Development in Roman stone arch bridges Colin O’Connor

The long success of the Roman Empire depended in large measure on the vast network of roads that was constructed. At its fullest extent this included more than 300 major stone-arch bridges, many still in use. In this article the author categorizes the bridges in terms of date and three basic parameters - maximum span, overall length, and height. This suggests two major periods of construction: the first century B.C. and the first half of the second century A.D. Large bridges are listed.

In a recent book [I], the author attempted to provide a thorough listing of Roman bridges. It included data for some 330 stone arch bridges, many of which are still in service, 34 timber bridges, and 94 aque- ducts, with locations ranging from Hadrian’s Wall, England, in the far north-west of the Roman empire, through Spain, Gaul, Italy, and Africa, to modern Turkey, Palestine, and Iran in the east. The technology and design of these bridges was examined, and the conclusion reached that, on all these grounds - number, geographical extent and technology - these bridges should be assessed as one of the most successful, extensive and lasting of all human, material achievements. Dates of construction were given where possible. This article takes this data for stone arch bridges and seeks to place them in chronological order, so that their development may properly be observed.

Size The most important parameters which describe a bridge are those concerning its size - chiefly its maximum span, overall length, and height. The span represents the first major difficulty for the designer and associated with this is the height, if it is great. Length in itself is not a design prob- lem, for it may be achieved by the repetition of smaller spans, but it does represent a logistical and supply problem, for the quantity of both materials and labour will increase with the length of the bridge. These three parameters alone are insufficient to

C. O’Connor, B.E., Ph.D., D.I.C.. B.D.. F.I.E.AUST.

Is Emeritus Professor of Civil Engmeering of The University of Queensland. After graduation, he worked for some years on bridge design and then joined the umversity. where he was a member of staff from 1954 to 1989. being Professor from 1970. He served as Head of Department and Dean of Engineering, and ha:. written technical papers and three major books: Design of Bridge Superstructures (1971), Spanning Two Centuries: Historic Bridges of Australia (1985); and, recently, Roman Bridges (1993).

Endeavour. New Series, Volume 16, No. 4. 1994 Copyright ep 1994 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. 0160-9327/94 $7.00 + 0.00.

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define entirely the size of the bridge; for example, there may be a succession of sub- stantial spans, none of which in itself is suf- ficient to be called extreme. It is useful to study also the sum of the two largest spans, the sum of the three largest spans, and so on. The data presented here include only the three major parameters, but studies have been made also of the others.

Figure 1 plots each of these parameters against date for many road bridges and some aqueduct bridges. It must be emphasized that, in many cases, the data are approxi- mate, but it has been considered better to include approximate values rather than exclude a bridge on the grounds of incom- plete data. The dating of a bridge may often be uncertain. It may be described, for

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Figure 1 Graphs of maximum span, overall length, and height against time

example, as Augustan, where this implies a date in the range 27 B.C. to A.D. 14. Here, the mean date, 6 B.C., is taken. The dating may be even more approximate, such as ‘in the first century A.D.’ - here it is taken as 50 A.D. - or again, a date given as ‘in the first or second centuries A.D.’ is taken as A.D. 100. It is believed that these approximations are reasonable, and in only one case is there an assumption that is sig- nificant. The bridge E23 is shown in the figure at A.D. 200. No real dating of this bridge is available, but it appears to the author that the true date is likely to be in the range 100-300 A.D. (with a mean of 200 A.D.). It can be seen that, for the purposes of this study, the approximation is unlikely to be important.

The heights plotted are intended to be from the top of the roadway (or waterway) to the level of the water in the stream below.

Figure 1 allows major bridges to be identified. They are marked and full data for them is given in Table 1. Twelve of the bridges listed in the table are shown in figures 2 to 13, in chronological order, so that any developments in their design may be observed. Not all are referred to in the text, but all are described in Table 1, and shown in figure 1.

