In this blog post, we will explain the structural principles of traditional wooden architecture and the secrets of the gajang technique, which reduces the number of pillars, in an easy-to-understand manner.
The structure of traditional wooden buildings can be divided into two main parts. One is the roof, and the other is the wooden frame that supports it.
First, the roof is covered with various types of finishing materials, such as roof tiles, and is composed of rafters, which are sloped structures that support these finishing materials. The wooden frame is the structure that supports the entire roof from below. The wooden frame consists of horizontal beams that are directly connected to the rafters and pillars and beams that support these beams.
Here, the pillars are the main structural members that are erected vertically on the ground, and the beams are structural members that are placed horizontally on the ground in the left and right directions (horizontal direction) and front and back directions (longitudinal direction) when viewed from the front of the building.
The most primitive form of wooden frame construction consists only of pillars and beams. When viewed from the side, the sloping rafters are supported directly by the beams, and each beam is connected to the pillars to transfer the load to the ground.
To form a sloped roof, three rafters are required: one ridge rafter placed at the top of the roof, and two eaves rafters placed on the front and rear pillars, which are lower than the ridge rafter. The height difference between these three rafters is used to adjust the slope of the roof.
In this structure, the longest pillar connected to the ridge beam located in the center when viewed from the side and two pillars located at the front and rear are used. The load of the roof is evenly distributed through these three rafters and the pillars supporting them, and then transferred to the ground. This structure consisting of three rafters is called a three-beam frame, which is the most basic form of traditional wooden architecture and is structurally very stable.
In a three-beam structure, there are three pillars on the sides of the building, which is recognized as a two-bay structure. Assuming that the front is also made up of two bays, there would be one internal pillar in the center supporting the ridge beam. However, such internal pillars can divide the interior space and hinder space utilization.
The structural device introduced to overcome this spatial limitation is called a “beam.” Beams support the ridge beam without internal pillars, thereby increasing the openness and usability of the interior space. Specifically, instead of installing the internal pillars that support the ridge beam all the way down to the ground, beams are used to connect the ridge beam to the tops of the two pillars facing each other.
The structural member that connects the beam and the ridge beam and replaces the internal pillars is called a daegong. The daegong supports the ridge beam and distributes its load to both pillars through the beams, effectively supporting the load of the roof without internal pillars.
This structural method of omitting internal pillars by introducing beams is called ganchu. When ganchu is used, the central pillar in the interior of a three-bay structure is removed, resulting in a single-bay interior cross section with the central pillar at the front as the axis. In other words, although there are two bays, the structure appears to have only one pillar.
Although reducing the number of columns has the advantage of providing a spacious and freely usable interior space, it may be slightly inferior to a two-bay structure with interior columns in terms of structural stability.
If the interior space of a building needs to be expanded, the roof area must also be enlarged proportionally. However, since there is a limit to the load that rafters can support, one middle beam is inserted between the ridge beam and both eaves beams to compensate for this. Then, by installing separate rafters between the ridge beam and the middle beam and between the middle beam and the eaves beam, the roof and wooden frame structure are expanded.
This structure is called a five-beam frame, which enables the construction of large buildings and the expansion of interior space. However, in order to maintain a five-beam frame without internal columns, additional structural reinforcement is required to support the load corresponding to each beam.
To this end, beams and braces are used. By installing ridge beams that support the ridge and middle beams that support the middle rafters, a stable structure can be maintained without internal pillars. By installing braces in various locations and omitting pillars, it is possible to create various spatial configurations in accordance with the design intent.
As the size of a building increases, not only does the roof area increase, but the load on the entire structure also increases dramatically. In this case, the number of internal columns naturally increases to ensure structural safety. If internal columns are placed randomly, problems with space utilization and structural balance will arise, so it is important to place them according to certain rules.
One of the most common methods is the regular grid system. In this system, internal columns are placed exactly at the intersection of the outer columns of the building when the outer columns are connected in a straight line. This method is widely used in traditional architecture because it forms a structurally very stable grid structure.
For example, if the principle of political placement is applied to a building with a structure of three bays at the front and three bays at the sides, four columns are installed in the correct positions inside. The side with the entrance to the building is called the front, and the opposite side is called the rear. The interior columns arranged according to the principle of political placement are called the front row and the rear row, respectively.
Removing these interior pillars entirely or in rows is also a type of gajung, which allows for more freedom in the layout of the interior space.
In addition, depending on the desired interior design or functional requirements, the interior pillars can be installed slightly offset from their exact intersection points. Although the structural stability of the pillar relocation method is somewhat lower than that of the pillar placement method, it has the advantage of ensuring the uniqueness and practicality of the space.
As a result, pillar placement and relocation are structural devices that have been developed along with various design techniques in traditional wooden architecture as options for finding the right balance between structural stability and freedom of space.
