An arch is a curved structure made from wedge-shaped units—typically bricks or stones—joined together with mortar. It is designed to span an opening while bearing the load above it, effectively transferring that weight to its supports, which are called piers or abutments. The unique shape of an arch allows it to distribute loads evenly, making it a strong and enduring architectural feature used since ancient times.
Components of an Arch
To understand how an arch functions, it’s important to become familiar with its individual parts. Each component plays a key role in the structural integrity and aesthetics of the arch.
A. Curves and Surfaces
- Intrados: This is the inner curve or underside of the arch. It forms the visible curve when looking at the arch from below.
- Extrados: The outer curve or top line of the arch is known as the extrados. It follows the same arc as the intrados but lies above it.
- Soffit: The inner surface of the arch, synonymous with the intrados, is referred to as the soffit.
B. Masonry Units
- Voussoirs: These are the wedge-shaped blocks that make up the arch. Each is cut to fit together with its neighbors, forming the curved shape.
- Keystone: Positioned at the highest point of the arch, the keystone is the central voussoir. It is the final piece placed during construction and locks all other voussoirs into position.
- Springer: This is the first voussoir placed at the base of the arch, adjacent to the skewback.
C. Structural Points and Lines
- Crown: The crown is the highest point along the extrados of the arch.
- Springing Points: These imaginary points mark the locations on either side of the arch where the curve begins to rise from the supports.
- Springing Line: A horizontal line connecting the springing points on both sides of the arch.
- Center of Arch: Also known as the striking point, this is the geometric center from which the arcs forming the intrados and extrados are drawn.
D. Supporting Elements
- Skewback: This is an inclined or sloping surface on the pier or abutment from which the arch springs or begins.
- Pier: A vertical structural support located between spans of an arch.
- Abutment: The end support of an arch that holds the lateral forces in place.
- Impost: A projecting stone or course of masonry located at the top of the pier or abutment, providing a base for the springing line.
E. Additional Features
- Spandrel: When two arches are side by side, the space between their extrados and a line drawn horizontally through the crown is known as the spandrel.
- Haunch: The lower half of the arch, between the skewback and the crown, is called the haunch. It handles the majority of the compression forces.
- Arcade: A row or series of arches supported by piers or columns is referred to as an arcade.
- Arch Ring: The curved course of masonry that forms the arch itself. It can consist of a single ring or multiple concentric rings.
- Bed Joints: These are the joints between the voussoirs, and they typically radiate from the center of the arch.
Dimensions of an Arch
Several key measurements define the size and shape of an arch:
- Span: This is the horizontal distance between two supports or abutments of the arch.
- Rise: The vertical distance from the springing line to the highest point of the intrados.
- Depth/Height: The perpendicular distance between the intrados and the extrados.
- Thickness: Measured perpendicular to the face of the arch, this is the breadth of the soffit.
Conclusion
Arches are not only beautiful architectural elements but also structurally sound systems that have stood the test of time. Each component, from the keystone to the spandrel, works in harmony to bear and distribute loads. Understanding these components helps engineers, architects, and enthusiasts appreciate the elegance and ingenuity behind one of the most iconic features in construction.
