Take the section of the standard timber, the cai, as the module of the building: it is 15 fen high and 10 fen wide, and every member is dimensioned in fen.
1 cai = 15 fen (height) × 10 fen (width)
The central pillar stands free
Run a single great pillar up the centre of the pagoda to carry the spire, but do not let the storeys rest on it: each storey stands on its own frame, stacked around the pillar.
The shinbashira at Hōryū-ji is a single cypress trunk whose felling has been dated by tree rings to 594. It stands on (or, in later pagodas, hangs above) a foundation stone and passes through the storeys without carrying them. In an earthquake the storeys sway in alternate directions like a stack of loosely fitted boxes, and the pillar limits their movement. This is one explanation of why so few timber pagodas have ever been felled by earthquakes. The engineers of the Tokyo Skytree (2012) cited the principle in its concrete central column.
Related rules
Between two stacked arms leave a spacer of 6 fen, the zhi; a cai plus a zhi, 21 fen, is a “full timber” (zucai) and the vertical step of each tier of a bracket set.
zucai = 15 + 6 = 21 fen
Build the bracket set (puzuo) of blocks (dou) and arms (gong) crossing at right angles, each tier projecting no more than 30 fen beyond the one below; a set of n projecting tiers is called an (n + 3)-puzuo, from four to eight.
projection per tier ≤ 30 fen · rise per tier = 21 fen · puzuo = tiers + 3
Where the eaves must project far, replace the outer arms by slanting levers (ang) that run down and out beyond the bracket set and rise inward to be held down under an interior purlin.
Shape the underside of every arm’s end in four or five facets, and reduce the upper third of a spindle column by the same method, so that no member ends in a hard corner.
Make the central bay (mingjian) the widest and let the bays diminish toward the ends; keep the eave columns no taller than the width of their bay.
Cite this page Chris Paton, “The central pillar stands free”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/shinbashira. Text, drawings and models licensed under CC BY-NC 4.0.