Arch

An arch is a curved vertical structure that spans an open space. These structures serve two main purposes in architecture. They can support heavy loads from above, or they can be used purely for decoration. While decorative arches date back to the 4th millennium BC, structural load-bearing arches became widely popular after the Ancient Romans adopted them in the 4th century BC.

Arches work differently than the trabeated system. In a trabeated system, builders use posts and beams, much like in ancient Greece, China, or Japan. An arch offers several advantages over a simple lintel or beam. With the same amount of material, an arch can span a larger distance. It can also carry more weight and is made of smaller, more manageable pieces. This makes masonry construction much more efficient.

A true arch is a load-bearing structure held together by compression. Compression is a force that pushes materials together. To understand the parts, we look at the individual blocks called voussoirs. These are wedge-shaped construction blocks. The very top block is called the keystone. The area around the keystone is known as the crown. The underside of the arch is called the intrados, or the soffit. The external surface is called the extrados. At the base, the arch rests on a block called an impost. The vertical distance from the springing level to the crown is called the rise.

The way an arch handles weight is called arch action.

When a vertical load is placed on the arch, the structure resolves that weight into both vertical and horizontal reactions at the ends. This creates a bending moment. However, the inward horizontal reaction from the abutments provides a counterbalancing force. This means the bending moment in any segment of the arch is much smaller than it would be in a beam. In a beam, the top is in compression but the bottom is in tension. In a true masonry arch, the elements are mostly subject to compression. Because masonry has very low tensile strength, the pieces must stay pressed together to prevent collapse.

In 1676, Robert Hooke discovered a fundamental symmetry in how these shapes work. He noted that a rigid arch stands like an inverted hanging chain. This relationship is called a funicular shape. The specific curve of an ideal, compression-only arch depends on how the load is distributed. Builders often use a pressure polygon to ensure stability. As long as this curve stays within the confines of the voussoirs, the construction remains stable.

Arches can be classified in many ways, including their shape, material, or structure. They can be rounded, such as the semicircular arches used by the Romans. They can also be pointed, which is a characteristic of Gothic architecture. Other shapes include the horseshoe arch, often seen in Islamic architecture, and the basket-handle arch.

You can also find segmental arches, which are less than a semicircle. These were used in Ancient Egypt as early as 2100 BC.

Architects often arrange arches in specific patterns. A sequence of arches can form an arcade. The Romans perfected this, using them in massive structures like the Pont du Gard.

Some arches are "blind arches," meaning the opening is filled in for decoration. Others are "relieving arches," which redirect weight to stronger points in a building. There are even "strainer arches," which were sometimes built as an afterthought to prevent supports from imploding due to miscalculations.

The role of the arch changed in the mid-19th century with the introduction of wrought iron and steel. These materials have high tensile strength, meaning they can be pulled without breaking. This allowed for long lintels and beams that did not require an arch shape. However, arches remain iconic and functional. Large-scale examples, such as the Gateway Arch in St. Louis, continue to demonstrate the power of this geometry.