Effect of Moisture Cycles and Timber Treatment on the Performance of Screwed Connections

IRG/WP 23-20694

L Yermán, M Xiao, Z Yan, L-M Ottenhaus

The impact of moisture in timber structures is associated with the deterioration of timber, and loss of mechanical performance. Methods have been developed to treat timber using chemicals to inhibit deterioration, however, water intrusion has much more immediate effects on the mechanical properties of timber associated with swelling and shrinkage of the wood. The effects of moisture on the mechanical performance of timber have been widely studied. Nevertheless, timber must be connected to create a structure and these connections are usually the most critical part of a structure. They are designed to fail in ductile mode as it allows enough time to identify further failures and prevent dangerous consequences for the occupants. Despite the critical importance of timber connections, the effects of moisture on their mechanical performance remain poorly understood. This work aims to assess the effect of repeated moisture cycles on the performance of laterally loaded single-shear nailed joints using LOSP-treated and untreated Radiata pine. The mechanical performance of the connections was evaluated through monotonic shear tests from which the maximum strength and initial stiffness were determined. A comparison of the experimental results with those predicted from the Australian and European standards is also presented. Results showed the mechanical performance of the connections was not affected by up to 8 moisture cycles. Specimens built with untreated wood showed a significant loss in capacity after 12 moisture cycles, whereas the specimens built with LOSP-treated wood showed a significant loss after 16 moisture cycles. This could be to the presence of residual solvent in the treated wood that prevents moisture ingression around the fasteners. The maximum strength after 16 moisture cycles was statistically the same for specimens built with both treated and untreated wood, which probably means the residual solvent was evaporated or leached. A comparison of the maximum strength with that from the Australian standard showed the current standard was unable to safely predict the loss of performance of the connection caused by moisture. The European standard utilises more system characteristics for the predictions but was not designed to be applied to Australian timber species, failing to produce safe estimations.

Keywords: timber connections, preservative treatment, moisture content, screws, timber standards

Conference: 23-05-28/06-01 Cairns, Australia

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