Pitman's Vol 14 - Arch Terminology

Front Cover of Volume 14

Building Educator
Volume 14

by R Greenhalgh (Ed)

Published by Pitman & Sons Ltd
16th April 1927

No ISBN number

Extract from page 648

Load on a arch
Fig. 52 - Volume 14 page 648

The load carried by an arch is the triangle of brickwork contained within the offsets, shaded in grey

In a segmental arch, this covers the entire span

With a semi-circular arch, it is shown by the tangents to the curve

The curved portion on both sides of the arch, from the springing-line to the tangent, is called the haunch

The remaining part, in the middle, is the crown

Parts of an arch
Image adapted from Civil Engineer

The pillars, or the parts of the wall, that supports the arch are called the abutments

Parts of an arch, including abutments
Image from Architechstok

The load carried by a semicircular arch rests upon the crown, and from thence is transmitted via the haunches to the abutments. This force on the haunches would throw the arch out of equilibrium and cause fracture, unless some opposite force, were there to resist it

This opposing force, or reaction, is provided by the mass of brickwork called the spandril, which is built at the sides of the arch, and which abuts the outer curve, called the extrados

The inner curve of the arch is called the intrados. The face of the arch under this inner curve is called the soffit

Components of an arch
Image adapted from Residential 101.com

1. Abutment
2. Centre
3. Crown
4. Extrados
5. Haunch
6. Height or Rise
7. Impost
8. Intrados
9. Keystone
10. Span or Chord
11. Spandrel
12. Springer
13. Spring line
14. Voussoir

Semi-circular arches are built from wedge-shaped blocks called voussoirs, starting from a seating which is at right angles, or 'normal' to the curve

In other words, the the seating or abutment is on a line, which passes through the centre of the curve

This horizontal line from which the arch commences is called the stringing line, and if the curve is less than a semicircle, the line of abutment is termed the skewback

The points on the springing line, from which the abutment starts, are called the stringing points

The voussoirs terminate at the centre of the crown and this central piece is called the key-stone