This blog post takes an in-depth look at how the three-bay structure, the foundation of traditional timber architecture, distributes loads and ensures the building’s balance and stability.
Traditional timber structures are fundamentally divided into two parts: the roof and the timber framework. The roof consists of various finishing materials like tiles and the rafters that support them. The timber framework supporting this roof comprises purlins that directly contact the rafters, along with the columns and beams that support these purlins. Pillars are members placed vertically on the ground. Purlins and beams are members placed horizontally on the ground, running left-right and front-back respectively when viewed from the front of the building.
The most primitive timber frame structure consists solely of pillars and purlins. When viewed from the side, the purlins must directly support the sloped rafters, and each purlin must be connected to a pillar. To form a pitched roof, three purlins are essential: one ridge purlin at the very top and two eave purlins placed at a lower height on the front and rear columns. The roof’s slope is determined by the height difference between the ridge purlin and the eave purlins. The sides consist of the longest central pillar connected to the ridge beam and two pillars at the front and rear. The roof load is distributed through each beam and pillar and transferred to the ground. This wooden frame structure with three beams is called a three-beam frame (三樑架) structure. It is the foundation of traditional wooden frame structures and a mechanically stable form.
In this case, three columns are placed on the building’s sides, forming a two-bay configuration. Assuming a building with a two-bay front, it can be inferred that one internal column supporting the central ridge beam would be placed in the center. However, the beam was introduced with the intent of eliminating internal columns within the building to maximize the usability of the interior space. Instead of an internal pillar extending down to the ground to support the central ridge beam, this method removes the internal pillar by connecting the ridge beam horizontally to the tops of two opposing pillars. The structural member connecting the beam and ridge beam, performing the same mechanical function as the internal pillar, is called the daegong (main beam). The roof load that the internal pillar would have supported is distributed and transferred to the remaining two pillars via the daegong and the beam. The introduction of the beam enables the reduction of interior columns (감주), resulting in a single-bay structure for the interior longitudinal section (a section cut vertically through the building parallel to the side, using the central front column as the axis) of the three-bay timber frame structure. While reducing columns resolves the segmented nature of space created by interior columns, it also has the limitation of offering somewhat less structural stability compared to a two-bay structure.
To further expand the interior space, the roof area must correspondingly increase. However, the load-bearing capacity of the rafters has its limits. Consequently, the five-bay structure was developed by inserting one intermediate purlin between the ridge purlin and each side eave purlin, and placing separate rafters between the ridge purlin and intermediate purlins, and between the intermediate purlins and eave purlins, thereby expanding both the timber frame and the roof. When beams are not used, the number of bays in the longitudinal section naturally increases. Therefore, beams are introduced to support the ridge beam (main beam) and the intermediate beam, replacing the internal columns. This method allows for various reductions in the number of internal columns within the longitudinal section of the timber frame structure, enabling the interior space to be configured according to its intended use.
Conversely, as the building’s scale increases, the load on the upper structure—including the roof and beams—also grows, necessitating more interior columns. In such cases, interior columns are arranged according to specific rules, considering mechanical stability. Connecting the positions of opposing columns at each point forming the building’s outer column row establishes a grid structure within the building. The method of proper placement (正置法) involves erecting interior columns precisely at the grid’s intersection points—the exact meeting points of each connecting line—forming the most stable structure. For example, when installing interior columns in a building with a frontage of 3 bays and a depth of 3 bays, a total of 4 interior columns are placed at the proper positions based on the grid intersections. The side with the building’s entrance is called the front, and the opposite side is called the rear. The arrangement of interior pillars placed according to the Correct Placement Method on the front and rear sides is referred to as the front row and rear row, respectively. The method of removing entire columns or entire rows is called the column reduction method. The method of slightly shifting the positions of the front and rear columns to partition space for specific purposes is called the column relocation method. While column reduction and relocation offer slightly less structural stability than the standard method, they hold significant importance for enabling diverse spatial arrangements.
