Experimental Study on Seismic Strengthening of Timber Beam-Column Joints by Lateral Confinement

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The beam column joints of a timber structural frame are subjected to axial force, shear force and flexural moment during earthquakes. In this study, two methods have been proposed to strengthen the bolt joints subjected to axial tension force. One method is to laterally confine the joints using a square hoop made of GFRP (glass fiber reinforcedplastics) and the other is to restrain the splitting of the joints using a transverse bolt with two large square washers. The effectiveness of those methods have been investigated by conducting axial tension tests on the joints.For the tests, 91 cedar columns with 105mm square section were prepared with the bolt connectors, which was called super-joints as the trade name. As the experimental variables, the thickness and width of the GFRP square hoops were varied from 5 to 7mm and 20 to 120 mm, respectively. Also, the sizes of the square washers for the transverse bolts were variedas 20x20x3mm^3, 40x40x5mm^3, 60x60x8mm^3. The test results indicated that those GFRP square hoops and the transverse bolts could improve the deformation capability of the joints. In case of the GFRP hoops, not only deformability but also strength of the joints were significantly improved when the width of the hoops were more than 80mm. However, the effectiveness of the transverse bolts were not so appreciable from the view point of a practical use because they became effective only after large deformation of the joints.As additional tests, 8 of three-story timber frames with full scale were constructed and tested under horizontal static loads which were applied to each floor level simulating seismic forces. The behaviors of the frames were predicted by frame analyses with the joint models which were established based on the above axial tension tests, The predicted behaviors were well agreed with the observation of the frame tests.