The hanging chain
Hooke’s rule and Gaudí’s method: a hanging chain, turned upside down, gives the arch whose line of thrust lies along its centre. Compare catenary, parabola and semicircle.
The rules it illustrates
As hangs the chain, so stands the arch. 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.
The line of thrust must stay inside the arch. 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.
Where it appears in the book
In the styles Art Nouveau.
In the chapters Frame and Skin.