The Problem Is Always the Span
Every building must hold a load and carry it to the ground. The structural system decides what spaces are possible.
| System | How it works | Limit | Characteristic space |
|---|---|---|---|
| Post and lintel | A beam across two supports | The beam's resistance to bending | Forest of columns, modest spans |
| Corbelling | Courses stepped inward | Very limited | Small chambers, false domes |
| Arch | Compression carried along a curve | Needs buttressing against outward thrust | Long spans, arcades |
| Vault | An arch extended or crossed | The same thrust, along a whole wall | Continuous covered space |
| Dome | An arch rotated | Thrust in every direction at the base | Great centralised interiors |
| Frame | A skeleton carries the load | Very few | Open plans, glass walls |
Post and lintel is the Greek system. Stone is strong in compression but weak in tension. A stone lintel fails by bending, so columns must sit close together.
The arch turns the load into compression along a curve. The price is thrust. An arch pushes outward as well as down, so buttresses or thick walls must resist it.
Vaults, Domes, and Frames
The vault extends an arch in depth or crosses two arches.
Gothic architecture combines three key structural inventions:
- The ribbed vault
- The pointed arches
- The flying buttress
Together, these concentrate loads into specific points. The wall stops being structural, allowing vast expanses of stained glass.
The dome rotates an arch. Its thrust acts all around the base. Byzantine builders placed round domes over square plans using pendentives.
The frame uses iron, steel, or reinforced concrete. A skeleton carries the load, turning exterior walls into lightweight screens.
Vitruvius
Vitruvius demanded three qualities in a building: firmitas, utilitas, and venustas. The structure settles its firmitas (solidity).
Common pitfall: treating structure as mere engineering separate from style. The style is what the structure looks like when working.