Degradation of shear performance of screwed joints caused by wood decay

Shear tests were conducted on screwed joints with decayed main members, and the degradation of shear performance of screwed joints by wood decay was investigated. The differences between decayed and control specimens in yield load ( P y ), and maximum load ( P max ) were both significant at the 1% level, while the difference in the initial stiffness ( K s ) was not significant at the 5% level. The extent of decay was evaluated through Pilodyn penetration depth and the decay depth. Pilodyn penetration depth had no significant relationships with K s , P y , or P max of screwed joints. It is inferred that no significant relationship was found between Pilodyn penetration depth and the strength properties because the range of the measured Pilodyn penetration depth was narrow (16–23.5 mm). The decay depth had a significant inverse correlation with K s of the decayed specimens. There was no significant relationship between the decay depth and P y , or P max . Multiple regression analysis was conducted on K s , P y , and P max using the decay depth and sound wood density (WD h ) as 2 explanatory variables. The multiple regression of K s , P y , and P max on the decay depth and WD h were significant at the 1%, 5%, and 1% level, respectively. K s , P y , and P max are inferred to be determined by both the decay depth and WD h . The strength properties of screwed joints with one screw obtained in this study were compared with those of screwed joints with seven screws obtained in a previous study. The difference between K s , P y , and P max of the screwed joints with one screw and those of the screwed joints with seven screws was not significant at the 5% level. It is inferred that shear performance of screw joints with multiple screws are sufficiently estimated from those of screw joints with one screw by simply multiplying the strength properties, even if the main members were decayed.