The mechanisms of moisture driven backout of nailplate connections. Solutions for outdoor environments and numerical modelling and predictions of moisture driven backout given climatic data

Abstract

Nailplated timber trusses, manufactured from timber members connected by nailplates, are widely used in the domestic and international housing market as part of the roofing and flooring systems. The extensive use of these structural elements is driven primarily by their cost efficiency, resulting from an efficient manufacturing process, and their structural efficiency. The use of such trusses however is typically limited to protected (or indoor) environments due to a phenomenon called “nailplate backout”. Nailplate backout is where the steel nailplate, used to connect two timber members together, separates from the parent timber. It is primarily caused by the repeated shrinking and swelling of the timber in response to changing environmental conditions. This moisture driven backout presents a significant hurdle to the expansion of the nailplated timber truss market for external use (and potentially application in the emerging mid-rise timber building market where the internal climatic conditions are relatively unknown). As part as a collaborative project between the industry, Griffith University and Queensland Department of Agriculture and Fisheries (DAF), this thesis aims at investigating solutions to both prevent backout of the nailplates and increase the performance of trussed joints when exposed to large moisture content variations and to develop the understanding of moisture driven nailplate backout through experimental and numerical modelling. Initially an investigation into redesigning the nailplate tooth to reduce the moisture driven backout was conducted. The proposed tooth redesign considered (i) two mechanical approaches consisting of redesigning the tooth profile and (ii) the application of an adhesive to a redesigned tooth profile. The effectiveness of the new designs was assessed using single nails, representative of a single nailplate tooth, with respect to their ability to resist moisture driven backout and their quasi-static withdrawal resistance after an increasing number of moisture cycles. Results indicated that the mechanical and adhesive approaches could effectively reduce the backout and obtain a higher withdrawal strength than currently used profiles. The re-designed tooth profiles were then adapted and implemented into a full nailplate to investigate if the results from single tooth would translate to a nailplate joint, particularly after the joints were subjected to severe accelerated moisture cycles. One mechanical-based and one adhesive-based nailplate design were considered and compared to currently commercially available nailplates. The backout of the nailplate was recorded at discrete intervals and the tensile capacity of the joint was also investigated. Findings included a reduction in the rate of backout and a statistically significant increase in tensile capacity in most cases for both proposed designs. Finally, this thesis proposes an analytical model to predict moisture driven backout as a function of the timber properties, tooth profile and climatic conditions that the joint will be exposed to. The model was validated against experimental data where the backout of a single tooth was pressed into a timber piece and monitored in real time using digital image correlation. The application of the model is then demonstrated by predicting the expected range of nailplate backout in two roof spaces.