Time dependent moisture driven backout of nailplates: experimental investigations and numerical predictions

Abstract

Nailplates are widely used in domestic and low-rise residential housing markets and have begun forming part of mid-rise developments. There is however a concern about nailplates backing out which has been observed in a variety of conditions. In-service backouts in excess of 2 mm have been recorded and can result in up to a 50 % reduction in joint performance. The amount of backout varies significantly from roof space to roof space and the physical phenomena leading to backout and their exact relationship to the climatic conditions are largely unknown. This paper experimentally investigates the time dependent backout mechanisms and develops a numerical model to predict the range of backouts that will be expected from a set of climatic conditions that the nailplates would be exposed to. Specifically, this paper initially investigates the backout of single nails, representative of nailplate teeth, by pressing the nails into pieces of timber and subjecting the samples to cyclic climatic conditions. The moisture driven backout of the nails was monitored in real-time using Digital Image Correlation for 48 days. Backouts of between 0.08 and 0.26 mm were observed. Results showed that during the drying phase, the timber surface tended to slide along the surface of the nail. A similar phenomenon was observed during the wetting phase but to a lower extent. This consequently led to a ratcheting mechanism that caused the nail to backout from the timber. A numerical model is then proposed to predict and replicate the time dependent backout behaviour of the nailplate tooth based on heat and mass transfer algorithm TransPore and Finite Element Software ABAQUS. The model was validated against the experimental results based on the timber properties and climatic conditions that the timber was exposed to. The model was able to accurately predict the time-dependent backout of the nails. To illustrate the application of the model, the expected nailplate backout for two different Australian roof spaces was investigated. A variety of parameters was investigated including the length and position of the tooth. It was found that longer teeth experienced higher rates of backout and significant variations in backout were encountered whether the tooth is closer to the edge or the middle of the timber.