718. Roman Bridges by Colin O'Connor

PONS AEMILIUS BUT NOW CALLED PONTE ROTTO

718.  Roman Bridges by Colin O’Connor.  This is a book well worth reading.  He begins:

The Romans did not invent bridges or roads but were the first to develop a system.  Bridges and roads go together for obvious reasons.  

A bridge is valued for reasons of its beauty, function, social reasons as it promotes trade, communication.  It also helps to create towns, cities. For the Romans bridges also possess religious significance.  Ancus Marcius, the fourth king of Rome, caused the construction of the Pons Sublicius.  Its construction and care were placed under the care of a priest, pontifex.  Pontifex means bridge builder.  Romans were animists and believed that rivers possessed a divinity and this divinity required permission to be crossed.

Romans avoided steep grades, often following river valleys.  They minimized river crossings and stayed on one side until a crossing was necessary or one road met another at right angles near a river.

O’Connor periodically covers briefly Roman history and then juxtaposes that history with road and bridge construction up to that time.

Rome commanded the Tiber River as it empties into the Mediterranean Sea.  To the north is Etruria which consisted of a league of 12 cities from the Tiber R. to the Arno River (in modern Florence).  To the south was/is Latium.  The people there had the same language as Rome.  This area stretches from the Tiber to the Pomptine Marshes and the Lepini Mts. just beyond Cape Circeo.

Rome grew under the Etruscan leaders it had.  In 530 BC the Cloaca Maxima was built this was a large open drain which made the area now called forum useable.  It was covered in the 4th century BC.  Tarquin (called the Proud by the Romans) was booted by the Romans in 510 BC.  He was not happy about this and went to Caere.  There he convinced Porsenna, the king of Clusium to help him regain his throne.

When Porsenna marched on Rome his ability to cross the river was hindered by the famous stand of Publius Horatius Cocles and his two friends, Spurius Larcius and Titus Herminius on the Pons Sublicius.  It is likely that inspite of the courage of Horatius, Porsenna occupied Rome.  But in 506 BC at the Battle of Aricia, the Latins defeated the Etruscans and at this point the Latin League was formed.  In 493 BC Rome and the Latin League combined.  In 360 there were some members of the league unhappy and this war, Rome won.

In 479 BC Rome attacked Fidenae in Etruria and failed.  But since Fidenae was critical for the security of Rome, it was taken in 426 BC.  In 405 BC Rome attacked Veii, the last Etruscan stronghold at Rome’s doorstep and took it in 396 BC.  

Rome’s advance into the south (remember the Latin League) involved them in three wars against the Samnians.  

The 1st Samnite War lasted from 343 to 341 BC.  The Sabellian tribe was absorbed.  The reason for this war was, as so many in which Rome was involved, the request of a nearby city.  The war began when Capua sought help from Rome against the Samnites.  Rome helped but the war was indecisive.  The 2nd Samnite war was fought 326 to 304 BC.  Rome was defeated at the Caudine Forks which is between Capua and Beneventum and at Lautulae.  At this point Capua joined the Samnites but Rome retook Capua in 314 BC.  In 304 BC the Samnites asked for peace.  3rd Samnite War was fought in 298 BC.  Lucania asked Rome for help.  The Samnites formed an army and it arrived north of Rome where the Romans defeated it at Sentinum.  Roman power stretched from Ancona on the Adriatic Sea to the Gulf of Tarentum.

Tarentum was concerned about the advance of Rome and formed an alliance with Pyrrhus, the King of Epirus in Greece.  The war lasted from 280 to 275 BC.  At Beneventum, the Romans won a major victory to win the war.  

Back to bridges.

The Romans built the Via Appia from Rome to Terracina on the Tyrrhenian coast, then to Minturnae, then to Capua and finally all the way to Brundisium on the heal of Italy.  In 314 the Romans retook Capua toward the end of the 2nd Samnite War.  So the Via is strategic for Roman security.  But it quickly became a trade route.  There is disagreement on when the road was paved but O’Connor is convinced that it was a substantial road from the beginning.  The road through the Pomptine Marshes was frequently flooded, parts of the road required boat transport.  Years later at Terracina, Trajan built a rock cut path for the Via Appia, visible today.  The road went through the pass at Lautulae (where the Romans were defeated) and crossed the Liris River (Garigliano)  The Romans established a colony at Brundisium in 244, thus the road was probably completed to that point by 264 BC.  From Beneventum there is a northern route along the Adriatic and this may be the Via Minturnae mentioned by Cicero.  These roads secured the area both for the Romans and for the inhabitants.  These also provided a means of trade and communication. The area south of Rome was secure.

