The Ancient Roman Bridge, a Timeless Engineering Feat
By: Richard Bruschi
Tags: Architecture, Roman History, Culture, History, Ancient History

The Ancient Roman Bridge, a Timeless Engineering Feat

Ancient Romans were the first major bridge builders. Through extensive use of the arch and concrete they perfected, they built the biggest and longest-lasting bridges of antiquity.

The Puente Romano de Mérida in Spain, completed in 117 AD, is the world’s longest surviving bridge of the ancient times. Ángel M. Felicísimo from Mérida, España, CC BY-SA 2.0, via Wikimedia Commons.

Stone arch after stone arch. That is the basics of an Ancient Roman bridge. Their shape and form don’t change much from that. Although it doesn’t sound fancy presented like this, it is yet another of the Ancient Romans' exceptional engineering feats. I’ll explain why.

The key to the Ancient Roman bridge was the “true” arch, built with stone, and the concrete they so expertly perfected, the Roman concrete or opus caementicium. Yet, alongside these two key components were important technological features and design details.

Origins

Arches were the first evolution away from the post-and-lintel structure that had existed for thousands of years in Neolithic civilizations (i.e.: until 2,000 BC in the Egyptian, Hellenistic, or Indian civilizations). It was initially a ‘corbel arch’ or ‘corbelled arch’, built as the higher the structure rose the more it offset towards the centerline. It is called a “false arch” because it is not self-supporting per sé.

The arch as we see it everywhere today, the “true arch”, appeared around 2,000 BC in Mesopotamia and it was built in bricks. An arch is a compression form, no tensile stresses happen (called “arch action”), transferring the forces all the way to the ground through the ‘springers’, the linear supports that begin when the curved part ends (e.g.: columns).

Completed in 3 BC, the bridge is in the pleasant countryside of southeastern France near Bonnieux, at the western edge of the Luberon Regional Natural Park. Its semicircular spans were built in stone blocks. . Photo by Hawobo, CC BY-SA 2.0 DE, via Wikimedia Commons.

Although the arch is self-supporting, there is a risk of collapse due to the lateral ‘thrust’. To avoid this, the structure is secured with ‘tie-rods’ or external supports such as abutments. Arches within a wall — or consecutive arches in a bridge — do not have that issue as there is another structure at its sides.

The arch requires a very simple construction method allowing for strength, flexibility, and aesthetics.

The Ancient Romans recognized the potential of this element and capitalized on its strengths. They implemented it by building tapered units called voussoirs with it, they then refined it and applied it systematically and to a wide range of structures such as aqueducts, bridges, and building entrances.

The arch bridge was not a new element and the Ancient Romans probably saw and assimilated it from the Etruscans, Greeks, and Mycenaeans. The Romans once again studied the design and the structure, assimilated it, greatly enhanced it, and adapted it to different conditions and needs.

Design and technological innovations

The bridge’s building unit was the self-supporting arch, therefore a bridge could be composed of one span or have many continuous spans in a row, without compromising its structural integrity. The arches could have different shapes depending on the structural needs, aesthetic goals, and terrain or the configuration of the body of water.

The basic arch was the semicircular one, but this meant increased height, therefore the segmental arch (i.e.: less than a semicircle) was preferred. This meant that they could cover wider spans as well as use less material. The increased load-bearing capability was also possible due to the use of the aforementioned voussoirs.

The ‘Ponte Pietra’ in Verona, Italy was completed in 100BC. It links the Roman ruins on the North side of the Adige river to what is now the historic center. The bridge views are completely different from another, with the Roman ruins, the Medieval neighborhoods, the river, the hills all combining differently. Photo by alex1965 via Pixabay.

Ancient Roman engineers included flood openings in the piers early on. This feature consists of an opening (or more than one) between one arch and the other, either within the width of the pier or as a ‘spandrel’ (the space between the top of the arch and an actual or imaginary rectangular frame). This means less material and less construction time, hence lower cost, but also a safer structure as the pressure of the water is reduced.

