The frame consisting of column and penetrating beam is one of the main horizontal load resisting elements of traditional wooden structures. More often, there is a connecting joint of the beam, which penetrates through the column. Therefore the shape of the joint is hidden inside the column, not visibly apparent. However, the shapes of joints of penetrating beam are various, and it is difficult to ascertain its shape only at the time of construction or dismantling. In order to clarify its shape in its existing state, X-rays inspection and other methods are required, which are expensive and labor intensive.
In order to evaluate the structural performance of such joints, this study aims to clarify the specification of joints of penetrating beam used in traditional wooden buildings.
For this reason, we collected information on joints of penetrating beam from documental reports on repair work. The target of research is on Buddhis temple halls built after 12th century, designated as Important Cultural Properties (Fig. 4)(Table 1).
Information on penetrating beams was obtained for 145 buildings, and it was confirmed that ryakukama joint is used in more than 80% of the buildings. We revealed that the shape of ryakukama joint used in penetrating beam can be divided into three major types: Type A, B, and C. At the same time, there were some joints which were difficult to determine whether the shape was A or B, which we named A-B. Before 1500, ryakukama joint was mostly type A. Type B start to appear in the first half of the 16th century. Results indicate that type C become the majority type in the second half of the 17th century (fig. 13). It is speculated that these changes in shape are due to the development of tools and the influence of building technology books called ‘hinagata-bon’ that prevailed from the 18th century. There was no correlation between the type of ryakukama joint and the location (region) of the building or the roofing material. The effect of year of construction is most pronounced.
Lastly, the dimensions of the ryakukama joints used in penetrating beam were investigated. There are two main types: height of joint is 0.45 and 0.8 when that of penetrating beam is 1(fig. 20). It is argued that the transition from type A and B to C was intended by the carpenters of the time to improve the structural performance, because the higher the joint compared to penetrating beam, the greater the bending strength. In Shape C, the ratio of the height of ryakukama joint to that of penetrating beam is 0.45, which is very similar to Shape B. Therefore, it is defined as Shape B-C and the changes in 5 types of ryakukama joint is shown in Fig. 23. There was a general tendency to shift from shape B to B-C and then to shape C. However, there was no unidirectional change in shape B-C in the second half of the 18th century, as shape B-C was more common than shape C.
There are many unclear points about the relationship between carving technology of penetrating beam, tools, and tree species from literature survey alone. Therefore it is necessary to discuss this point through interviews with experts and carving experiments. In addition, it will be necessary to clarify the structural performance of the frame consisting of column and penetrating beam on the results obtained in this study. In addition, it is necessary to understand how the differences in specifications of joint of penetrating beam affect the overall performance of the building.
