What is the term for perpendicular distance between intrados and extrados?
📐 Understanding the Geometry of an Arch
An arch is defined by several key geometric properties. This question focuses on the dimension that defines the thickness of the arch ring itself.
(D) Depth: The Thickness of the Arch Ring
The Depth (or sometimes called "height") of an arch is the correct answer. It is the perpendicular distance measured between the inner curve (Intrados) and the outer curve (Extrados). This dimension essentially represents the thickness of the voussoirs that make up the arch, which is critical for its load-bearing capacity.
(A) Key: The Central Stone
The Key or Keystone is the wedge-shaped stone at the very top (crown) of the arch. It is a component, not a measurement of distance.
(B) Pier: The Intermediate Support
A Pier is an intermediate vertical support for a series of arches (an arcade). It is a structural support, not a dimension of the arch itself.
(C) Rise: The Vertical Height
The Rise is the clear vertical distance from the springing line (the level where the arch begins) to the highest point on the intrados (the inner curve). It defines how tall the arch is relative to its span.
🖼️ Visual Diagram of Arch Dimensions
This diagram clearly illustrates the key dimensions of an arch, including its Depth, Rise, and Span.
📖 Glossary of Arch Components and Dimensions
Key Curves and Surfaces
- Intrados: The inner curve of the arch.
- Extrados: The outer curve of the arch.
- Soffit: The physical inner surface of the arch (corresponding to the intrados).
Key Dimensions
- Depth: The perpendicular distance between the intrados and extrados.
- Rise: The clear vertical distance between the springing line and the highest point on the intrados.
- Span: The clear horizontal distance between the supports.
- Thickness: The horizontal distance measured perpendicular to the front and back faces of the arch.
Key Components
- Voussoirs: The wedge-shaped masonry units that form the arch.
- Key / Keystone: The final wedge-shaped unit fixed at the highest point (crown) of the arch.
- Crown: The highest part of the extrados.
- Springers: The lowest voussoirs, located immediately adjacent to the supports.
- Spandrel: The curved triangular space formed between the extrados and the horizontal line through the crown.
Support Components
- Abutment: The end support of an arch that resists both vertical load and horizontal thrust.
- Pier: An intermediate support for a row of arches.
- Skew back: The inclined, splayed surface on the abutment which is prepared to receive the arch's thrust.
- Springing Points: The points on the supports from which the curve of the arch begins.
- Springing Line: An imaginary horizontal line joining the two springing points.
- Arcade: A row of arches supported by piers or columns.