These Roman engineers were the first, and until the Industrial Revolution the only, to consistently design and build the bridges with concrete, called opus caementicium. This was extensively used in construction in 150 BC already. It was durable (basically having the consistency of hard rock) and resistant to both erosion and cracking. It was also inexpensive, adaptable, easy to integrate with stones and bricks, and easy to decorate with cladding. Last but not the least, some types could set underwater which, of course, was essential in bridge building.

Aesthetics were also important and most bridges were cladded. The cladding was in brick or dressed (i.e.: squared) stonework either in ashlar or rustication work. The first has a refined and usually textured yet a flat face and smoother finish. The second is any work that keeps the blocks’ exposed face to varying levels of controlled roughness and protrusion, such as in ‘bossage’.

The construction of a bridge was a communal regional effort. Usually, more than one municipality was involved in the funding of the works. The names of the commissioning entities and the builders were recorded in documents and on the bridge itself, in an inscription and onto the stonework for the latter.

Although the literal and figurative building blocks of the bridge were only a few (stone and concrete taking the shape of an arch) technical detailing was also a reason for the quality, and therefore longevity, of an Ancient Roman bridge. The span of the bridge sloped gently to allow for water drainage, stone blocks, the critical ones or all of them, would have been linked together with lead clamps, bracing might have occurred, and stonework was linked with specific joints (e.g.: dovetail) at important junctures. If rocks with underwater concrete couldn’t be used to build the foundations of the piers and the piers themselves, the builders used ‘cofferdams’, timber boxes sealed with clay which were driven into the riverbed and filled with concrete.

The experience of walking over Ponte Sant’Angelo leading straight to Castel Sant’Angelo is even better when you are the only one on it. Via Pxfuel.

Today

The Ancient Romans built 931 bridges that we know of, 800 of which in Europe and 431 of those in Italy. Many are still standing today, and many are still in use.

There are many interesting Roman bridges still in use. Most might not seem much, at first glance, but just knowing that a bridge is 2,000 years old instantly gives it an aura of respect. The appreciation also comes from a combination of various aspects. Its architectural and engineering design. The surrounding natural landscape, as for the Ponte di Augusto in Narni, Italy. Alternatively, its urban setting can alter our perception of it, as for Ponte Pietra (‘Stone bridge’), built in 100 BC over the Adige river in the historic center of Verona, Italy. There’s also the experience of walking over its ancient surfaces, as in the famed Ponte Sant’Angelo in Rome, completed in 134 AD by emperor Hadrian and which leads straight to the famed Castel Sant’Angelo.

The majestic view of Ponte Sant’Angelo leading to Castel Sant’Angelo, one of the most famous shots of Rome. CC0 Public Domain via Pxhere

The following are some other notable mentions of fine Ancient Roman bridges.

The Ponte Fabricio (‘Fabricio’s bridge’)in Rome was completed in 62 BC. It is one of the world’s oldest major bridges still standing. It stands today basically as originally built and it only received ornamental modifications, the first ones of which only in the mid-1400s AD. It has been in continuous use and it is also the oldest bridge in Rome.

The Puente Romano (‘Roman bridge’) over the Guadiana River at Mérida in southwest Spain is the world’s longest surviving bridge from ancient times. Sixty spans of the original sixty-two, for a total of 721m instead of the original 755m, remain. Completed in 117 AD during the reign of Trajan, its semi-circular and short arches are cladded in white stone. It is very elegant. The bridge is still in use and it was recently made available for pedestrians only.

The Merchants Bridge in Narbonne, France. Didier Descouens, CC BY-SA 4.0, via Wikimedia Commons.

The Pont des Marchands (‘Merchants bridge’) was built in the 1st-century AD in Narbonne, southern France. It was originally composed of six spans each with a segmental arch, but only one is visible today, as the others are incorporated in the adjacent construction. It is nevertheless well preserved. In Medieval times buildings up to four stories were built over it, making it one of the few bridges in the world to have shops lined on it.