Caratterizzazione di pannelli x-lam in pino marittimo sardo
Background and Purpose: The aim of this work is to present the idea of a short procurement chain of timber as a means to provide an increased value to Sardinian forests. It is based on the evidence that timber buildings are increasingly useful for a number of reasons including sustainability, the speed of erection, and excellent structural performance. However, most of the timber currently used in Sardinia is imported from outside this area. The idea is to use the best part of locally-grown trees to produce timber boards, while all the remaining part of the tree including the production waste is used as biomass for energy production. Important issues to address are the generally low mechanical properties of timber from locally-grown Sardinia trees such as Maritime Pine, which would make some wood-based products like glue-laminated timber not technically viable. Cross-laminated timber panels seems to be a possible solution to this problem because this wood-based product is manufactured in such a way that even with low-quality timber boards it is possible to obtain a medium quality panel. The panel is made of layers of timber boards with the adjacent layers glued under pressure at a right angle. Another issue is the need to grade the local timber, for which a number of specimens must be tested on destruction in order to identify a visual or a machine stress grading procedure. Last but not least, the panels must be tested on destruction to correlate their mechanical properties to the properties of the boards. Materials and Method: the research has been developed through the following steps: 1) two maritime pine plantations with stands suitable for logging and processing were identified, extensively surveyed and sampled. On selected standing trees, based on measurements taken at different heights, the first preliminary grading was applied by sorting for structural and energy use. 2) Trees were harvested by a local company and the logs were finally assorted based on their size and their external defects. 3) The logs were then transported to the local sawmill, where different boards size required to build the grading rules and to produce the CLT panels were cut. Each board was then subjected to a non-destructive measurement of the Modulus of Elasticity using acoustic tool for measuring stress wave velocity (Viscan-Microtec) 4). After kiln drying, the required boards (approximately 840) were subjected to non-destructive measurements of their physical properties (density, humidity, defects etc.) using the machine purposely developed by Microtec. The aim was the calibration of this machine in order to enable the machine strength grading of Sardinia maritime pine. 5) The required boards were visually characterized and then tested to destruction in order to measure their strength and correlate this values to the presence of defects such as knot diameters and positions, grain deviations, etc. 6) Based on the results of phases 4 and 5, the visual and machine based grading rule for Sardinia Maritime Pine have been developed. 7) By applying the newly developed grading rules, some boards 5 have been selected among the available ones and used for the production of some prototypes of CLT panels. 8) In order to determine the structural performance, 68 panel have been tested to destruction. Testing was carried out in accordance with EN 408 on specimens with a span to depth ratio equal to 18 to determine the bending strength and stiffness, and on specimens with span to depth ratio equal to 9 to determine the shear strength. A number of different methods exist for the analysis and design of CLT elements, including the Shear Analogy Method and the Mechanically Jointed Beams Theory (Gamma Method). These methods have been considered in this study and a relative comparison have been presented in order to determine which method is most suitable when considering CLT formed using Sardinian grown timber.. Results: It was found that Maritime Pine as structural material is limited by stiffness rather than strength or density. The effective bending stiffness of CLT is a measure of the material stiffness in relation to the cross sectional build-up of the panel. To be competitive on the market, a Sardinian CLT product will have to compete with imported CLT panels, which are usually made from C24 graded material (spruce). In most cases this is simply because the C24 material is widely available on the market rather than a specification from the designer. The performance of panels made of Maritime Pine boards has been compared directly with that of imported products, demonstrating that an increase in the Sardinian panel depth of just 15% is sufficient to match the stiffness of the imported panels, which is the most important design property. Conclusions: This work lays the foundation for the development of a short procurement chain of wood in Sardinia. The EDENSO project developed in parallel to this doctorate study is still in progress and further tests on maritime pine CLT panels are planned. A short procurement chain of timber is a possible means to create job opportunities and reduce depopulation, particularly important in some area of the island. By adding value to the forests by means of timber production used in prefabricated components employed in low-rise timber buildings, it is also possible to improve forest management and even extend forested areas, which have many positive effects on the environment, the landscape and the reduction of hydrogeological hazard.
biomass, classificazione biomassa, cross-laminated panels, forestry, pannelli x-lam, short procurement chain, timber grading, wood, Settore ICAR/09 - Tecnica delle Costruzioni
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