Arch Components

Base Components

• The construction of an arch begins with the abutments, which are the stones at either side of the arch on the bottom most level, acting as a base. At the inner top corner of either of the abutments is an imaginary line called the spring line which is half of the span, or distance between the abutment stones. Atop each abutment is a stone called a "skewback"; it has one flat side and one angled side, the beginning of the sides of the arch.

Haunches of the Arch

• Wedge-shaped stones comprise the sides of the arch and are referred to as "voussoirs" or "ring stones." Depending upon the kind of arch, the ring stones may vary in size but are generally made to be uniform in shape and size. Voussoirs are commonly referred to as the haunches of the arch and include all of the stones from the skewbacks to the keystone. The keystone is the uppermost stone of the arch which holds the structure together.

Conceptual Components

• Lines are named to represent certain aspects of the arch, including curve and height. The underside of the arch is referred to as the "soffit." A line called the "intrados" represents the curve of the soffit and a line called the "extrados" represents the outer curve of the voussoirs. The rise of the arch is the vertical line from the springline to intrados at the center of the keystone.

Natural Arches

• A natural arch is a rock formation found in nature and not constructed by man. There are five components of a natural arch. The opening is the space within the frame of the arch The entrance is found at either end of the arch where it meets open air. The lintel is the volume of rock that is supported by the arch at the base. The base is the surface that lies beneath the lintel and opening.The abutment is any part of the rock frame that supports the lintel but is not part of the base.