Question

Outer curve of an arch is called -

• A. Intrados
• B. Evolute
• C. Involute
• D. Extrados

Hint:

In arch design, always distinguish between the intrados and extrados for understanding stress distribution.

Answer 1

The Correct Option is D

Step 1: Understanding arch terminology.
The outer curve of an arch, which is subjected to tensile forces, is known as the extrados. This is the counterpart to the intrados, which is the inner curve of the arch.

Step 2: Analyzing the options.
(A) Intrados: Intrados is the inner curve of the arch, not the outer curve.
(B) Evolute: Evolute refers to a curve derived from the envelope of tangents to another curve, unrelated to the outer curve of an arch.
(C) Involute: Involute is a term used for a curve generated by a point on the end of a string unwinding from another curve, not related to arches.
(D) Extrados: Correct — The extrados is the outer curve of the arch, which carries the compressive forces.

Step 3: Conclusion.
The correct answer is (D) Extrados, as it is the outer curve of an arch.

Answer 2

Arch terminology can be worked out from the prefix of each word and the structural role of each curve, without relying purely on memorized labels.

1. Intrados: The prefix "intra-" means inside, so this names the inner curve of the arch, the surface facing the space below, not the outer curve.
2. Evolute: This is a term from differential geometry describing the locus of the centers of curvature of a given curve, a mathematical concept unrelated to arch construction terminology.
3. Involute: Also a geometry term, describing a curve traced by unwinding a taut string from another curve, again not an architectural term for an arch surface.
4. Extrados: The prefix "extra-" means outside, so this names the outer, convex curve of the arch, the surface that mainly experiences compressive forces as the arch pushes outward.

Since "intra-" and "extra-" directly translate to inner and outer, and the other two options are unrelated geometry terms, the outer curve must be the extrados.

Therefore, the correct answer is Extrados.