Roman Arch Bridges: How Much Weight Can They Hold?

Why were arch bridges so crucial to the Roman Empire, and how did they survive relatively in-tact even to modern times?

At the core of the Roman Empire was their engineering prowess, and most notable of all their infrastructure advances was the Roman arch.

The arch bridge and arched structures allowed the Romans to construct buildings with a far greater ratio of wall openings to height than had ever been possible before. The evidence of such architecture is found in not only the Roman Coliseum but also the labyrinth of arched catacombs that lie beneath historic Rome. Focusing in on the arch bridge, it was a technology never seen before, one that allowed boats to pass under walkways and roads and one that enabled Roman’s famous series of raised aqueducts.

Why was the arched bridge so crucial to the Roman empire, and what structural properties of the arch have that enabled Roman architecture to survive relatively intact even in modern times?

An arch bridge was, and is, so revolutionary to structural design because the elements of which function almost entirely in compression. Due to the distribution of both dead and live loads on arches, stresses are always translated in compression, allowing for materials such as rock, or unreinforced concrete, to be used effectively. If you know anything about concrete’s and rock’s material strengths, you likely know that neither function practically in tension loading. Nowadays, concrete beams are reinforced with rebar to allow for tension loading, but the Romans didn’t have that ability.

As an arch’s radius of curvature increases, it begins to behave slightly more like a beam, therefore low compression forces or tension forces, begin to appear on the underside of the arch. The Pantheon, still the biggest unreinforced concrete dome structure in existence is estimated to have been the largest domed structure the Roman’s could have built without collapse.

Examining how much load an arched bridge can hold is a little tricky. Since all of the components of an arch function in compression loading, the maximum loading values of any given arch is essentially equivalent to the shearing point of any material. Granite, for example, would be a far better arch construction material that sandstone. Even still, the ability for arches to hold load is far beyond any other structural element, even those today.

A well-built arch from the stone doesn’t even need mortar to connect the parts, rather the friction forces from compression keep the structure stable. Rather than spend hours determining the maximum load of an arch constructed from a given stone, we are going to settle with a maximum loading value of a really big number. For the Romans, and even engineer’s today, a solid arch structure’s yield point is far beyond realistic loads that structure would ever see.

These same principles that made the arch so strong also made them last so long. When a structure created from arches undergoes a series of loads creating low material stresses and strains, fatigue was seen in the arch over time is very minimal if nothing. Since arch’s yield points are so far beyond practical loading values, they tend to last until the rock or structure is weathered. In turn, a very long time.

The Romans did use concrete to build many of their structures, like the Coliseum, which is known to be about 10 times weaker than modern concrete. However, while the concrete was weaker, it was far more resistant to weathering than modern concrete due to the abundance of volcanic ash used in its construction. Through this increased weathering capabilities and the strength of solid arch structures, Roman architecture and buildings are still around today, in nearly all of their original beauty.

Sources: SmithsonianHesstonHow Stuff Works

This article has been published in Short Sleeve and Tie Club.

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