Reinforcement for compression perpendicular to grain in column-base and -beam joints using partially threaded and fully threaded screws

Abstract

Reinforcement using self-tapping screws is an effective strategy for augmenting the bearing capacity and stiffness of timber beams subjected to compression perpendicular to the grains. This paper presents an experimental study on reinforcement using self-tapping screws for compression perpendicular to the grain. The bearing capacities and initial stiffness depended on various factors: wood species, screw information (diameter, arrangement, and thread configuration), and load conditions. For specimens with fully threaded screws in column-base joints, the experimental outcomes diverged from the predictions when a previously published calculation method was used for two primary reasons: (i) the predicted capacity for timber failure at the screw tips was less than the experimental values, and (ii) the capacity of single screws for buckling failure was underestimated. Therefore, we propose an enhanced equation to refine the calculation method ($R^2=0.83$). It was observed that partially threaded screws provided a reinforcement effect, albeit inferior to that of fully threaded screws. This was owing to a deficiency in the pushing-in capacity of single screws. Additionally, the reinforcement effect in the column-beam joints was less than in the column-base joints. This is attributed to a gap in the strain distribution. Thus, in this study, we developed a new equation for partially threaded screws and column-beam joints.