There are three ways to organize physical structures:
- Masses: entirely relying on solid bodies of material for support. engineered earth, masonry
- Frames: material formed into edges and nodes. tent frame
- Shells: material in a thin membrane enclosing a volume. aluminum baseball bat
Universal Principles of Design; Lidwell, Holden, Butler
The analytic framework around structural forms breaks down when applying the framework to the same structure at different scales. This is particularly common in nature, where fractal structures are common.
Wood is a good example. Cut, dried, and dimensioned; a solid pillar of wood might be considered a mass structure. However, at a microscopic scale, wood is composed of many bonded tubes and fibres. At this scale, it could be considered a type of frame. Additionally, each tube could be considered a shell.
Looking back at the pillar of wood as a whole, it becomes problematic to simply describe it as either a mass, frame, or shell structure.