Bracketing is a unique feature of Chinese and Japanese architecture. Bracket systems help to solve four problems that arise as buildings get bigger: how to reduce beam spans, how to brace wall sections above columns, how to support a wide eave, and how to strengthen the frame. We will look at how bracketing systems evolved, and how brackets fit together.
There is some archaeological evidence of Persian and Indian origins of brackets in their simplest form. The earliest evidence in China now dates brackets to 1100 B.C. These early brackets functioned in the wall-plane. They were simple bolsters acting like capitals between column and architrave, or they supported horizontal beams above the architrave.
From 200 B.C. to 600 A.D., Chinese builder-architects adapted brackets to function perpendicular to the wall-plane as cantilever beams. They sometimes added a cross-yoke onto a perpendicular arm. Now a bracket set could support a wall and a small roof eave. At this point, the bracket set became a three-dimensional cluster of parts.
During the Tang, Liao, and Sung dynasties (600-1200 A.D.), the art of bracketing reached its zenith. Builders wanted to erect bigger structures. They enlarged the elements (columns, beams, rafters) while preserving the same proportions. The basic building format, i.e., a rectangular plan wider than deep enclosing a single space, did not change. The roof and roof-overhang (eaves) needed to be proportionally larger, though. Builders had to figure out how to construct a wider eave. So they invented a larger bracket system that could cantilever farther out from the wall-plane and they added a new structural member—the eave-purlin—to carry rafters beyond the wall purlin. Voilà! Wider eaves. Thus builders were able to preserve the graceful, almost floating effect of the curved roof.
The sloping bracket arm, or lever arm (angtou, odaruki) materialized in early Tang as a special bracket member. It was held parallel to the roof slope rather than horizontal like all the other bracket arms. Some bracket sets had two or three angtou, one above the other. The angtou appeared to slice through the horizontal bracket arms, but actually the length and position of the lower bracket arms were adjusted to give it proper support. While perched on a stack of bracket arms, the angtou balanced the eaves-purlin outside and the interior aisle purlin inside. The wall-plane acted as a fulcrum. The roof load could be 400 kg./m.2or 80 lb/sq. ft. The eave could cantilever 10 to 12 feet beyond the wall line.
Now we also see the cross-yoke arm (choumu, funahijiki) bearing purlins and longitudinal tie-beams, and providing better bearing where beam ends joined.
During this creative period in Chinese architecture, builders grew more confident of their geometry and layout skills and designed fancier bracket sets. One sees corner sets and intermediate sets where some bracket arms and blocks are at 45 degrees to each other. Hexagonal, octagonal, round buildings, you name it, builders had figured out how to make bracket sets for any situation.
As the years and centuries rolled by, changes in taste and building design made large bracket sets obsolete. Ming dynasty bracket sets became smaller and more profuse. They were used as brightly-colored decoration in the Chinese version of Victorian gingerbread. The mighty angtou was deleted and replaced with an impostor, a horizontal bracket arm with a sloping nose to look like an angtou. Fortunately, some thousand-year-old timber frame buildings have withstood war, weather and neglect to thrill us with their beauty.
A bracket set is a group of wood blocks (dou) and short beam-arms (gong) cut so they interlock and form a unit when stacked up together. Bracket sets are assembled piece by piece in the wall-plane, usually on top of a column or beam. A bracket set springs from a single point of support called the base block. The first arm nestles in the base-block. The ends of each arm carry bearing blocks upon which the next arm sits. Each succeeding arm is longer than the arm below it. The bracket set grows in alternating upward and outward steps. A set can have up to five tiers of bracket arms. The top arm supports a beam or purlin. Sometimes intermediate bracket arms also support a structural member. Bracket arms have names for their position in a set. There are pumpkin arms, flower arms, kidney arms, wall arms, the sparrow, monkey or insect head, and the bird’s-beak head. Bracket sets can be made of stone in stone structures, but they still remind one of wood.
When a bracket arm is perpendicular to the wall-plane, it acts as a cantilever beam. On the outside of the wall it can support an eave-purlin. The distance from the eave-purlin to the wall is equal to the extra eave overhang. Bracketing with the eave-purlin allows for a greater outward span of theroof. So, the first function of brackets is to support the roof and extend the eaves.
The bracket arm also extends inside the building, where it either supports an interior frame member or is held in place by one. Often a tie-beam takes the place of one bracket arm and thus ties the bracket set to an interior part of the structure. Corner bracket sets carry a single or double hip beam. When sets are connected with transverse and longitudinal ties, they strengthen the frame.
Besides holding up the roof, bracket sets also stiffen and brace the upper wall section. This upper wall follows the perimeter columns and adds height from the architrave level to the roof. The height of the upper wall above the architrave is about half of the column height, a 1:2 ratio. A 20-ft.-tall column means a 10-ft.-tall upper wall. The upper wall is constructed by stacking horizontal beams on top of each other. Bracket arms and transverse tie-beams perpendicular to the wall plane lap these upper wall beams and lock the wall in a vertical plane. Bracket sets also connect the upper wall to the roof structure and to the interior frame. They keep the upper wall from rotating and falling off of the columns. The second function of brackets is to stay the upper wall section.
Bracket arms under beams transfer loads back to columns and reduce the span of beams between columns. This third function of brackets is not very obvious visually, but as Chinese forests were depleted, beam size and span became an important issue.
Bracket arms, blocks, and cross yokes parallel to the wall plane increase the number of bearing surfaces for framing members such as purlins and longitudinal tie-beams and increase the number of lateral connections. The cross-yoke looks like a small bracket arm and typically gives three points of support to the longitudinal member that it holds. Especially where purlins or ties-beams join end to end, it is helpful to have more bearing surface underneath the joint. The fourth function of brackets is to strengthen the frame by increasing the number of connections between members.
All these joinery connections unify the frame. They make the frame more resilient without necessarily becoming rigid. This joinery is flexible and even a bit elastic, partly because wood is by its nature resilient, and partly because, while small woodwork requires tight-fitting joinery, large-scale building uses loose joinery. Chinese building philosophy is similar to T’ai Chi. The force of an attacker, or earthquake, is not confronted or resisted. A person sidesteps the attacker. A building flexes and lets the force pass by.
Bracket sets in large buildings can number tens of thousands of pieces, all connected by joinery. Bracket pieces are small relative to the size of framing members. Building with brackets is an efficient use of materials. Brackets sets may thus offer a shock-absorbing capacity. In a region as prone to earthquakes as Asia, brackets may help buildings to survive.
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