The Via Annia joined Capua to Rhegium.  Parallel to Annia is Via Domitiana (imperial road) giving access to Puteloli (Pozzuoli).  This area had the valuable ash which Romans added to concrete to allow the concrete to dry in wet environment.  In the same area is Via Herculia.  It is a late road.

The Via Salaria began at the Pons Sulpicius and followed the Tiber.  This road was important as a trade route.  The Via Latina is connected with the fight against the Aequi for the pass at Algidus.  A battle was fought there in 431 BC.  This is the battle at which Cincinnatus, elected dictator in an emergency, was victorious.  The Via Valeria was built in 307 BC to Carsioli and a colony established in 303 BC.  The Via Clodia went north at Satrum to help control the area.  Via America went north to Horta in 240 BC.  The Via Cassia went north to Florentia (Florence). The Via Aurelia went north along the coast on the west side of Italy.  The initial road was constructed by Caius Claudius Cotta in 241 BC. Eventually this road was extended all the way to Spain.  The area north of Rome was secure.

More history.  In 276 BC Rome had control from Eturia to Brundisium.  Messana on the tip facing of Sicily facing Italy was attacked by Hiero, the King of Syracuse.  Messana asked Rome for help.  Thus the 1st Punic War, for there was a strong Carthaginian presence in Sicily.  Rome had naval victories at Mylae in 260 BC and at Ecnomus in 256 BC.  Rome invade Africa in 256 BC.  It was here that Marcus Atilius Regulus was captured and displayed such courage.  But Rome won the war and Sicily was seeded to Rome.  Soon after Sardinia and Corsica were acquired.  

In the north.  Gauls crossed the Alpes into Italy, advanced south and defeated the Romans at Allia River in 390 BC.  After a ransom was paid (all this was was a plundering expedition), the Gauls withdrew.  In the 3rd Samnite War the Samnites were aided by people in the area of Sentinum north of Rome. Thus at the end of the 1st Punic War the Gauls again invade Italy and made it all the way to Telemon on the Adriatic coast just below Ravenna.  The Romans drove them back and began establishing colonies along the Po River.  By 220 BC Placentia, Cremona, Luna and Genoa had been established.

Roads.  Thus the Via Flaminia.  This road left Rome at Pons Mulvius.  It was built by Flaminus when Censor in 220 BC.  The road was extended to Ariminum on the Adriatic.  There is a Roman tunnel through the Furlo Pass. 

History-  In Spain Saguntum asked for an alliance with Rome.  Granted.  The 2nd Punic War commenced when Hannibal besieged the city.  Hannibal initially victorious in Italy, his reinforcements were defeated at the Metaurus River (Romans would have been helped by the Via Flaminia).  The Romans were also successful in Spain and Africa.  The war was over in 202 BC.  After this war the Gaull in northern Italy attacked colonies in northern Italy.  2 armies were sent and were victorious.  

Romans added new roads.  Via Annia (two by that name)  which went from Aquilia to Bologna.  Via Postumia went from Genoa to Verona.  Via Popilia went from Ariminum to Ravenna.

In the beginning the sea was the easy route to Gaul.  The Romans had a long, valuable alliance with Massilia.  The rivers were used as trade and convenience up the Rhone.  But after Arausio October 6, 105 BC where the Romans were defeated by the Gauls.   The Romans won at Vercellae and Aquae Sextiae soon after.  There were and had been for far more than a hundred years major problems with Gauls.  So the Romans took the whole of far southern France.  A network of roads were built.

In Spain a network of roads for strategic reasons and commerce- there were mineral deposits of gold, silver, copper, mercury and iron in Spain.

Northwest Africa saw the construction of roads for defense, commerce and agriculture.

In AD 43 Britain was invaded.  A web of roads was built.  

Builders of roads

In the Republic the roads were built by 12 consuls, 6 censors and 2 praetors.  Appius and others may have done more than just say build the road.  There are two reasons for this:  the highest ranks of Roman elective service required training and experience as of 180 BC.  Some expectations surely exited previous to this. The lowest office was that of Quaestor.  Minimum age was 25.  Next Curule Aedile at an age of 36.  The Praetor required an age of 39 and the consulship 42.  Before these offices were pursued, the Romans had a habit of expecting those on the move to serve as assistants to general, governors and office holders.

