as aggregate in certain parts, and was almost all refaced with brick. The wear and tear on the bridge over the years reveals these layers, which in turn shows the change of constructions styles.
The Pons Fabricius today is a tangible reminder of the undeniable greatness that the Roman Empire once was. Much like the Pantheon or the Aqua Claudia, the builders of the Pons Fabricius used the limited engineering knowledge they had to create an incredibly strong and durable structure. When talking about roof vaults, it is said that if it stands for five minutes, it can stand for five hundred years. This does not apply to masonry bridges. Complex loading and erosion and damage caused by water and flooding add many factors that can be unpredictable. The fact that time, natural disasters, and use for a continuous 2000 years has not phased the bridge is astonishing. The Pons Fabricius is truly an engineering marvel.
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