Arches – Definition of the depth (thickness) of an arch ring In arch terminology, the “depth” (also referred to as the thickness of the arch ring) is defined as the distance between which two surfaces or lines?

Introduction / Context:
Accurate definitions are essential in detailing arches. The “depth” of an arch relates directly to its capacity to carry compressive thrust and affects the geometry of voussoirs and the weight of the ring.

Given Data / Assumptions:

• Masonry or concrete arch with defined intrados and extrados.
• Standard geometrical terms used in architectural engineering.
• Depth measured normal to the soffit in common practice.

Concept / Approach:

The inner curved surface of the arch is the intrados (soffit), and the outer curved surface is the extrados. The distance between these two surfaces is the thickness or depth of the arch ring. Other lines such as the crown, springing line, and ground level define position and support conditions, not the ring thickness.

Step-by-Step Solution:

Verification / Alternative check:

Construction drawings and specifications denote ring thickness explicitly as the intrados–extrados distance, often constant for simple arches.

Why Other Options Are Wrong:

Ground and crown/springing lines are geometric references for level and rise; they do not measure ring thickness.

Common Pitfalls:

Confusing “rise” (crown above springing) with “depth”; mixing up soffit/intrados terminology.

Final Answer:

Intrados and extrados