Round small-diameter timber for construction: Final report of project FAIR CT 95-0091




Ranta-Maunus, Alpo


The use of small-diameter timber in construction has been investigated. The aim of the work is to increase the use of the wood harvested in forest thinning in construction applications. The work has covered a wide range of aspects, from availability of the material to design of the structures. This publication summarizes the results in following areas: availability, dimensions and quality of conifers harvested in forest thinning, cost of harvesting and woodworking, comparison of drying methods: seasoning, warm-temperature and high-temperature kiln-drying, improving durability, strength of round small-diameter conifers, potential types of structures to be built from round timber and new mechanical joints. The tree species that were included in the study are Scots pine, Norway spruce, Sitka spruce, Larch and Douglas fir. The main reasons why round timber is rarely used in construction can be summarized as: the material is not available via the normal commercial routes, the roundness requires special methods and systems that are not known by architects and carpenters, the strength values of timber connections are not available for engineers, the lack of standards and models. This research aims to produce information needed in the use of small roundwood in load-bearing structures in order to remove the obstacles mentioned above. Results concerning availability of construction-quality round timber in the first commercial thinning reveal that the resource itself is vast: millions of cubic meters in Finland alone. The yield per hectare is, however, limited and dependent on the dimensions required. When the diameter of the final product is adequate at less than 100 mm, the first commercial thinning is also economic for the harvesting of construction timber. The economics of manual and mechanical harvesting have been compared. When larger dimensions are needed, the second thinning is more likely to produce the required material. The cost of producting round timber is primarily dependent on the surface quality needed: timber peeled cylindrical is twice as expensive as material that is only debarked. Both of these have their own market. The cost of construction is dependent on labour costs, which at the moment is higher for round timber than for sawn timber because conventional systems are not suited for the use of round timber. Additional costs may arise from the deviations of cylindrical form. Drying is a critical phase of production, which determines how much checking is observed. In this respect, high-temperature drying gives much better quality than normal, commercial warm-temperature kiln-drying or natural seasoning. Accordingly, the drying method should be chosen based on the surface requirements. End-cracking also affects the capacity of joints. The strength of small-diameter timber was observed to be higher than expected. Characteristic values are presented as well as a proposal for visual strength-grading. A method for non-destructive mechanical strength-grading based on X-ray is also proposed. A statistical analysis is presented which indicates the dependence of strength and stiffness on different factors such as density, knots, moisture content, diameter and age. New mechanical connections have been designed and tested. For engineered structures, a round form enables the use of steel lacing around the wood, which considerably increases the load capacity of the joints. The largest quantities of round timber are used, and can be used, in non-structural applications and in small, traditional-type buildings. Smaller in volume but important for the image of roundwood is its application in the architecture of medium-sized leisure industry buildings in which the load-bearing structure is visible. As part of the project, designs for a footbridge and a watchtower have been made.
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