Consider, first, the maximum spans. The bridges at the extreme left, I22 and 172, are not large bridges, but are inserted to form a boundary at the ‘old’ end of the range. Both are plotted at 173 B.C. The identify- ing numbers are those used in the book [ I] ; any others could have been used, but these may be more convenient. The prefix, I, means Italian. Similarly, G is for Gaul, SP for Spain and E for the east (effectively Asia Minor, Palestine and further east). The following numbers are, in the book, in citation order, but this has no significance here.

I22 is the small Ponte di Nona, now built into the foundations of the later large Ponte di Nona, still carrying the traffic of the Via Praenestina, east of Rome. 172 is the Ponte San Lorenzo (or d’Ercole, or Camillario) in Etruria, to the north of Rome. M.E. Blake [2] refers to ‘the systematization of the roads and bridges [in Etruria] undertaken by the Romans in 174 B.C.‘. Both bridges were built about this time. They are the oldest bridges for which reliable data have been found and provide, in a sense, a terminus, but there is no doubt that there were earlier Roman bridges. The Via Appia, generally regarded as the oldest Roman road, was commenced in 312 B.C., and, although it would have come into use before it was complete, it is only to be expected that its earliest stages would have included bridges of some sort.

In figure 1, in the graph for maximum span, a line has been placed to join I22 to 15, which is identified in Table 1 as the Pons

Figure 2 II, the Pons Mulvius in Rome, was probably built in its present form about 109 B.C.

TABLE 1 LARGE ROMAN STONE ARCH BRIDGES (LARGE DIMENSIONS ARE PRINTED IN HEAVY TYPE)

Date Bridge Maximum span Length Height (ml 0-N (ml

144-140 B.C. AQla P. [l] San Pietro

144-140 B.C. AQlb P. Lupo

142 B.C. 15 P. AemiliuslRotto, Rome

109 B.C. I1 P. Mulvius, Rome

90 B.C. 165 P. dell’Abadia

100-70 B.C. 131 P. Catena, Cori

62 B.C. I3 P. Fabricius, Rome

27 B.C. I87 P. d’Augusto, Narni

25 B.C. SP15 P. Romano, Merida [2] (1st)

c20 B.C. 1128 Porta Cappuccina, Ascoli Piceno

18 B.C. AQ30 P. du Gard

27 B.C.-A.D. 14 1155 P. St Martin

3 B.C. 1159 P. del Pondel

A.D. 14-37 G9 P. de Sommieres

cA.D. 40 SP15 P. Romano, Merida (1 +2)

A.D. 38-52

A.D. 38-52

A.D. 30-52

cA.D. 100

A.D. 104

A.D. 98-l 17

A.D. 98-l 17

A.D. 98-l 17

A05 P. San Antonio

A060 Segovia

AQ4 Aqua Claudia, Capannelle

SP13 P. de Alcantara, Toledo

SP21 P. de Alcantara, Alcantara

SP20 P. de Alconetar

154 P. Rotto, Cervaro R.

SP15 P. Romano, Merida (1 + 2 + 3)

A.D. PndCent

A.D. 200 (?)

A.D. 193-211

cA.D. 260

cA.D. 380

A.D. 560

AQ89 Aspendos

E23 Limyra

E36 Severus bridge, Kahta

E42 Band-i-Kaisar, Shushtar

E28 Taqkliprii, Adana

E7 Sakarya, Sangarius R. 24.5 429 -

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1 Pons, Ponte, Puente 2 Built in three stages 3 Dimensions of 2nd or 3rd stage

Figure 3 165, the Ponte dell’Abadia, built about 90 B.C., has a single span that is both large and high. The increase of the width of the arch in its lower courses is unusual.

Figure 4 13, the Pons Fabricius of 62 B.C., crosses to the lsola Tiberina, an island on the Tiber in Rome, by two equal spans. One of the largest dangers to these bridges was scour around the foundations and protective measures around the piers can be seen, together with the floodway provided through the pier itself.

Aemilius, or Rotto (broken bridge), in Rome. The line is probably a fair upper bound to achievement in this period. It is extended then to I155 at the year 6 B.C., this being the Pont St Martin (figure 7), with the largest span achieved by the Romans. It can be seen that these upper bounds were challenged only by 165, the Ponte dell’ Abadia at Vulci (figure 3) and the later E36, the Severus bridge at Klhta, near the R. Euphrates, north-east of Adana in modern Turkey. A dotted line is also shown on the graph to represent the decline in maximum span after 155, proceeding to E36 and then to E7, the Sakarya or Sangarius River bridge of A.D. 560.

