An arch is a structure constructed in curved shape with wedge shaped units (either bricks or stones), which are jointed together with mortar, and provided at openings to support the weight of the wall above it along with other superimposed loads.
Because of its shape the loads from above gets distributed to supports (pier or abutment).
Different Components of an Arch
The following are the different components of arches and terms used in arch construction:
The inner curve of an arch is called as intrados.
The outer curve of an arch is termed as extrados.
Soffit in an Arch
The inner surface of an arch is called soffit. Soffit and intrados are used synonymously.
The wedge-shaped units of masonry which are forming an arch is called as voussoirs.
Crown of an Arch
The highest part are peak point of extrados is called crown.
The wedge shaped unit which is fixed at the crown of the arch is called keystone.
Spandrel in an Arch
If two arches are constructed side by side, then a curved triangular space is formed between the extrados with the base as horizontal line through the crown. This space is called as spandrel.
This is an inclined surface or splayed surface on abutment, from which arch curve starts or ends.
The imaginary points which are responsible for the springing of curve of an arch are called as springing points.
The imaginary line joining the springing points of either ends is called as springing line.
Springer in Arches
The first voussoir at springing level which is immediately adjacent to the skewback is called as springer.
The lower half of the arch between the crown and skewback is called haunch. Highlighted area in the below fig is haunch.
The row of arches in continuation is called arcade.
The circular course forming an arch is called as arch ring. An arch may be formed by one ring or combinations of rings.
The projecting course is provided on the upper part of a pier or abutment to stress the springing line. This course is called impost.
The joints between the voussoirs are called bed joints. These bed joints are radiate from center of arch. The red color lines in the below figure are bed joints.
Center of an Arch
The geometrical point based on which the arcs forming intrados of arch, extrados of arch and arch rings are described is the center or striking point.
Span of an Arch
The clear horizontal distance between the supports or abutments or piers is termed as span of an arch.
Rise of an Arch
The clear vertical distance between the highest point on the intrados and the springing line is called as rise.
Depth or Height of an Arch
The perpendicular distance between the intrados of arch to the extrados of arch is called depth of an arch or height of an arch.
Thickness of an Arch
This is the breadth of soffit which is measured perpendicular to the front and back faces of an arch. Colored area in the below figure is the thickness of an arch.
Pier and Abutment of an Arch
The intermediate support of an arch is called as pier. The end support of an arch is called as abutment.