A frame structure carries loads through connected members, a shell structure through a thin outer surface, and a solid structure through its mass. A combination structure uses two or more of these structural systems together.
How Each Structure Works
The key difference is the load path: the route through which forces are transferred to the supports or ground. This affects material use, mass, strength, stiffness, manufacturing, and failure.
| Structural type | Load transfer and characteristics | Examples |
|---|---|---|
| Frame | A network of beams, columns, struts, or ties transfers loads. Members may experience tension, compression, bending, or torsion. Frames often have a high strength-to-weight ratio but depend on strong joints. | Bicycle frame, roof truss, crane |
| Shell | A thin, continuous surface distributes loads across its skin. Curved shells can be especially stiff while using little material, but dents or cracks may cause buckling. | Eggshell, drinks can, monocoque body |
| Solid | Loads pass through the bulk of the material. Solid structures are usually stable and resistant to compression, but they use more material and have greater mass. | Dam, brick wall, stone column |
| Combination | Different structural types work together so that each performs a suitable function. This can improve stiffness, strength, protection, or material efficiency. | Aircraft fuselage with frames and skin, framed building with load-bearing panels |
Material selection must match the forces involved. For example, a frame member under tension requires sufficient tensile strength, while a column under compression needs compressive strength and resistance to buckling. Shells require suitable stiffness and toughness because local deformation can weaken the whole surface.
A common misconception is that every hollow product is a shell structure. A structure is classified by how it supports and transfers loads, not simply by whether it contains empty space.
IB Exam Technique
When asked to distinguish these structures, define each one using its load path, then give a relevant example and explain one advantage or limitation. Do not identify a structure only from its appearance; connect its form, material properties, and structural function.