Similarly, in the next graph, for overall length, a line has been drawn from I5 (the

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Pons Aemilius, as above) to SP15, the first stage of the Puente Romano, at Mtrida in Spain. This remarkable bridge was extended twice, as shown in the graph, and Table 1, to reach the remarkable overall length of 721 m. The central part of the bridge was built first, to cross the Guadiana River. Unfortunately, the river thought better of it and bypassed the bridge, to the north. The Romans extended the bridge, as a second stage (figure 9), but the river then went round to the south, requiring a further and final extension. The overall length is extra- ordinary, with a total of 60 spans (a few of which are now buried in the southern abutment).

In plotting an upper bound to overall length (for road bridges) one should then

Figure 5 Much of the mighty bridge 187 at Narni (27 B.C.) has collapsed, chiefly because of scour. It was a great bridge in terms of its maximum span, overall length and height.

Figure 6 AQ30, the Pont du Gard aqueduct bridge, believed to have been built by Agrippa about 18 B.C., is large and unusual in form. Its height, 47.4 m, is about that of a modern, 15storey building.

follow the line shown to the final bridge SP15, whose length was never exceeded by the Romans. The graph, however, shows also a dotted extension and three longer aqueducts.

There is a sense in which the bridge at Merida consists of three bridges, rather than one, with those built second and third having lengths of 239 and 222 m, each substantial in themselves, but shorter than the 260 m

_- ..--_ - _-.I _. - - - _-. --- __.. - ____ --. Figure 7 1155, the Pont St Martin, has the largest span, 35.6 m, of all Roman stone arch bridges. It was built in the period of Augustus (27 B.C.- A.D. 14), and is slightly segmental.

Figure 8 One of the features of 1159, the bridge at Pondel, is its remarkable height, 40-45 m. It was built privately in 3 B.C.

of the original bridge. The dotted line proceeds instead to 154, the Ponte Rotto, now in ruins, but originally crossing the Cervaro River on the Via Traiana, the alternative route to the Via Appia from Rome to Brindisi. It proceeds then to E30, the remarkable, multiple-span, segmental bridge at Limyra, referred to again below, and then to E7, the Sakarya bridge in western Turkey, with 12 spans, 7 of which were in excess of, or close to, 20 m. Its overall length was 429 m. E28, the old bridge at Adana, close to the birthplace of Paul, the apostle, was also large. More unusual again was E42, the dam bridge of Band-i-Kaisar, over a branch of the Karun River, at Shushtar or Susa in modern Iran. About 300 m long, its importance was chiefly as a dam, for it supplied water to a major irrigated area. The Roman emperor, Valerian, with some of his troops, had been captured by Shapur I about A.D. 260 and forced to build the bridge.

The graphs are not intended to show all aqueduct bridges, and these bridges differed in one noticeable respect from road bridges: in their width, for they were typically more narrow, However, their construction was an important part of Roman engineering exper- ience and would have influenced the design of road bridges. The graph for overall length shows, first, AQ30, the Pont du Card near Nimes in the south of France (figure 6), one of the greatest of all Roman achievements in bridge building and then AQ60, the mighty aqueduct at Segovia in Spain (figure 11). But the longest of them all is shown as AQ4, with an overall length of 1375 m. This is the long and high arcade formed to carry the water of the Aqua Claudia north from Capannelle, as it left higher ground to cross the lower plains on its final approach to Rome.

There were also other long works in stone, of which some examples may be given here. There was in Pergamum a structure which, viewed end on, was a two- span bridge, with spans of 12.1 and 12.5 m. More truly, it was a large drain or tunnel, for it passed beneath the Red Courtyard, and the vaults had lengths of about 195 and I8 I m. South of Rome, the old Via Appia was built up between two major retaining walls near Ariccia, rebuilt by Augustus [2], and these were about 200 m long. There was also, in the south of France, a viaduct some 1500 m long, called the Pont Serme or Pons Selinus. Its form is not known.

