EXPERIMENTAL VERIFICATION AND SEISMIC STRENGTHENING DESIGN OF BI-DIRECTIONALLY EFFECTIVE WOODEN BEAMS (<i>NAGESHI</i>) WITH HIGH STRENGTH SEMIRIGID CONNECTIONS

 The seismic strengthening method by using Nageshi, which is a traditional wooden beam, is sometimes adopted in the traditional wooden structures such as temples and shrines. Generally, the reinforcing construction method by Nageshi is effective only in one direction, however, we proposed the new seismic strengthening method with bi-directionally effective Nageshi by using joint metals (see Fig. 2) as seismic strengthening scheme of an existing temple (see Fig. 1, Photo1). In this paper, we verified the structural performance of bi-directionally effective Nageshi and revealed the structural mechanism of Nageshi joint and proposed the design method of the new strengthening method by Nageshi. In addition, the design example of the actual building was shown.<br> We conducted the in-plane shear tests of seven specimens as shown in Fig. 3 and Fig. 5; one is unreinforced specimen for comparison, the others are reinforced specimens with various configurations of Nageshi. As the results of reinforced specimens as shown in Fig. 6, the stiffness and strength of all specimens were much greater than those of the unreinforced specimen, so the method to reinforce by Nageshi was proved to effective. Also, all specimens showed a stable load versus story drift angle response until large deformation area.<br> The effectiveness of the new strengthening method by two-tiered Nageshi we proposed was confirmed, because the stiffness and strength of the specimen with two-tiered small Nageshi (75-150 cross-section) were comparable to those of the specimen with normal Nageshi (150-150 cross-section). However, the bending deformation of small Nageshi was shown at the end stage due to a lack of flexural rigidity compared to the rigidity of the joint (see Fig. 9). In addition, as the results of the specimen with three-tiered small Nageshi, the bending strains of the columns increased due to the high rigidity of the joint (see Fig. 8). The elastic frame analysis, which simulated the experimental specimen, was conducted in order to verify how the flexural rigidities of Nageshi and column are affected by the rigidity of joint. The characteristics of the bending rigidity of the joint were calculated from the mechanical model based on the Inayama theory of compression perpendicular to the grain in wood (see Fig. 13). As the analytical results, it was proved that the stiffness was considerably reduced if the flexural rigidity of the column is low (see Fig. 16). Therefore, if the flexural rigidity of the column is insufficient, the performance of Nageshi joint cannot be maximized. On the other hand, if the flexural rigidity of the column is sufficient, the analytical result changed with the difference of the bending rigidity of the joint; the stiffness of the frame decreased with the decreasing the flexural rigidity of Nageshi.<br> Finally, an example of the reinforcement design of the actual temple was shown. Because Nageshi reinforcement method is used under floor, it has become possible to reinforce almost all of columns. As the result, the eccentricity ratio of the whole structure was improved.