Bridge Construction Methods: Why Are Roman Bridges So Stable?

Roman architecture is at the centre of Europe’s visual allure. Sophisticated Roman structures have survived hundreds of centuries without renovation, withstanding Christianity, the Franks, the conquests and even Boris Johnson.

Remember how proud you were when you built that bridge in your son’s Lego city? Well in 103AD Roman engineer Apollodorus of Damascus built a 1.1km bridge from stone and wood. Trajan’s Bridge was 19m from the surface of the river, 15 metres wide and capable of supporting the weight of hundreds of traversing Roman soldiers. It was only a few feet shorter than the Sydney harbour bridge.

The Romans built a lot of the West’s longest standing buildings. They were also the first civilisation to make bridges from concrete. Some of these ancient bridges stand now as they did the day they were built. This is thanks to the structural innovations that were first used by the Romans. And innovations that eventually helped shape contemporary bridge engineering. The Alcántara Bridge is such a bridge, standing in Spain since 104AD. Roman civil engineer Caius Julius Lacer was the man behind the bridge. His tomb stands nearby, with an epitaph that reads “I leave a bridge forever in the centuries of the world”. He wasn’t wrong.

Why Are They So Strong?

There is a rumour doing the rounds on the web that the Roman engineers in charge of building the bridges had to stand beneath them as the scaffolding was removed. Apparently the trepidation of tonnes of rock and debris falling and crushing you lead to some pretty tight structural planning.

A more likely history is found in the Roman Empire’s military expansion. To improve Roman access lines the empire formed guilds of skilled workers and thinkers who shared ideas and building principles. These early engineering guilds made important discoveries in structural design, in the materials and in the piers that supported the legs of the bridges.

The Voussoir Arch

The Romans had improved the traditional footbridge by creating a bridge that maintained its structural integrity through the centre. To achieve this the Romans did not rely on steel beams running through the stone members, but instead on the tensile strength of the stones themselves.

The shape of the arch allowed the bricks to be inserted at a curved angle until they joined at the peak of the arch with a keystone. This keystone was shaped as a trapezoid that used the weight of the stone and concrete in the bridge to compress the tapered stones together. This pressure formed a structure in the arch that required a tremendous amount of force to rupture. Where traditional bridges were at their weakest in the centre, the arch was at its strongest.

If reading that felt like reading the actual instructions the Roman engineers were giving their workers in 100AD, watch the video below.

Pozzolana Cement

The Arch was a structural innovation in building design. But it wasn’t the only thing the Roman Empire contributed to construction. The Romans were also unique in the materials they chose to build with. A natural cement called pozzolana was used by the Romans as mortar for the piers (the legs) of their bridges. Not only is this cement said to be ecologically cleaner than today’s cement mixtures, but its also a cement that grows stronger over time.

Pozzolana is still used in some countries. It’s made by combining two parts pozzolana (which is a type of slag that forms naturally from volcanic rock) with one part powdered lime. As early as the 3rd century B.C the Romans used pozzolana instead of sand in concrete in their construction. This gave their structures supreme strength and stability.

Cofferdams

As not every bridge built by the Romans had the luxury of building its piers on land, Romans used cofferdams where the piers would fall within a body of water. Romans used the cofferdam as a temporary structure that allowed the construction of a bridge pier in a space of water.

The cofferdams that were used by the Romans were simpler than the ones used in contemporary construction, but their function is identical. First, the Romans would dig a ring of timber logs into the river bed. They would then fill the gaps between the logs with clay for waterproofing, before pumping the water from inside the circle of logs. Upon the newly dry riverbed, the Romans would construct pozzolana and stone piers. After the construction had been completed the ring of logs was removed and the piers stood in the riverbed like magic.

The Romans Themselves

The Romans were a proud and prosperous civilisation. They didn’t just stumble upon their architectural achievements. Before they were perfecting bridges they had already stolen and improved some of the Greek’s best structural ideas. The Romans also had high levels of civil craft and a military background that had an associated understanding of how to build the strongest fortifications and walls. Like most early civilisations, they also didn’t have any trade unions. Meaning they never saw cost and labour as a roadblock when creating their monolithic structures.