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Roman bridges

This post is also available in: Polish (polski)

Relief on Trajan’s Column showing Trajan’s Bridge.

Bridges were an integral part of the road system. Without them, the famous Roman roads would lose much of their value. The ability to build even the most magnificent routes will be of no use when you encounter an obstacle in the form of, for example, a river. It is significant that while the older routes passed around obstacles, the younger Roman roads were led in a straight line, coping well with all obstacles. Let us refer to the example of Via Claudia Valeria, on a short distance of which we find as many as 43 bridges! Let us add that while in the construction of Roman roads we can see Etruscan, Greek and Persian influences, and the achievement of the Romans itself is primarily an unprecedented number of routes built, the Romans surpassed the entire world of that time in terms of building bridges, setting the bar high even today’s engineers.

The topic of Roman bridges is a topic for many books, if only because of the large number of surviving or reconstructed copies, as well as archaeological sites. So, of necessity, I will discuss only the most characteristic structures, which at the same time will introduce us to a few basic, both for historical and technical reasons, types of bridges.

So first a bit of history

The first Roman bridges were constructed of wood, taking into account both the ease of processing and the availability of material. For this reason, later on, wooden bridges appear most often wherever there is a need to quickly transfer troops to the other bank. An example of such a construction is the Rhine Bridge[1], discussed below, by the legions of Caesar. Of course, there are also gaps in knowledge in the field of stone processing, so it is not surprising that the first bridges in Lazio were wooden. The pons Sublicius of the 6th century BCE is considered the oldest bridge in Rome. mentioned in Livius. It was just a wooden structure. In the times of the republic, it was always rebuilt in the same form, i.e. from wood. Why? According to legends, it was this bridge that was demolished when Horatius Kokles fought the attacking soldiers of Lars Porsenna, and each bridge in ancient Rome had not only practical but also religious significance, which will be discussed later.

A higher form of bridge construction was the replacement of wooden piles driven into the bottom of the river with stone pillars. The first such bridges appear in Italy around the 3rd century BCE, which does not change the fact that during this period the Romans continued to perfect the techniques of building wooden bridges. The pinnacle of the method of joining stone pillars and wooden spans was the bridge over the Danube[2] in Drobeta (now Turnu Severin), built by the troops of Emperor Trajan. Here, too, the use of such a structure was due to practical reasons, not to ignorance of better techniques.

The final formula for erecting bridges should be all stone structures, implemented both with the use of immobilizing stone blocks with the use of metal clamps and fillers[4], and of course Roman cement, which after the tests turned out to be quite good from the modern Portland cement. The bridges of this structure have survived to our times in an almost unchanged form, the best example of which is the bridge in Alcantara on the Tagus[3], which was said to have suffered more from modern wars than from natural conditions (i.e. from the passage of years).

At a time when the Romans were reaching for power over the world known at that time, the Etruscans offered mainly simple bridge crossings, of which two stone bridges in Bieda, small like most Etruscan structures, have survived to our times. The most famous bridge achievements of Greek engineers include two pontoon crossings. He constructed the first for Darius in 514 BCE. Mandrokles of Samos, thanks to which the army of the Persian ruler defeated the Bosphorus and went to war against the Scythians. Darius’ successor, Xerxes, in 480 BCE he set off for Greece via the Hellespont on a pontoon bridge built by the Greek Harpalos.

In the Aegean culture, simple stone bridges were known, in which a block of stone was thrown over the empty space. Egyptians in the 6th century BCE they used simple arched structures that transferred the load to the pillars, thus making it possible to increase their span. This made it easier to cross larger rivers. This solution was taken over by engineers of Hellenic culture, and in the 2nd century BCE. the Romans got acquainted with them and developed this technique to an impressive size. As you can see, history does not know of any earlier bridge structures that could match later Roman achievements.

