Aquila:Trajan's Bridge (Nova Roma)
|This article is from the Nova Roma publication "Aquila".|
Following the Roman War against the Dacians and thier surrender to Emperor Trajan, peace again settled on the area. However this peace lasted only two years before Decebalus, the Dacian king, raised the banner of revolt. He took prisioner, the garrison commander of the Roman forces left in Dacia. This comander by name was Longinus and he was a close friend to the Emperor Trajan.
The Dacian king believing that he had the advantage of the Emperor now tried to negotiate better terms with Trajan. Trajan entered into peace discussions , but making no promises. Meanwhile Longinus realizing that he was the factor holding back action by Trajan, took his own life , in a decidedly courageous and selfless act, thereby leaving Trajan free to pursue his intentions against the Dacian King.
Trajan ordered a bridge built across the Danube River, and this bridge was to be a permanent structure, consisting of a wooden bridge superstructure resting on some twenty huge stone piers, with a massive stone abutment and arch at each end of the bridge. The bridge was constructed at the Iron Gates, just below the entrance to the Djerdap Gorge, where the Danube had sliced it's way through the Western Carpathian Highlands. The bridge was 1,500 meters in length, 50 meters in hieght, and had a width of 20 meters.(1)
The bridge was built by Apollodoros of Damascus, Trajan's famed engineer and Hellenized Syrian in 102-104 AD and it was one of the largest (if not the largest) military engineering work of it's time. It is not known exactly which legions built the briidge, however we do know that there were eleven legions that could be placed in that region between Vienna and the Black Sea. The I Italica and V Macedonia were on the lower Danube River; Five legions; I and II Adiutrix, IIII Flavia, VII Claudia, and XIII Gemina were concentrated in the 100km between Belgrade and the Djerdap Gorge. A further four legions were encamped between Belgrade and Vienna. The X Gemina and the XI Claudia, posted in from Germany and may not have arrived before AD102. (2)
The pictures accompanying this article are a model of the bridge which can be found at the Iron Gates Museum in the city of Turnu Severin, Romania. What remains of the Trajan Bridge is an abutment on the Romanian side of the Danube which stands just in front of the museum. We are told that everyone in that city knows what it is and admires it greatly. (3) The remains of the twenty stone bridge piers are now underwater, and only the abutment on the river bank, mentioned above, are visible today . A Roman historian writing over 100 years after the death of the Emperor Trajan indicates the size of the bridge piers were 60 feet (18 meters) wide, 150 feet (45meters) high and 170 feet apart. Furthermore, these piers were set up where the Danube was deep and swift, with a muddy bottom. Apparently the bridge did not remain in use for very long. The historian, Cassius Dio, writes that Emperor Hadrian after giving up the province of Dacia, had removed the bridge superstructure, to prevent the Dacians from raiding across the Danube into Moesia, and that it was missing during the historian's lifetime. (4)
The construction of the bridge piers alone must have been a herculean task. First a boat (pontoon) bridge would have been built across the Danube to provide a structure on which to move materials to the planned positions of each bridge pier. If a pontoon bridge could not be built due to the powerful river flow, then all materials would have had to be delivered to pier site by barge or boat. This was probably the situation in the case of Trajan's Bridge due to the size and volume of the Danube River.
Cofferdams were constructed at each planned point of a stone pier. A cofferdam is a watertight box built of piles driven into the river bottom, which is bigger all around than the projected size of the pier itself. The piles were driven into the river bed by a pile-driver. Mounted on a raft which was anchored in place over the pile. The pile driver itsel, was a large heavy stone suspended from a derrick by a rope or cable. The stone was lifted to it's highest point by slave labor and then let fall onto the top of the pile, driving the pile into the bottom of the river. These piles were oak, if available, or alder which Vetruvius allowed, "will last forever in the water." and were sharpened to a point at the bottom. Each cofferdam was strengthened inside by heavy timber supports. The Romans would nearly always construct their cofferdams with double walls. This is further described by Vetruvius: "Double-walled cofferdams, bound together with planks and chains, are to be put in, and clay is to be pressed down as tightly as possible between them. Then the enclosure is to be emptied of water with waterscrews and waterwheels." The waterscrew to which Vetruvius refers would have been an Archimedian Screw, which was particularly useful, as it is today, since it could be turned by slaves working on dry land or in a barge tied up alongside the cofferdam. The waterwheel would probably have been used only on the largest projects since waterwheels would be very large and expensive.(5) The cofferdam was built in a shape around which the river could flow easily and then chained together, The water was then pumped out of the cofferdam while laborors filled the cracks between the vertical piles with clay. Once the cofferdams were clear of water, a foundation of tar-covered pilings was built and on this foundation the pier was constructed of large carefully cut stone on the outside and smaller uncut stone in the center. The mortar used between the stones contained Pozzolana which was a special ingredient which made the mortar very hard. (6)
When the piers reached the designated height, then the preconstructed wooden arches were hoisted into place.
On the stone piers was built a series of wooden arches, and these in turn supported a wooden deck. In the previously mentioned relief of this bridge on Trajan's column, if the sculptor who carved the relief was accurate, Appolodoros used diagonal bracing in the wooden part of the bridge. Aa far as can be determined this is the first definite example of the truss which depends for its strength upon the rigidity of three beams fastened together to form a triangle. (7)
I especially wish to provide my thanks to Prof. Popa of the University of Bucharest for his forwarding to me the pictures of the model of Trajan's Bridge. I had previously tried to contact the Iron Gates Museum by phone and by Fax, and could get no answer from either medium. It was the receipt of these two pictures of this fabulous model in the Iron Gates Museum, which inspired my writing this article.
1. P. Connolly, "Greece and Rome At War ISBN 185367303X
2. P. Connolly; "The Cavalryman" ISBN 0199104247
3. Letter from Professor Popa, University of Bucharest (22 Oct. 05)
4. P. Connolly, "The Cavalryman," L.S. De Camp, "Ancient Engineers" ISBN 0345320298
5. L.A. and J.A. Hamey, "The Roman Engineers" ISBN 0521225116
6. David Maccaulay. "City -- A Story of Roman Planning and Construction" ISBN 0395349222
7. L.S. de Camp, "Ancient Engineers"