Effect of moisture and rate of loading in the withdrawal capacity of screws in Cross Laminated Timber (CLT)

Cross Laminated Timber structures are often exposed to undesirable moisture conditions, typically during construction and from water leakage during their service life. Due to timber’s hygroscopic nature, the moisture impact is more critical than in other common building materials and can lead to local failures, especially at the connectivity level, which is considered a critical point. In this study, the effects of moisture and displacement rate were studied in the axial withdrawal capacity of self-tapping screws in Cross Laminated Timber. The effect of displacement rate was also considered in the tensile mechanical properties of screws. Two types of screws, three distinctive moisture contents, and displacement rates were investigated. Specimens with one and two screws were also tested. A worst-case scenario of water leakage and poor waterproofing was considered as the upper boundary of moisture content. The experimental findings showed an apparent decrease in both the axial withdrawal strength and stiffness at high moisture contents above the saturation point and an increase in the axial withdrawal capacity at high displacement rates. High displacement rates decreased the ductility and increased the yield load capacity of screws in tension. The effective number of screws proposed in EC5 agreed well with the experimental results for two screws. However, a lower number was observed at high displacement rates attributed to non-uniform load distribution. Modification factors were proposed for the withdrawal stiffness and strength considering both moisture and displacement rate alone and combined effects and agreed well with the literature but not with EC5. The experimental results were compared with design equations for the axial withdrawal stiffness and strength, as found in the literature. The equations proposed in the draft version of the new EC5 agreed well with most findings.