Solutions to reduce moisture driven backout and improve withdrawal strength of nailplates: experimental investigations

Timber trusses, typically manufactured from timber members connected by nailplates, are widely used in the domestic housing market. Their use is however limited to indoor environments. The exposure of timber trusses to environments where the timber experiences high amounts of moisture content (MC) variations causes the nailplates to be driven out from the surface of the timber, a phenomenon commonly referred to as “backout”. As part of a collaborative project between the industry, Griffith University and Queensland Department of Agriculture and Fisheries (DAF), this paper aims at investigating solutions to both prevent backout of the nailplates and increase their withdrawal resistance under large MC variations. The nailplate teeth were redesigned following (1) a mechanical approach consisting of redesigning the tooth profile and allowing the nails to resist the withdrawal force by both friction and mechanical action and (2) the application of an adhesive to a redesigned tooth profile, allowing the adhesive to penetrate the timber with the nail. The efficiency of the new designs was experimentally assessed using single teeth (representative of nailplate teeth) with respect (1) to their ability to resist backout resulting from accelerated MC cycles and (2) their quasi-static withdrawal resistance after increasing numbers of moisture cycles. Results showed that the proposed mechanical designs reduced the backout by up to 50% when compared to currently used tooth designs. The application of an adhesive prevented moisture driven backout. The newly investigated tooth designs resulted in higher withdrawal strengths to currently used nails. It was identified that subjecting the nails to only one moisture cycle reduced the withdrawal resistance of currently used and glued teeth by up to 60% while the withdrawal resistance of the proposed mechanical designs was not affected by the number of cycles.