More interesting Roman bridge structures

1. Caesar Bridge on the Rhine

Initially, there was a fierce dispute over this bridge. On the one hand, we have a detailed testimony, on the other hand, as we know, the author himself testifies, and his words were also questioned. The current findings suggest that both the number of soldiers available to Caesar (estimated at 40,000) and the level of engineering knowledge at the time did not prevent this bridge from being built. His drawing reconstructions were attempted several dozen years ago. The photo shows a model that can be seen at the Museo nazionale della civilta romana in Rome. A detailed description of the construction can be found in Caesar’s memoirs of the Gallic War, book IV, 17-19. This bridge was built in 55 BCE. between Koblenz and Andernach, and as Caesar himself admits, it was not necessary from a military point of view, Roman troops at that time could easily cross on rafts and boats. Its construction is, as we would say today, a purely PR activity. The idea was to present the Germans the engineering and organizational skills of the Roman army, which, according to Caesar’s description, had the desired effect. Contemporary researchers were most doubtful about the comparison of the parameters of the bridge with the pace of its construction. Some analyzes show that the bridge could be up to 400 meters long, 7 to 9 meters wide, and the river was 9 meters deep. Such a sizeable structure was to be built in just ten days! Contemporary calculations, taking into account the technical knowledge of the time, showed that it was possible. It should also be remembered that, after all, the author’s account was widely known in antiquity, while no one accused Caesar of lying in this matter. Archaeological research also shows that when it comes to the construction of wooden bridges, it is not the pinnacle of Roman engineering. The Caesar Bridge on the Rhine stuck in the memory of posterity mainly due to the detailed account of its construction.

Permanent wooden bridges were also erected throughout the empire, as evidenced by the mention of the renovation of the bridge at Mutina on the via Aemilia route in CE 259, as well as the archaeological work at Le Rondet in Switzerland at the remains of the bridge over the Broye River, which showed that the bridge was was built in the time of Augustus, and renovation works were carried out even in the middle of the 3rd century CE

2. Trajan’s Bridge over the Danube

Trajan’s Bridge on the Danube

The work of the famous Apollodorus of Damascus, who is considered (together with Vitruvius ) to be the greatest architect of antiquity. The author of Trajan’s Forum, along with the famous column (on which the aforementioned bridge was also carved, as well as the earlier pontoon crossing), being a kind of court architect of Emperor Trajan, accompanied him in both expeditions against the Dacians led by Decebal. During the first expedition, he designed and supervised the construction of a pontoon bridge, during the second, he climbed to the heights of bridge engineering, building a bridge that was a connection between stone pillars and wooden spans.

The bridge was built between 103-105 CE, in Drobeta, now Turnu Severin. The most difficult stage was the construction of twenty pillars, spaced every 51 meters at the bottom of the Danube. The technique of their construction was discovered in 1909 when it was decided to dismantle two pillars that were obstructing navigation. In the first stage, a waterproof cover was erected, filling the space between two rings driven into the bottom of piles with concrete. Later, water was taken out and the construction of a brick wall of the pillar began. Its interior was then filled with concrete mixed with stone rubble, and then the entire pillar was faced with stone slabs, connected with metal clamps. In this way, a structure measuring 18 by 33 meters was achieved. Only the bridge abutments on both sides of the river were made entirely of stone blocks. We know the wooden spans from the description of Cassius Dio and the sculptures on Trajan’s column. They consisted of three parallel arches connected by struts. The pavement and the railings of the bridge were also made of wood. The reconstruction of the bridge span in the form of a mock-up can be seen today at the Museum of Iron Gates in Turnu Severin. The entire bridge was 1,135 meters long, 13 to 19 meters wide, and rose about 46 meters above the Danube level.

Why was timber used for the spans, although the Romans already successfully used brick and cement? The Decebal Dacians were not an easy people to subdue, Emperor Domitian trumpeted their victory over them, ignoring the fact that it was a de facto truce paid by Rome with a high tribute. It took two wars for Trajan to finally show off Decebal’s great treasure during his triumph. The bridge had wooden spans so that it would not be used by the enemy in a counterattack. It was easy to burn it when needed. The pillars themselves, although they are a marvel of architecture, would be of no use to unfamiliar barbarians.