There is another reason that people such as Appius knew more than just how to say build the road.  Marcus Vipsanius Agippa, 2nd in command to Augustus, supervised the construction of a fleet, equipment, was responsible for two aqueducts, the cleaning of the cloaca maxima (for this he actually boarded a row boat and personally looked at the cloaca and the Pons Agrippae.  He had a map made of the entire empire with details of cities, roads, terrain.  The ruins of his bridge can be seen 160 meters northwest of Ponte Sisto in Rome. Agrippa’s interests show that he was more than an administrator.  Sextus Julius Frontinus wrote on surveying, the art of war, military science, farming, boundaries, roads, colonies and aqueducts. This last survives, De Aquis.  He was consul, general, writer and governor.  In the De Aquis he states that he had a habit of learning everything he could about whatever he was assigned. Pliny the Younger, when he was governor in Bithynia in northern Turkey went on site to the springs for an aqueduct to figure out a problem.  When younger, he was in charge of the drains and Tiber in Rome.

The Curator Viarum, caretaker of roads, was an important position.  Apparently it was not a political payoff position.  In most cases this position followed the Praetorship.  Engineers (fabri) often came from the army.  These people would have had a great deal of experience.

Labor was supplied by soldier, slaves, free citizens.  There was a huge carpenter association in Rome called the collegium fabrum tignuariorum.  

So O’Connor’s point seems to be that many people in positions of building knew a fair amount about the job before they took on the position.  (I know from reading Cicero’s letters, for example that he clearly knew a great deal about architecture and construction.)

Roman Technology

There are designer constraints:  ability to shape the material, transport and handle.  Roman tools:  hammer, axe, adze, pick, drill, file.  For these a hardened iron was essential.  The old view that the Romans only borrowed and had poor regard for practical skills are buried by the evidence presented by O’Connor.  He shows that there was no shortage of ideas, skills and principles as evidenced by what they built.  He interestingly works backwards to prove this, since no manuals survive.  

Romans lacked extensive mechanical power, hard metals.  And for the Romans employment was a major concern.  And not having a modern capitalist system, they did not want to eliminate workers.  A large cash of tools was found in Scotland and huge numbers of nails of all sizes. These indicate the existence of some kind of assembly line set up.

They possessed great skill in ship building.  Frames were held together by wooden pegs.  Planks held in place by pegs.  Planks were mounted edge to edge, not overlapping, with biscuit wedges.  The wooden pegs holding the planks were pierced by bronze spikes to make a very tight fit.  It is clear that the spikes were made in bulk.  So his point is that there must have been some kind of production system in place to do this.  He also gives high marks for such ship construction.

The Romans knew of the male screw but not of the female nut.

To give a comparison O’Connor mentions that in 1586 the obelisk of Nero’s Circus was moved 240 meters by Fontana. It weighed 310 tons. It took five months and was considered a great feat.  The Romans had moved this obelisk down the Nile River, across the Mediterranean and by land to Rome.  By the way the ancients made rope to the same standards as modern rope.  

There is a crane carved on the tomb of Haterius.  It is in the Vatican Lateran Museum.

In the construction of bridges there was need for a reach of 15 to 18 meters from pier to pier.  The Romans as the tomb shows and the bridge makes clear had the equipment to move large rock the distance and also possessed the pile drivers necessary to create a means to install piers in the middle of a river.  Oak piles have been found with iron tips embeded in rivers.  The Romans some how learned of pozzolana (a tufa, volcanic rock) which allows concrete to harden in wet areas.  It also made Roman concrete very hard, even by modern standards.

Tufa with lava, white limestone and other elements is called peperino.  Travertine is a form of limestone.  It is easily damaged by fire but as bridge material, no problem.  The Pons Mulvius is made from it (109BC).

There are 91 bridges in Italy.  32 use Travertine, 29 use limestone, 23 use tufa, 7 use a conglomerate.  The Romans apparently figured out that the hardest stone did not make the best material for bridges.  Does this make sense?  It will.

A stone with a high proportion of very small pores is less durable than stone with large pores.  Large pores allow space for trapped moisture to expand and contract without doing damage.  The saws used by the Romans to cut limestone, etc. was used well into modern times.  Sand was washed into a cut by water flow.  The different styles of stone laying indicates a knowledge about the material and an ability to adapt different methods to different needs.

Concrete.

