Optimising laminated high-mass timber components assembled from a fibre-grown resource for building applications

In recent years, there has been a growing interest by the timber industry in Australia and worldwide in developing higher-value structural mass laminated timber products from fast-growing, fibre-managed plantation eucalypt resources (i.e., pulpwood eucalypt). The plantation eucalypt resources in Australia are predominantly managed to produce woodchips, which is considered as a low-value product. The timber from such plantation resources is of low-structural grade and contains a substantial number of strength-reducing features such as knots that can complicate its wider applications for structural purposes as individual boards. This doctoral thesis aimed to develop and optimise higher-value structural nail-laminated timber (NLT) floor panels from fibre-managed plantation eucalypt timber. The target specie was Eucalyptus nitens (E. nitens), which is the main plantation hardwood specie in Tasmania. In parts of the experimental investigations of this thesis, however, tests have been done on plantation Eucalyptus globulus (E. globulus) as well to create a basis for comparison with E. nitens‚ÄövÑvÆE. globulus is the main plantation hardwood specie in Australia. The experiments in this thesis were performed within three phases. The general aims and main findings of each phase are described as follow: ‚Äö In the first phase, the effectiveness of conventional timber processing regimes (i.e., harvesting, sawing, drying, and visual stress-grading) on fibre-managed plantation eucalypt and the physical and mechanical characteristics of the recovered sawn boards from this resource were determined. The aim of this phase was to investigate if the plantation eucalypt can be properly harvested, sawn, dried, and graded to produce sawn boards suitable for NLT production. Another aim of this phase was to create a profile of material characteristics for the plantation eucalypt sawn timber that can assist the development of NLT floor panels by better understanding the resource. Based on the results obtained in this phase, the conventional timber processing regimes used resulted in nominal sawn timber recovery rates of 25.8% for E. nitens and 31.8% for E. globulus. Considerable amounts of mechanical damages from debarker and sawing machines were detected in the recovered timber boards. Unrecoverable collapse, distortions, and surface checking were the main drying defects in the recovered boards. The visual stress-grading method showed no reliable accuracy in structural grading of plantation E. nitens timber. Alternative grading systems need to be developed for the structural use of this resource. A profile of visual, physical, and mechanical properties was created for the plantation eucalypt resources in this phase. ‚Äö In the second phase, NLT floor panels, with and without concrete topping, were constructed using different cross-sectional configurations and span lengths. Conventional methods were used in manufacturing these panels. The panels were subjected to both vibration and short-term four-point bending tests. The aim of this phase was to determine the short-term structural capacity of the NLT and NLT-concrete floor panels made of randomly selected boards and to identify the best configuration for NLT production from the plantation eucalypt resources. The second aim of this phase was to identify any issues associated with the development of such products that can be addressed in the third phase of this doctoral research. Based on the results, all the test panels successfully passed the serviceability limit state (deflection and vibration) and strength limit state (bending moment capacity) requirements for structural floors in residential and office buildings. The best configuration of the panels was identified. A relatively high variation was observed in the bending characteristics of the panels. The conventional screw-type shear connectors used in producing the NLT-concrete panels resulted in a low composite action. ‚Äö In the third phase, based on the results obtained from the previous phases, innovative approaches were developed for the effective use of sawn fibre-managed plantation E. nitens in NLT and NLT-concrete production. The configuration and assembly methods of the NLT and NLT-concrete elements were optimised by developing appropriate timber-concrete connections and a new lamination system. The NLT panels (without concrete topping) produced using the new lamination system were subjected to short-term four-point bending test and long-term serviceability bending loads for 90 days to determine the bending creep deflection of the panels. The proposed lamination system reduced the variation in the MOE of the NLT panels. The results suggest that this lamination system can both customise the MOE of the panels and effectively maximise the use of lower-grade boards in NLT production. Overall, the fibre-managed plantation E. nitens timber demonstrated appropriate short-term and long-term structural performances to be used in the construction of NLT floor panels for both residential and office buildings. Producing structural products from fibre-managed plantation eucalypt can create new markets for this resource and ensure a reliable supply of timber for the building construction sector in Australia. The use of such plantation resources will also be associated with some difficulties. These difficulties were discovered and are discussed in this thesis and relevant suggestions to address these difficulties are made through an agenda for future research.