3. Bridge over the Tagus in Alcantara

Bridge over the Tag in Alcantara
Author: Dantla from de.wikipedia | GNU Free Documentation License, Version 1.2

Considered the most magnificent Roman bridge, at least among those that have survived our times in the best condition. The bridge was 194 meters long, the 8-meter wide roadway is 47 meters above the bottom of the river. Erected at the beginning of the reign of Emperor Trajan. On one of the arches we find the inscription: PONTEM PERPETUI MANSURUM IN SAECULA MUNDI FECIT DIVINA NOBILIS ARTE LACER, which means “The bridge destined to last forever through the ages of the world was built by Lacer”. Thanks to this, we know that its builder is a certain Gaius Julius Lacer, who must have been proud of his work, because he ordered to be buried nearby. The size of the individual span arches is admirable. The two middle ones reached over 30 meters of span, thanks to the use of cement. This bridge has survived to our times (although, like the bridge in Trier, repaired many times) mainly due to the engineering excellence of its structure. The demolition of one span did not cause the other spans to collapse, nor did the war damage during various conflicts (including the wars of the Spaniards with the Portuguese and the French in the 18th and 19th centuries), in which one of the arches was destroyed to make it difficult for the enemy to cross, nor did they destroy the entire bridge.

The middle of the bridge was formed by a road, in Latin called iter, it was accompanied by two-sided pedestrian sidewalks (decursoria), and was topped with full or openwork balustrades called plutei.

In the middle of the bridge, there is a triumphal arch, 14 meters high, with the inscription: Imp. Caesari. Divi. Nervae. F. Nervae Traiano. Aug. Ger. Dacio. Pontif. Max. TRIB. TEST; VIII Imp. V.Cos V. P.P.

4. Bridge over the Moselle in Trier

Bridge over the Moselle in Trier

One of the two was once located in this city. The older one, built-in 44 CE, which has not survived to our times, was of a mixed structure, made of wood and stone. The wooden structure, 300 meters long, 10 meters wide, rested on eleven mighty stone pillars, pentagonal in shape, with a blade directed against the river, made of blocks of stone fastened with metal clamps. These pillars were additionally supported by wooden piles. There were about 250 piles per pillar.

In the 2nd century CE Another bridge was erected when the older one, possibly damaged, still existed. The new bridge, including access roads, was 350 meters long. The structure itself, although slightly shorter – supported by seven pillars and two abutments – was much more solid. During construction, so-called curtain walls made of double rows of piles sealed with clay were used, which allowed water to be removed from the place where the pillars were erected. These were built of three types of stone – basalt, limestone and sandstone, which were connected in the form of blocks with metal clamps, and the joints were flooded with lead. The arches of the bridge are made of concrete. The bridge has been repaired many times, for the first time already in the 4th century, despite this, its solid structure has survived to our times and is one of the main attractions of Trier, it should be noted that the original ancient lineage has only five pillars, the rest are later repair work and reconstructive.

Estimates of the materials needed for its construction are 6,245 kg of wood, 1,100 kg of clay, 11,706 kg of stone and 25,500 kg of iron and as much lead. It is also estimated that a group of 100 workers would need six months to build the bridge.

Religious aspect

As I mentioned earlier, the bridges of the Romans were not only strategic but also religious. Their construction was, in a sense, an expression of human opposition to the intentions of the deities who were responsible for the existence of an obstacle in a given place, e.g. a river. The construction of the bridge, as an act against the intentions of the gods, was therefore preceded by propitiating religious rites, often combined with human sacrifice. The custom of throwing small dolls into the Tiber on May 14 is considered to be an echo of this practice. The high priest of Rome was pontifex maximus, or the supreme bridge builder. The college of priests, which was bound to care for the oldest bridge, pons Sublicius, the college of bridge builders (collegium pontifices) had initially three priests, to reach sixteen in Caesar’s day. From the time of Caesar and Augustus, the title pontifex maximus is also a dignity held exclusively by the emperor. In 375 CE the Christian ruler Gratian refused this dignity as pagan. The “stray title” was taken by Pope Leo I (440-461) and has been worn by every pope ever since. A horse with a row for this Catholic who remembers about the bridge pedigree of this title.

Author: Gajusz Juliusz Krisbaum (translated from Polish: Jakub Jasiński)
  • English-language Wikipedia articles
  • Gaius Julius Caesar, The Gallic War
  • Wielowiejski Jerzy, Na drogach i szlakach Rzymian, Warszawa 1984

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