Following are the terminologies used in arch work (Fig. 19.1).
Figure 19.1 Various parts of an arch
1. Abutments
These are the outermost supports of an arch from which the arch starts. That is, this is the wall which supports the arch.
2. Springing Line
It is the horizontal line joining the two springing points.
3. Span
This is the width of the opening covered by the arch.
4. Intrados
The inner curve of the arch.
5. Extrados
The outer curve of the arch.
6. Arch ring
This is the curved portion of masonry forming the arch.
7. Voussoirs
Wedge-shaped blocks of stone or brick masonry of which the arch ring is made.
8. Key Stone
The central voussoir at the central point (crown) of the arch.
9. Crown
The highest part of the extrados.
10. Soffit
The inner or under surface of the arch.
11. Skewbacks
This is the springing point of an arch at the support.
12. Springing Points
The points at the intersection between the skewbacks and the intrados.
13. Springer
The lowest voussoir next to the skewback.
14. Haunch
The lower half of the arch between skewback and crown.
15. Rise
The vertical distance between the highest point of intrados and springing line.
16. Spandrill
The triangular space between the back of the arch ring and a horizontal plane tangent to it at the crown.
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