A chain hanging freely takes the one shape in which it is in pure tension; turn that shape upside down and it is the shape of an arch in pure compression for the same loads.
free-hanging chain (catenary) ↔ ideal arch under its own weight
Rules · Materials · Structure · Theory
Use iron as iron
A new material calls for new forms: iron should be used in slender members that show its strength in tension and bending, not cast in imitation of stone columns and vaults.
Viollet-le-Duc, who had spent his life studying the logic of Gothic construction, argued that the Gothic builders would have used iron without hesitation had they possessed it, and that nineteenth-century architects, who did possess it, dressed it up as masonry instead. His lectures proposed halls in which thin iron struts carry masonry vaults, each material doing what it does best.
The great sheds of the century had already shown the way: Paxton’s Crystal Palace of 1851, assembled in a few months from standard cast-iron columns, girders and panes of glass on a 24-foot module, was a building that no stone-built precedent could have produced. The engineer’s repetition of identical parts became an architectural principle in its own right.
Related rules
Hang a chain (or a model of strings loaded like the building) from two points; the curve it takes, turned upside down, is the ideal shape of an arch carrying the same loads, in pure compression.
A masonry arch stands if a line of thrust (the path of the compressive force through it) can be drawn entirely within its thickness; the catenary arch contains its own line of thrust exactly, the semicircle only if it is thick enough.
catenary: thrust line = centre line · semicircle: needs t ⁄ R ≳ 0.11
In a framed building the columns and beams carry every load; the outer wall can be reduced to a light cladding hung on the frame, floor by floor, and need grow no thicker as the building rises.
load-bearing wall: thickness grows with height · frame: cladding thickness constant
The outward form of a building should arise from, and express, what the building does and how it is made, as the form of every living thing expresses its life.
Instead of applying ornament to a structure, shape the structure itself, especially exposed iron, into lines that are both constructive and decorative.
Cite this page Chris Paton, “Use iron as iron”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/iron-as-iron. Text, drawings and models licensed under CC BY-NC 4.0.