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 · Ornament · Structure · Materials
Let the structure become the ornament
Instead of applying ornament to a structure, shape the structure itself, especially exposed iron, into lines that are both constructive and decorative.
Victor Horta in Brussels and Hector Guimard in Paris read Viollet-le-Duc as licensing a new architecture of exposed iron. In the Hôtel Tassel (1893–94) Horta let a slender iron column rise in the stair hall and sprout curling tendrils that carry the beams, while the same whiplash line runs across the mosaic floor and the painted walls. Structure, ornament and furnishing were drawn by one hand in one continuous line.
The Art Nouveau façade uses the same method outside: a bowed oriel of glass framed in thin iron, railings bent into plant-like strokes, window heads that swell and flow rather than following the canonical arches.
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
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.
Omit applied ornament from buildings and everyday objects: it wastes work, money and material, dates quickly, and adds nothing that fine material and exact workmanship cannot give.
Begin every design from its construction; the architect’s task is to turn the construction into art-form, never to hide it behind forms borrowed from another material.
Cite this page Chris Paton, “Let the structure become the ornament”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/structure-as-ornament. Text, drawings and models licensed under CC BY-NC 4.0.