The last graph shows developments in bridge height. Shown again as starting from 15, the boundary line proceeds first to 165,

Figure 9 This photograph of SP15, the Puente Roman0 at Merida in Spain, shows only the second stage, built about A.D. 40. The initial bridge, of 25 B.C., is beyond the ramp visible in the distance. It was cut off by flood, requiring an extension at this side, and later (A.D. 98-l 17) required a further extension at the far end. Its final length was 721 m, the largest for a Roman stone bridge.

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Figure 10 Not all the spans of G9, the Pont de Sommieres, are now visible, for they extend beyond the modern river training wall at the right. It dates from about A.D. 14-37.

Figure 11 The lofty aqueduct at Segovia in Spain, AQ60, built in the time of Claudius (AD 41-54) was, at least until recently, still in use. It is high, but its spans are much smaller than those in the Pont du Gard (AQ30, figure 6).

the Ponte dell’Abadia (figure 3), now adjoining the Etruscan museum at Vulci. Again it branches, with the full line pro- ceeding to SP2 1. another of the great stone bridges, the Puente de Alcantara in Spain (figure 12). Of the bridges shown in the graph, two have captions that are under- lined, and are distinguished because their structure does not proceed down substan- tially to water level. 131 is the Ponte Catena, outside one of the gates of ancient Cori, south from Rome; Blake [2] has a photograph. The other, 1159, the Ponte de1 Pondel (figure 8), west of Aosta, is another remarkable bridge, for three reasons - it was privately built, its deck lies far above the stream tumbling through a rocky gorge below, and its walkway was enclosed.

Again, however, these traffic bridges

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were surpassed by an aqueduct, AQ30, the Pont du Gard, seen before. It is appropriate to make a comment here on the Roman aqueducts. The Pont du Gard is significant among aqueducts primarily because of its large span, coupled secondly with its great height. Table 1 shows that the later aqueduct, AQ60 at Segovia, had much shorter spans, and the same is true of AQ4, the grand Claudian arcade, and most other Roman aqueducts. To find similarities to the Pont du Gard, one must look to AQla, the Ponte San Pietro on the old Aqua Marcia, and to a lesser extent, AQ5, the Ponte San Antonio on the Aqua Anio Novus, both near Rome.

The graphs, particularly the first, also show the numbers of bridges and the dis- tribution of this number with time. The two

major periods of Roman traffic bridge con- struction were the first century B.C., par- ticularly its latter half, and about the first half of the second century A.D. There was a similar trend with aqueduct construction, although there were also others, such as the Aqua Claudia (AQ4) and Aqua Anio Novus (AQ5) in Rome, and the Segovian aqueduct (AQ60), built in the first century A.D.

Segmental construction The normal Roman arch was semi-circular; that is, it included an angle of 180°C. An arch formed as part of a circle, but includ- ing a smaller angle, is referred to as seg- mental. It is commonly asserted that the Romans did not build segmental arches, but in the author’s view this is incorrect.

Apollodorus of Damascus was a Greek engineer who worked chiefly for Trajan (A.D. 98-117). About A.D. 104-105, he built a bridge across the Danube, the greatest of all Roman bridges, with 2 1 or 22 spans, generally about 32.2 m between the faces of the piers, totalling 1070 to 1100 m. It has not been included here, for the spans were timber arches, but it is noticeable that these were segmental, with an included angle of about 64”. I f the Romans could build a seg- mental arch in timber, there is no reason why they could not build them also in stone, and there is evidence that they did indeed do so.

The Ponte San Lorenzo in Padua is now partly buried, but has three, segmental stone arch spans (12.7, 14.4 and 12.5 m) with an included angle of about 113’. It was built between 47 and 30 B.C. Pont St Martin (1155), of 27 B.C.-A.D. 14 (figure 7), was slightly segmental (144”), as was a nearby arch in Aosta. More impressively, SP20, the Puente de Alconetar (figure 13), now relocated, seems without doubt to have included a segmental arch in its northern approach, built in the period A.D. 98- 117, with an included angle of 120”. Finally, the remarkable bridge at Limyra (E23), in south-west Turkey, had 26 or 27 segmental arch spans, each typically 10.7 m clear, with an included angle of only 83’. Expressed differently, this angle describes an arch which, at the centre, rose only 0.189 X span above the springings. Unfortunately, as mentioned previously, the dating of the bridge is not known. There is another, later Roman segmental arch near Tyre.

