In this blog post, we’ll explain the history and physical principles of arch structures—which have been in use for over 6,000 years—in a simple and accessible way.
The Pont Neuf in Paris, France; the Ponte Vecchio in Florence, Italy; and the Richmond Bridge in Australia are all considered historic and beautiful bridges in their respective regions. A common feature of these bridges is their arched structure, which curves like a bow. The word “arch” derives from the Latin “arcus” and refers to a familiar bridge shape that has been used for a long time in both Eastern and Western cultures.
Traces of the arch form can be found in the Mesopotamian region as early as around 4000 BCE, and its full-scale development originated in Etruscan architecture. Since then, the arch has been consistently adopted in architecture and bridge design worldwide, and the reason for its enduring use lies in its structural advantages and its ability to effectively utilize the properties of materials.
In the past, materials obtained from nature—such as stone, earth, and wood—were often used as-is without processing. While these natural materials are prone to breaking when split or bent by hand, they are relatively strong when subjected to compressive or tensile forces from the sides. Today, materials such as rebar and concrete are combined to increase strength, but in the past, people designed arches to maximize the inherent strengths of the materials.
From a structural perspective, the upper part of a bridge directly bears the load, while the lower part transmits that load to the ground. The forces acting on a bridge can be categorized into bending moments (forces that cause a material to bend), shear forces (forces that cause a material to slip in opposite directions), and axial forces (forces that push or pull a material through compression or tension). For example, if you extend your arm horizontally and lift a heavy object, you can feel a significant bending moment acting on your arm and shoulder. If you press your fists together and push them in opposite directions, your fists will try to slip apart; this is a tangible example of shear force. When you press down on a piece of taffy with your hand, you feel compressive force, and when you pull on it, you feel tensile force.
Natural materials such as stone and wood are weak against bending moments and shear forces but strong against axial forces. Arch structures take advantage of this very characteristic. When a horizontally placed beam-type bridge bears a load, the bending moment and shear force cause the material to bend or break; however, an arch distributes the load laterally, causing the material to be subjected primarily to compressive force.
You can verify this difference with a simple experiment. If you hold a flat sheet of paper horizontally and place an object on it, the center will bend or sag; however, if you bend the same sheet of paper into an arch shape and secure both ends, it will maintain its shape even when a much heavier object is placed on it. In an arch, the load from above is transferred to both sides, creating a force that presses down on the bridge horizontally—this effectively utilizes the material’s strength under compressive force.
It is easier to understand when explained numerically. For example, suppose wood has a resistance of 100 to shear force and bending moment, and a resistance of 200 to axial force. If a weight of 150 is applied to a bridge, a flat beam-style bridge must withstand 150 as bending moment or shear force, which exceeds the material’s limit (100), causing it to bend or break. However, if the same material is used to construct an arch, the load is converted into an axial force (compressive force) component, allowing it to withstand up to 200; thus, it can stably support a load of 150.
In this way, the arch is an efficient structural method that avoids the weaknesses of the material and utilizes its strengths. The fact that sturdy structures can be built by utilizing the properties of natural materials—without requiring advanced fabrication techniques or composite materials—is why people in the past widely used arches. Because of these advantages, arch-shaped bridges and buildings can still be easily found all over the world today.
