Question

Arches and shells use primarily which type of strength in their design?

• A. Tensile
• B. Compressive
• C. Bending
• D. Shear

Hint:

Structural Geometry Advantage: - Beams span spaces by bending (creating heavy internal tension and compression). - Arches and Shells span spaces by squeezing together (pure axial compression). This structural squeezing action allows weak materials like masonry or unreinforced concrete to span massive gaps safely.

Answer 1

The Correct Option is B

Concept: Form-active and surface-active structures utilize specific geometric shapes to convert external gravitational loads into purely internal axial forces, avoiding inefficient structural states like bending moments or heavy shears. An arch is designed specifically to redirect downward vertical gravity forces into a curved path that runs along its profile down to the ground support foundations.

Step 1: Analyzing the mechanics of an Arch.
When a gravity load is applied to the top of a correctly curved arch, the geometry forces the stones or masonry units to press tightly against one another. This transforms the load into an unbroken internal line of thrust characterized entirely by axial compression. Because the structure stays in compression, ancient builders could construct massive stone bridges and cathedrals using materials like brick and stone that have high compressive strength but almost zero tensile capacity.

Step 2: Analyzing the mechanics of thin Shell structures.
Thin shell structures are curved surface-active structures. Because their thickness is exceptionally small compared to their overall span, they cannot resist significant internal bending or shear stresses. Instead, their spatial curvature allows them to transfer distributed surface loads down to their supports primarily through clean in-plane axial forces, dominated heavily by membrane compression.

Step 3: Conclusion on design behavior.
By utilizing pure compression, arches and thin shells reduce material requirements while maximizing structural efficiency, allowing them to span vast distances without sagging. Therefore, they rely primarily on compressive strength.