Arches and domes

These are the ruins of the Colosseum, one of the most visited tourist attractions in Europe. The outside walls are probably the most spectacular part of this amphitheatre. They are 48 meters high, and consist of four floor levels, made of stone. Before the building collapsed, each of the three bottom levels was made up of 80 curved window-like openings. Each opening is formed by a stone structure that looks like an inverted U.

This structure — an arch — is very strong and capable of supporting large loads. How does it work? Arches in the Colosseum are made of wedge shaped stones — wider at one end and narrower at the other. These stones, called voussoirs, fit together, forming a semi-circle or an arc. When there is a load applied from above, the central voussoir, known as a keystone, presses against the two neighbouring stones on the sides.

These, in turn press on the voussoirs below them. This way, the load is distributed evenly and transmitted to the sides of the inverted “U” — the vertical supports. Because the voussoirs are pressed together, the only forces present in arches are squeezing — compressive — forces. What if instead of an arch, there was one straight stone block — a beam? Unlike an arch, a beam is subject to both compressive and stretching — tensile — forces.

Stone is a material that withstands and carries compressive forces very well. But it’s much weaker when it comes to tensile forces. So, in buildings made of stone, arches prove to be stronger than beams. Arches were commonly used by ancient Romans to build bridges, amphitheatres and aqueducts. But even today, arches are often used to support certain structures, especially those that spread over a long distance, such as bridges.

There is another structural element that works in a similar way to the arch. Imagine you take an arch and rotate it around its central axis. This would result in a three dimensional shape that looks a bit like an upside-down bowl, hollow on the inside. This is a dome. Similar to arches, domes are made from fitted pieces that rest one on top of the other, forming a semi-sphere.

The design allows the load to resolve into compressive and tensile forces. In a dome, the forces are directed downwards and all around the dome’s perimeter. Domes have been popular in architecture, because they can enclose large spaces while remaining stable and strong. Apart from that, they can also be very beautiful and impressive! Therefore they have been especially common throughout the ages in large buildings with cultural significance, such as temples, museums, government or university buildings.

Over time, many different types of domes have been invented and a wide range of materials and designs have been used to build domes. Today domes cover stadiums, serve as greenhouses, and provide other types of enclosures. Throughout history, the arch and the dome have contributed to the function and beauty of structures around the world. It's possible that some buildings you see in your daily life have these important architectural elements! Take a look and see.