Calcium carbonate (lime) is burnt.  Burnt lime is calcium oxide.  When water is added to it it is calcium hydroxide.  This reacts with carbon dioxide in the air to form calcium carbonate.  Add pozzolana and it hardens in water.  Pozzolana causes the lime to be taken up and this adds strength.  The Romans had little idea of the chemistry but knew the results.

The Romans kept aggregate separate from the concrete mix.  These were mixed together at the site.  The Romans liked to lay brick molds, a wall in which to place the concrete.  These were made simultaneously to make sure that the wall which served a s a mold (and pleasant visible surface if so desired) bonded.

In vaulting and such timber was used to form the molds.  In bridges exposed concrete is not used.  The durability of Roman concrete is famous and in some cases it is better than modern concrete. But the physical structure and pore size count more than simple strength.  Roman concrete contains air holes which help to reduce internal pressure due to freezing and thawing.  Crystals which absorb salts also do less damage.

Romans used timber for centering, scaffolding, cranes and for final structures they did build many bridges out of wood and for good reason as we shall see). Romans were well aware of the qualities of different woods.  Vitruvius lists twenty different kinds of timber.  The strongest was ash (fraxinus).

The surveying tools were the groma.  It allowed a straight line to be followed or right angle.  There was a leveling device, chordates and the dipotra which was more elaborate.  Typically aqueducts dropped 1 foot for 500 feet.  Try to imagine the problems of maintaining this over up even terrain with rivers, ravines and mountains.  The Romans also had a hodometer.  A carriage with 4 wheels, each wheel four feet in diameter.  400 revolutions of the wheel caused a drum to rotate 1 time.  A stone in the drum fell out into a bronze container.  The noise would indicate one Roman mile (5,000 feet).

Computations.

Did Romans produce drawings?  Yes.  The tools to do so survive.  The Abacus makes it clear that the Romans could add, subtract, multiply and divide by simple numbers.

Masonry Bridges.

Pons Mulvius- the Via Flaminia begins here.  It has 6 spans.  Spans 2 and 3 are faced with Travertine.  Livy mentions this bridge (27.51.2).  M. Aemilius Scaurus restored it in 109 BC.  There are flood openings in the middle of some piers to relieve the strain during floods.  

Pons Fabricius was built in 62 BC.

Ponte Rotto was built in 179 BC.  Marcus Fulvius Nobilior and M. Aemilius Lepidus were the Censors.  The original one had a timber deck.  The stone arches now visible were added by Scipio and Mummius in 142 BC  (This is the Scipio which captured Carthage in the 3rd Punic.)

The oldest bridge is the Pons Sublicius.  600 BC.  It was made out of wood.  One was still there is AD 400.  No longer exists.  Evidently it was maintained as a wooden bridge for centuries.  It was located near Forum Boarium, down stream from the Pons Aemilius.

Ponte del Diavolo.  One arch rib remains.  It connected to Via Latina which connected Cassinum with Interamna Lirenas.  Near Arpinum it crossed the Liris via Ponte Marmone (This is the bridge near Cicero’s home).

Ponte di Sperlonga is unusual.  Built 1st century BC. 

Past Minturnae Via Appia heads to Sessa Aurunca.  And there in a farmer's field is Pons Aurunca, 1 kilometer west of town.  Follow right bank of the stream down from a minor road which leads to route 7.  Access is along old Via Appia. 

Via Flaminia has more bridges than any other road.  But it is sad to see how many bridges were destroyed during World War II.

Gaul- roads came later here.  Water ways had provided means of commerce and travel.

Spain perhaps the greatest of all Roman bridges is here- Alcantara over the Tagus River is spectacular.  Merida may be the longest, Puente Romano, 3,000 feet plus.

Turkey

There is an arch in a theater with voussoirs stepped at mid length, perhaps to deal with earthquakes.  This technique may have been used in bridges.  But it certainly shows Roman attention to various variables.  There is a bridge at Kahta, one of the largest spans at 34.2 meter.  The largest surviving is Pont St. Martin at 35.6 meter.

Timber Bridges

Very common.  There was plenty of lumber.  There was technology to cut and work wood.  Quick to build.  Besides Romans built wooden structures to hold stones in place as stone bridge was built.  Columns in Rome show wooden bridges.  In Great Britain there were numerous.  For sure wooden bridges were built first when a road was first constructed.  Are there other reasons for wooden bridges? Romans did not build bridges out of stone near seas.  

