Mechanical and artificial improvement of nailplate connected timber truss joints

Abstract

Timber trusses are widely used in the housing market, which members are often connected by nailplates. However, the use of this type of connection is restricted to environments where the moisture content variation is somewhat limited (such as the building envelope). This is due to the nailplates being driven out from the timber as a result of the mechano sorptive withdrawal effect, commonly referred to as “backout”. Recently, this backout was observed in more controlled environments thus prompting a linked project between industry and Griffith University to investigate solutions to increase the tooth withdrawal resistance under common design loading conditions and cyclic moisture contents. Two different design approaches are taken: (i) by mechanical means through a redesigned tooth profile allowing the nails to resist the pull out force by both friction and mechanical action and (ii) through application of an adhesive to two different tooth profiles. The new designs were experimentally assessed using single nails with respect to their quasi-static withdrawal resistance and their ability to resist the backout through accelerated moisture cycling. Results showed that the proposed mechanical designs have comparable performance with currently used nails, while the application of adhesive significantly decreased the backout.