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What is an arch?

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 load coming from above will distributed to supports (pier or abutment).

What are different Components of an Arch?

The following are the different components of arches and terms used in arch construction:

Intrados

The inner curve of an arch is called as intrados.

Components of an Arch - Introdes

Extrados

The outer curve of an arch is termed as extrados.

Components of an Arch - Extrodes

Soffit in an Arch:

The inner surface of an arch is called soffit. Soffit and intrados are used synonymously.

Voussoirs

The wedge-shaped units of masonry which are forming an arch is called as voussoirs.

Voussoirs

Crown of an Arch:

The highest part are peak point of extrados is called crown.

Keystone

The wedge shaped unit which is fixed at the crown of the arch is called keystone.

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.

Spandrel in an Arch

Skew Back

This is an inclined surface or splayed surface on abutment, from which arch curve starts or ends.

Skew Back

Springing Points

The imaginary points which are responsible for the springing of curve of an arch are called as springing points.

Springing Line

The imaginary line joining the springing points of either ends is called as springing line.

Springing Line

Springer in Arches

The first voussoir at springing level which is immediately adjacent to the skewback is called as springer.

Springer in Arches

Haunch

The lower half of the arch between the crown and skewback is called haunch. Highlighted area in the below fig is haunch.

Arcade

The row of arches in continuation is called arcade.

Arcade in Arches

Ring

The circular course forming an arch is called as arch ring. An arch may be formed by one ring or combinations of rings.

Impost

The projecting course is provided on the upper part of a pier or abutment to stress the springing line. This course is called impost.

Impost

Bed Joints

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.

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.

center-of-an-arch

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.

Rise and Span of an Arch

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.

Piers and Abutments of an Arch