Impacts of elevated moisture levels and subsequent drying on screwed

IRG/WP 23-40970

C M T Roder, C D Gerber, J J Morrell

Mass timber (glue-laminated timber, cross laminated timber (CLT) and laminated veneer lumber) are increasingly used in mid-to high rise structures in Australia where the climate is mainly sub-tropical with elevated moisture levels and high termite risk. Moisture intrusion poses a major challenge anywhere but becomes more critical under these conditions. Untreated/non-durable materials are considered dry-use products (<15 % MC), but moisture can accumulate during construction or through in-service leaks. While reports of moisture intrusion in mass timber are limited, prior experience suggests that failures will eventually occur. The risk of wetting creates a disconnect between dry-use expectations and reality. Moisture accumulation, especially in CLT poses a challenge owing to directional swelling and its effects on the glue lines and composite integrity, but a more important concern is the effect on connector performance. Understanding the effects of moisture intrusion on connections is critical for ensuring structures remain fit for purpose over their design life. Few studies have examined moisture intrusion effects on mass timber connections and most examined softwood CLT. There is growing interest in using hardwoods such as Eucalyptus nitens for CLT, but few reports on moisture effects. This study examined the effects of wetting on performance of wall-to-floor connections with self-tapping screws in E. nitens CLT. Wetted/dried assemblies experienced substantial internal cracking, glue line failures and splitting coupled with loss in ultimate capacity. The results highlight the importance of avoiding moisture intrusion for optimal performance. Further studies to better characterise the role of moisture distribution on performance are underway.

Keywords: CLT, E. nitens, connections, moisture, durability

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

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