ESTIMATION OF LOAD–DEFORMATION CHARACTERISTICS OF NAIL/WOOD SCREW PLYWOOD-TIMBER JOINTS AND PLYWOOD-SHEATHED SHEAR WALL CONSIDERING THE LOW CYCLE FATIGUE CHARACTERISTICS OF FASTENERS

Abstract

Plywood-sheathed shear walls are widely used in timber structures as resistance elements against horizontal loads such as seismic and wind forces. When an in-plane shear force acts on a plywood-sheathed shear wall, the nails/wood screw joints between the plywood and post-and-beam resist the shear force, thereby securing the load bearing capacity and toughness required for the shear wall. However, the toughness cannot be sufficiently secured in some cases, for example, positive and negative cyclic loads such as the seismic force. In such a case, cyclic bending deformation occurs in the fastener and fatigue failure occurs because of low cycle fatigue. Because this phenomenon causes vulnerability to the mechanical behavior of the joints or the shear wall, the low cycle fatigue characteristics of the fastener in the load-bearing performance evaluation should be considered. Therefore, in this study, the load–deformation characteristics of the shear wall and its joints were estimated considering the low cycle fatigue characteristics.

The performance of the joints was estimated by modeling the load–deformation characteristics of the joints and multiplying the reduction coefficient considering the low cycle fatigue characteristics. The performance of the shear wall was estimated by the finite element analysis using this model. The results of the constant-amplitude cyclic bending test of the fastener were used for the low cycle fatigue characteristics of the fastener.

The single shear test of the joints and static lateral loading test of the shear wall were performed using different loading protocols (such as those according to ISO and Japanese Industrial Standards) for comparison with the estimated results. The fastener used in the test entailed 4.1×38 mm and 4.5×50 mm wood screws and CN50 nails. In the joints test, Japanese cedar lumber was used as the main material and coniferous plywood as the side material. In the shear wall test, Japanese cedar lumber, Douglas fir lumber, and coniferous plywood were used for the columns and base, beams, and face material, respectively. Additionally, the nails and wood screws were fastened at intervals of 150 mm for the face material.

The load–deformation relationship could be roughly estimated in the joint estimation. In the estimation of the shear wall, when the fracture properties in the experiment such as punching out or pulling out were different from the fracture of the fastener, the estimated toughness was sometimes lower than that of the experimental results. The properties that often indicate the breakage of the fastener could be roughly estimated.