The Romans and the Chinese understood this well. To control their vast empire and defend it, it takes roads, to be able to move quickly, and bridges, to overcome obstacles.
A simple stone slab makes it possible to straddle a short distance, but the weight of the rock imposes significant compression and tension forces in the top and bottom of the slab, respectively.
For a span of more than 2m, the architects of antiquity developed arch bridges.
Their characteristic shape makes it possible to distribute the weight from the center to the banks, where the abutments are located.
To build such a bridge, a temporary wooden structure, called a “falsework”, is positioned at the exact location of the vault. Perfectly cut stone blocks are mounted along the form and on one another. The keystone is placed last to ensure the cohesion of the whole and the wooden form can be removed.
Arch bridges are very strong because the stone handles compressive forces well. Especially when the space around the vault is densely filled.
Many of these ancient works are still in use today.
To cross great distances, it is possible to use multiple arches, but the many piers of the bridge then constitute an obstacle to navigation.
It was not until the invention and mastery of new materials that new bridge structures could be built.
At the time of the Industrial Revolution, iron and cast iron bridges appeared.
But, these materials are too expensive on their own.
With steel and concrete, civil engineers of the 19th and 20th centuries made major advances:
Concrete piers are very resistant to compression.
Steel cables are resistant to tension.
It is thus possible to combine the advantages of these two materials by making reinforced concrete.
With metal reinforcement it is now possible to make structures lighter and more robust.
All of these materials technology innovations together with the power of computer-aided design make it possible to simulate and then build impressive structures.