The columns of Trajan and Aurelius make clear that the Roman army owed as much to the spade as to the sword.  The carving accurately reflect bridges and physical evidence has been found.  The mason for the columns clearly copied from sketch books to display the bridges.

It is possible that Apollodorus may have used trusses on his bridge across the Danube, perhaps the greatest bridge the Romans ever built and that out of wood.  Timber girder bridges are found on the columns.  There is precedent.  Caesar built a timber bridge across the Rhine in 10 days.  It may have been 260 meters long. Apollodorus' bridge- one pier remains.  It crossed the Danube.  Dio Cassius says that there were 20 piers 150 ft high, 60 ft wide and 170 feet apart.  It was about 3,600 feet long.  

Via Appia at Minturnae crosses the Liris (Garigliano ).  Here was the Pons Tirenus.  It was built in 295 BC.  It was known to exist in AD 548.  The Romans used wood where stone was not suitable.

More evidence of application of rules for construction:

The Romans used inverted syphon when bridge height required exceeded 45.7 meter.  Such a height made a bridge impractacle.

Romans used two kinds of aqueduct designs.  One- used numerous arches with short spans for long stretches across gentle valleys.  Two- large spans across large rivers to reduce number of spans.

Rules for bridge design.  This was difficult for me, I did not understand the math.  But did try to get the implications.

The rules appear only by studying the bridges.  

Abutments (end of bridges) constructed first.

Then the foundation for the piers.

Then the piers.

The arch ribs consist of springing voussoirs with radial joints. and keystone.  The underside of the arch rib is called intrados and the upper is called extrados.  The space between the one extrados and the next is the spandrel.  This was filled in either with dirt of concrete.  The weight of the spandrel pushes down and sideways.  The arch pushes inward.  Thus there is a balance.  The end pieces must be wider to help control the end arches.

Virtuvius says 3 types of foundations:

1.  Build directly on the surface on solid material.

2.  Timber piles driven into soft earth

3.  Foundation on water with forms made of beams driven into the water.  Water is pumped out and the space filled with concrete or masonry.

Piers could have vaults piercing in order to deal with floods.  Triangles pointed up stream to break the force of water. The thickness of the piers is 1/3 of the span distance.  Sometimes it is 1/2 and a few 1/5.  The Romans clearly understood the relation between the two.

Arch design.

Arch consists of arch rib, spandrel and material in the spandrel.  Ratio of rib thickness to span is important.  Ranges from 1/10 to 1/20.  This is a wide range.  How does this indicates that Romans had a sense of arch design?  If a span is 50 ft or less, most bridges are 1/10.  

A voussoir with a width of 3 1/2 feet by 5 ft by 5 ft would weigh 9,438 lbs.  This is near the limit of what a crane could handle.  But the keystone of the Ponte Manlio is is 20,328 lbs.  So there must have been special cranes developed for unusual situations.

O’Connor demolishes the idea that segmented arches came to Europe from the Chinese.  In China there is a bridge built in AD 600 which is segmented.  There are far too many segmented bridges in Europe from the Roman period to allow this.  Prime example is Apollodorus bridge across the Danube.  It clearly is segmented.

Roman Ribs.

Different types.

Most common- stretcher bonds with staggered rows.

Arch construction.

Cane could reach 11.5 meter.  First bridge was usually of wood.  This allowed materials and equipment to be moved about.

My observation- it is clear that Romans mastered the problem of form and function.  For those aspects of a bridge which add texture with its shadowing and appeal are directly related to the function and construction of the bridge.

Falsework (the wooden framework which held stone in place until ready to take on the weight of the bridge) supports are often found part way up the springing voussoirs because these could be constructed without support.  Many place survive on bridges where protruding stone was used to support falsework or the holes into which supports for falsework was placed.  Thus the spandrel may have been filled before arch rib was completed.  It may have added strength.  Falsework could maintain huge pressure, so when it was removed, there must have been a way to do so without causing pressure on the stone about in uneven ways.  It is important to remember with cranes that two may have worked on opposite ends of an arch.

Analysis of an arch.

O’Connor here gives mathematical explanation for ability of Roman arches to function.  It was beyond me.

Conclusion.

The Romans probably built thousands of bridges.  Their achievement is measured by shear numbers, extent (all over Europe, Asia, Africa) technology.  It is good to remember that spans achieved by the Romans were not exceeded until 1755.  “The most successful, extensive and lasting of all human, material achievements”.