ESTIMATION OF STRENGTH PROPERTIES OF TENSILE BOLT JOINT FOR CLT CONSTRUCTION

 Cross Laminated Timber (CLT) panel is consisted of several layers of lumber stacked crosswise and glued together on their surfaces. In Europe, mid-to-high-rise building using CLT has been already built. Recently, CLT structure attracts attention in Japan as one of the means to promote Wooden Public Buildings. Since the lamina can utilize small or middle diameter wood as the raw materials, expansion of CLT demand would promote local forestry and wood industry. There are various joint methods of CLT such as LSB joints, screw joints with steel side-plates, and so on. In this study, we examined tensile bolt joint which is one of the effective joint methods of the end of panel when the wall panel showed rocking behavior, assuming architectural structure using the panels of around 1-2m in width. Research process of this study is as follows:<br> 1) Comparison for calculated results and test results on partial compression of CLT materials.<br> 2) Test results of tensile bolt joint with CLT.<br> 3) Proposal of design method for estimating strength properties of tensile bolt joint with CLT and the verification of its suitability.<br> In study 1), partial compressive strength of CLT panel was calculated by combining estimated results of the longitudinal and lateral compression stiffness of lumber (Eq. 1). These estimated results were compared with experimental values of the paper (ref. 12), and estimated results were calculated slightly lower.<br> In study 2), cyclic loading tests were conducted tensile test on tensile bolt joint with CLT. The parameters of the experiment are as follows:<br> 1) Vertical end distance between lower end of the CLT and rectangular through-hole (l_s), horizontal edge distance between side edge of the CLT and rectangular through-hole (l_n), strong-axis direction or weak-axis direction and positioning of edge-glued lamina (Table 2, Fig. 7)<br> 2) Thickness of the CLT and number of the bolt (Table 3, Fig. 23)<br> In consequence, it was found that the above all parameters of experiment had an influence on the strength performance, particularly, the end distance (l_s) and the edge distance (l_n) were important elements for affecting the change of the fracture mode.<br> In study 3), the estimating results calculated with proposed design method were compared with the experimental results. It was quoted that the stiffness of joint is obtained by sum of the compression of CLT, tension of CLT and tension of bolt, those stiffness in series (Fig. 30, Eq. 1～Eq. 4). The yield strength of joint assumed it compressive strength of parallel layers (Fig. 31, Eq. 5). Fig. 32 shows the three kinds of fracture mode, and the maximum strength of joint was assumed to the minimum of those maximum strength (Fig. 32, Eq. 6～10). As a result, maximum tensile strength, fracture mode, yield strength and stiffness were close to these estimations. However, estimated stiffness of test specimen ”P77” was higher than experimental result. It was estimated that the reduction rate of the stiffness per unit area accompanying increase in the compressive area of the parallel layers was higher than expected.