Basics in Building Construction

Building Components And Types

Lesson 17 and 18 : Arches

Technical Terms

The definitions of technical terms used in connection with the arch are presented below.

• Intrados: The inner curve of the arch.


• Soffit: The inner surface of the arch. Sometimes, the terms, intrados and soffit are treated as synonymous.


• Extrados or back: The external curve of an arch.


• Voussoirs: The wedge-shaped units forming the courses of an arch.


• Skewback: The inclined or splayed surface on the abutment. It is prepared to receive the arch and form it, springs the arch.


• Springer: The first voussoir at springing level on either side of an arch.


• Crown: The highest point of the extrados.


• Key: The wedge-shaped unit at the crown of an arch. It is sometimes made prominent by making it of a larger section and projecting it below and above the outlines of an arch.


• Abutment: The end support of an arch.


• Piers: The intermediate supports of an arcade.


• Springing points: The points from which the curve of an arch springs.


• Springing line: The imaginary horizontal line joining the two springing points.


• Span: The clear horizontal distance between the supports.


• Rise: The clear vertical distance between the highest point on the intrados and the springing line.


• Centre: The geometrical centre of the curve of an arch.


• Ring: The circular course forming an arch.


• Depth or height: the perpendicular distance between the intrados and extrados.


• Spandril: The irregular triangular space formed between the extrados and the horizontal line drawn tangent to the crown.


• Haunch: The lower half portion of the arch between the crown and the skewback.


• Arcade: Arrow of arches supporting a wall above and being supported by piers.