Performance of axially-loaded self-tapping screws in hardwood: Properties and design

The stock of deciduous tree species is raising. Some of them feature outstanding high mechanical properties and are thus predestined for load bearing applications in timber engineering. Although the pressure to use these species is growing, the timber industry and engineering sector still stays observant. For some products made of deciduous timber costs are reported, which prevent their competitiveness against products made of coniferous timber and other building products. One way to improve the competitiveness and use of products made of deciduous timber is the development of innovative and specific single fastener and joint solutions with the aim to maintain at its best the high load bearing capacity of these products also at the joints. In this paper the outcomes from a comprehensive testing campaign conducted in frame of the long-term research project hardwood_SCREWS and for two types of self-tapping screws exposed to axial loads are reported. One screw is of commercial type and used as benchmark. The other screw is an innovative alternative and especially developed for applications in deciduous timber. Three groups with specific parameter settings and aims were tested to analyse the following influences on insertion and withdrawal properties: the wood anatomy, the thread-grain angle and the predrilled insertion. The degree in relationships between investigated physical/mechanical properties is quantified by power coefficients. Compared to coniferous timber, these power coefficients outline a disproportional capacity of screws inserted in deciduous timber. Following this, the paper concludes with a generic model approach for calculating the withdrawal strength of axially-loaded self-tapping screws inserted in structural timber or glued laminated timber products made of either coniferous or deciduous timber.