Trajan's bridge relief on the Trajan's Column in Rome
EMPEROR TRAJAN AND HIS DANUBE BRIDGE

How the Roman conquest of Dacia resulted in the construction of a 1,158m long super-bridge in Eastern Europe?

Introduction

Judging from territorial area, Rome reached its maximum extent during Trajan (53 - 117 CE)'s rule. Trajan was a legendary Emperor who would be forever remembered as the optimus princeps, meaning the best ruler, and the first Emperor born outside of Italy.

Trajan was born in a Roman colony near modern-day Seville, Spain, but he was fully Italian and was born with Roman Citizenship. His career path involved serving in the Roman Army in some of the most troubled frontlines, and he had plenty of hand-on combat experiences. In summer 97 CE, Emperor Nerva's health was failing and he adopted Trajan as his heir, thus paving the way for Trajan's succession. Adoptive succession was nothing new in Roman Empire, but from 96 - 180 CE, the "Five Good Emperors" formed an uninterrupted line of adoptive succession, which was unprecedented in Roman history.

Despite hailed as a role model of peaceful transition of power, historians always suspect that Nerva was pushed to adopt Trajan as his heir, and Trajan's relation with the despised tyrant Domitian was played down by Classical authors. However, regardless of all the conspiracies surrounding Trajan's ascendance, his achievements and qualities as a leader were beyond reasonable doubt.

After Trajan succeeded Nerva, he adopted an aggressive expansionist policy. As a battle-hardened soldier who served in Germania, he believed that only conquest could bring Rome glory and the best defense was a good offense. His first target was the Dacian Kingdom, the only state in Europe with an organized government apart from Rome.

Marble bust of Trajan

Marble bust of Trajan

Construction of the Trajan's Bridge

Trajan's interest in Dacia was due to both its strategic location and the tremendous wealth and Gold/Silver mines within its border. He campaigned twice in Dacia: the first time succeeded in making Dacia a Roman vassal, the second time complete annexation of Dacia into a Roman province.

 

During Trajan's Dacian campaign, Rome constructed one of the most impressive engineering feat in the Early Empire period: the Trajan's Bridge. The Trajan's Bridge was the first permanent-structure bridge in history to surpass 1km in length. The construction of its 20 masonry pillars and segmental arches stood as a testimony of Roman engineering excellence.

Trajan ordered the construction of the bridge in 103 CE, as an aftermath of his inconclusive first Dacian campaign. The Dacian capital Sarmizegetusa Regia was well-defended and surrounded by a network of fortified strongholds. The Roman Army desperately needed to get the heavy siege equipment across the Danube quickly before it launched the second invasion. He entrusted the architectural and engineering work to a man called Apollodorus of Damascus(Greek: Ἀπολλόδωρος ὁ Δαμασκηνός), an ethnic Greek engineer from the province of Syria. Construction work continued until 105 CE and once completed, the bridge would serve as the primary, if not sole, Danube crossing for the Roman Army when they launch invasions into the Kingdom North of the river.

Artist's concept of the construction site of Trajan's Bridge

Artist's concept of the construction site of Trajan's Bridge

 Trajan's Bridge: a Roman engineering marvel

Archaeological works determined the total length of the bridge to be 1,158.33m, 1,097.5m spanned the river. The masonry pillars were made of a concrete core with a brick face, and the spacing between the pillars was remarkably uniform. Among the pillars surveyed, the average center-to-center spacing was 54m±2%, this implied an average span of each segmental arch of 55m (The wiki number of 38m was a gross underestimation), a number unrivaled anywhere in antiquity. To reach this remarkable span, the arches were constructed of timber (likely Oak wood) connected by mortise and tenon and possibly reinforced with Iron.

We know that Iron was used in the construction of Roman bridges, it reflects the scale of mass-production of Iron in Roman Empire. In Antiquity, Iron was expensive and difficult to work with, and was usually used to make military equipment and tools. However, as one of the top pre-industrial Iron producers, Rome was able to use Iron extensively in monumental buildings. The Colosseum once had 2-300 tons of Iron clamps for fixation of marble panels. It is thus understandable that Iron was possibly used in the construction of Trajan's Bridge, a flagship project vital for national interests.

Calculated mechanical properties showing an arch of Trajan's Bridge under load.

Trajan's bridge mechanical properties.

Trajan's Eastern Ambitions

Trajan's ambition was far greater than the conquest of Dacia, his true strategic goal was to settle the "Parthian problem" once and for all. He used succession crisis in Armenia to legitimize his campaign and invaded Mesopotamia with a huge Army in 113 CE. Trajan succeeded in capturing the Parthian capital Ctesiphon for the first time in history and advanced as far as the Persian Gulf. Trajan also briefly took the old Persian/Elamite capital of Susa and Roman Army was knocking at the door of Iran proper. However, he failed to take the stronghold of Hatra and a Jewish Revolt cut his campaign short.

Trajan briefly established the provinces of Mesopotamia and Assyria, he made Parthia a Roman vassal and installed a client King. In 117 CE Rome controlled 5+mil km  of landmass and 2.5mil km  of the Mediterranean, with a population potentially in excess of 70 million. 1/4 of the world's population once lived under the rule of Trajan. His achievements in the East was short-lived though. During Trajan's Parthian campaign, a Judean revolt called Kitos War broke out again, forcing Trajan to face a two-fronts war. Nevertheless, Trajan managed to reach the Persian Gulf and his army briefly advanced as far as the Old Elamite Capital of Susa. Trajan's life-long dream of repeating Alexander the Great's conquests would never materialize. After he died of illness on 11 Aug 117 CE, his successor Hadrian decided to give up his Eastern conquests. Armenia's situation largely returned to the status quo established after the Treaty of Rhandeia in 63 CE. Only Dacia was retained as a Roman province.

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Fate of the Trajan's Bridge

The Trajan's Bridge was intentionally destroyed by the Romans themselves to prevent it from being used by "Barbarian" tribes North of the Danube to invade Roman territory. Historical records told conflicting stories about who ordered its dismantling, some said Hadrian and some said Aurelian. Judging from the fact that Hadrian did not abandon the Dacian conquest, and Dacia was administrated as a Roman province until Aurelian gave it up 170 years after Trajan, it is less likely for Hadrian to destroy such a high-value infrastructure asset that facilitated and reinforced Roman rule in Dacia.

Surviving ruin of the Trajan's Bridge in modern Romania.

Surviving ruin of the Trajan's Bridge in modern Romania.

References and footnotes

  1. Mehrotra A. and Glisic B. (2015) Reconstruction of the appearance and structural system of Trajan's Bridge. Journal of Cultural Heritage 16(1), pp.65-72. https://doi.org/10.1016/j.culher.2014.01.005