ROTATIONAL EMBEDMENT MECHANISM OF TRADITIONAL WOODEN STRUT AND ITS FORMULATION

The important structural elements of traditional timber structures in Japan are rotational resistances of column-beam joints. The restoring forces characteristics of their structures depend on the rotational resistances of the joints. Therefore, the elasto-plastic restoring force characteristics of embedment of joints is the most significant in order to evaluate the seismic performances of the traditional timber structures. The authors have already applied Pasternak Model (abbreviated to PM) to the embedment and established the formulation of embedment mechanisms of partial compression of wood, which is the most appropriate embedment mechanical model for embedment problems. Furthermore, they developed the Elasto-plastic Pasternak Model (abbreviated to EPM) for the elasto-plastic embedment mechanism considering the strain hardening behavior. However, there are many types of joints and their resisting mechanisms are different. Thus, the mechanisms of different types of joints should be made clear. The authors have already established the formulation of crosspiece joint applying the EPM of the mechanism and confirmed the formulation based on the static loading tests. In this paper, the authors focus on the strut which has not been considered as a structural element so far, and propose the formulation of embedment mechanism of the strut, as a model of rotational embedment under the constraint conditions. Then, they carried out loading tests of the struts, and obtained their restoring forces characteristics. The loading test of two types of struts were carried out; one is Y-series (six specimens) which support the floor beams standing on the base stone. The other is T-series (three specimens) which stand between two beams, called, “Taihei tuka”. The dimentions of the strut are: depth:120mm, breadth:120mm, height:280mm. Beams depth:180mm(Y-1～3) and 90mm(Y-4～6), and 150mm (T-1～3). All species of specimens are Japanese cedar (Sugi). The test results showed that the maximum horizontal resistances of Y-1～3 were 8～11kN, the maximum normal loads were 36～50kN. The maximum horizontal resistances of Y-4～6 were 6～7kN, the maximum normal loads were 18～28kN. T-1～3 were the maximum horizontal resistances of 12kN, the maximum normal loads were 48～53kN and had the large deformability up to 200mm. The resisting mechanism of strut joints due to horizontal loads is “Diagonal Effect” which push up the beam, rotating and embedding into the beam in accordance with the rotation of the strut. The mechanism is due to geometrical fact that the diagonal line of the strut is longer than the height of the strut. Another mechanism is the distance between beams decreases due to inclination of column. The same mechanism is found in short beams between inclined columns and panel walls within the frames. The authors considered the mechanism of struts is rotational embedment of strut into beams under the constraint conditions and proposed Elasto-plastic Pasternak Model formulation established by the author, in order to analyse them appropriately. Then the formulation is discussed and verified by the simulation based on the Elasto-plastic Pasternak Model formulation. As a result, the deformability is very large and the strut should be considered as a structural element for exact seismic estimation of traditional timber structures.