Experimental study on shear performance of nail and screw-laminated timber-steel composite and timber-timber systems using low-grade timber and mechanical fasteners

Constraints on high-quality timber resources have created an imperative for the timber industry to improve the utilisation of their huge existing low-grade timber resources. One of the most promising and innovative methods to address this need is combining low-grade timber and sheet steel using advanced manufacturing technologies to develop cost-efficient and sustainable composite sections that benefit from both materials' structural advantages. Besides conducting standard tests on materials and fasteners used in this study, two different test setups were developed and 90 specimens with 18 different configurations were designed, fabricated, and tested to evaluate the structural performance of this novel adhesive-free Timber-Steel Composite (TSC) system and compare it with traditional Timber-Timber (TT) configurations. These systems were manufactured by combining low-grade timber and sheet steel utilising self-drilling screws and D-head shape nails as new types of materials and fasteners in this kind of application. By conducting standard pull-out tests, the influences of key parameters such as steel plate thickness, and fasteners' type, size, pattern and spacing on the shear performance of these systems were investigated. After recording and classifying different failure modes of fasteners, the test results were analysed in detail and key mechanical properties and slip parameters of all tested specimens were calculated. Moreover, obtained experimental results for different configurations were compared against both the theoretical method and analytical prediction models. Test observations demonstrated the failure modes of specimens relied entirely on the type of the system as well as the fasteners' type that they were built from. Comparing the test results showed that using steel plate significantly improved the shear capacity and stiffness of the TSC sections compared to traditional TT sections. The application of self-drilling screws created a much stronger connection with higher shear capacity and clamp load, while D-head nails performed much better in terms of ductility. Increasing the steel plate thickness and fasteners' diameter improved the load-carrying capacity of TSC sections. However, doubling the fasteners along the grain spacing adversely affected the shear capacity and stiffness of the TSC sections. Comparing the test results of the TSC specimens with various fasteners' patterns and numbers demonstrated adding more fasteners while providing enough spacing between them linearly increased the shear capacity of the TSC sections. Comparison between test results and the theoretical approach suggested by Eurocode 5 showed that this method could reasonably estimate the load-carrying capacity of TSC sections. It was also found that by increasing the fastener diameter, the predicted theoretical values got closer to the experimental results. Moreover, comparing the experimental results with the analytical model presented by Foschi demonstrated that this model could closely simulate the nonlinear load-slip behaviour of both systems up to failure with sufficient accuracy.