Effect of moisture on screw and nail connections in timber, OSB, and hybrid aluminium–timber assemblies

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Abstract

This study investigates the effects of moisture exposure on selected timber connection configurations, with a focus on withdrawal and embedment strength and stiffness of screws and nails in sawn timber and oriented strand board (OSB). A comprehensive experimental campaign was conducted on selected configurations involving screws and nails in both solid timber and OSB, under three hygrothermal regimes: constant typical room conditions, prolonged wetting, and cyclic dry-wet-freeze exposure. Configurations involving a combination of solid wood, OSB and metal plates creating tight joints with restricted movement were also investigated. Insertion torque effects were investigated separately for a specific screw-with-metal-plate withdrawal configuration in treated timber under Room and Wet regimes, including cases of over-torquing that resulted in thread stripping. The results show that increased moisture content significantly reduces both strength and stiffness of connections involving sawn wood and OSB. For the cyclic exposure protocol used in this study, sawn timber specimens recovered strength after drying, whereas OSB showed irreversible degradation. For the torque-controlled screw–plate–treated-timber configuration, over-torquing, including cases in which torque exceeded the reference safe insertion threshold, was found to significantly reduce withdrawal capacity and stiffness. One instance of brittle screw failure was also observed and SEM analysis showed intergranular fracture features, which are consistent with hydrogen embrittlement. These findings highlight the importance of adequate protection from moisture ingress and the use of safe torque limits in structural timber design, particularly in light of future climate trends and increased use of engineered wood products.

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