There is a possible trend line if these included angles are piotted against date. Following the Ponte San Lorenzo. there is a return to a less segmental form at Pont St Martin, but then there may be a significant trend towards a reduction in the included angle: from 144” for the Pont St Martin, to 120” at Alconetar, and then 83” at Limyra. This was a significant development in technology. In his book, the author argues how it may have begun.

Other parameters There are other factors which should be included in a full discussion of development

Figure 12 The bridge SP21 of AlcBntara in Spain is one of the great stone bridges. Its spans are large, it is high and long and is still in use, carrying a major road from Portugal to Spain across the R. Tagus. It was built in A.D. 104. A modern dam has raised the level of the water beneath the bridge.

Figure 13 SP20, the bridge of AlconBtar, has been relocated and preserved. It dates from the period A.D. 98-117 and is long. One of its most interesting features is shown in this photograph, a small arch at the northern end that is markedly segmental.

in Roman stone arch bridges. Two only will be mentioned here.

The use of lime to form a hardening mortar was taught to the Romans by the Greeks. The Romans were then fortunate to find, by chance, a naturally occurring pozzolan, which would, if mixed with lime, form a hydraulic cement; that is, one capable of hardening under water. The original

possibly about 200 B.C., but it was later found (about 60 B.C.) that a red pozzolan from the vicinity of Rome gave better results. There is no doubt that these early pozzolans were used in bridge construction, in the foundations and piers, and in the infill above the arches. However, Blake [2] (p. 341) refers to the engineers of Agrippa as not having mastered hydraulic concrete in

The periods of major bridge construction appear to have been the first century B.C. and the first half of the second century A.D. Major bridges after that date, although they exist, are rare.

References [I ] O’Connor, d. ‘Roman Bridges ‘, Cambridge

University Press, 1993. 121 Blake, M.E. ‘Ancient Roman Construction in

Italyfrom the Prehistoric Period ro Augustus ‘. pozzolan came from Pozzuoli, near Naples, 33 B.C. (in the sewers of Rome), and that Carnegie Institution of Washington, 1947.

their work needed to be redone about 1 l-4 B.C. Her implication is that it was not until that date that concrete was properly used, and this has some relevance, partic- ularly in a study of 1155, the Pont St Martin Lll.

Another major parameter in the design of arch bridges is the ratio of rib thickness to span. From a study of the proportions of 42 bridges, the author has suggested that the rules used by the Romans were:

1. Use as a guide the ratio for rib thickness to span of li 10, knowing that this could be reduced to as low as l/20.

2. Avoid stone depths greater than 5 Roman feet (1.5 m).

It appears that this ratio does not relate directly to date. More truly it is dependent on span, as indicated by the second part of the above rule. The limiting stone depth of 1.5 m appears to have been determined by the capacity of Roman lifting equipment and the weight of the stones. A ratio of I/ 10 and a depth of 1.5 m gives a span of 15 m. For greater spans, a rib thickness held at 1.5 m will give ratios less than 10 - for example, 1120 at 30 m - and there is evidence in the Pont St Martin, with a large span and a slender arch rib, that the Romans took special steps to address this problem.

Figure 1 shows a tendency for maximum span to increase with time, until about 6 B.C. This would lead, then. to an expec- tation that the rib thickness ratio would tend to reduce with time, for the larger bridges.

Conclusions This study has allowed the identification and listing of major Roman stone arch bridges. In two important respects, the design of these bridges peaked at about the birth of Christ, for both the maximum span, 35.6 m, and maximum height. 47.4 m, were achieved about that time. This should not be taken to imply a subsequent immediate decline in technology, for there were at least two later developments - the overall length of bridges continued to rise, and there was the development of the segmental arch.

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