This paper deals with the development of models for twist in structural timber. Twist was measured on 240 studs of Norway spruce (Picea abies). Several material parameters were also measured, such as spiral grain angle, shrinkage in all three directions, annual ring width and density. Twist in the studs was measured at four different times at different moisture contents. The amount of twist correlated well with the moisture content and was reversible throughout several moisture changes. When the moisture content decreased, the twist increased and vice versa. About 50% of the variation in twist could be explained by a single parameter, i.e. the average growth ring curvature. All studs with severe twist were cut with its centroid within a radius of 75 mm from the pith. A statistical analysis of the data shows that growth ring curvature and spiral grain angle together explained about 70% of the variation in twist. Other parameters, such as shrinkage strains, density and ring width, did not increase predictability. When using a model developed by Stevens and Johnston (1960), about 66% of the variation in twist could be explained. The model also explained twist quantitatively well. The model included curvature of the growth ring, spiral grain angle and the tangential shrinkage strain. Verwerfung von Fichtenschnittholz. Teil 2. Simulation der Verdrehung Diese Arbeit behandalt dia Entwicklung von Modellon fu Èr die Verdrehung in Schnittholz. Die Verdrehung wurde gemessen an 240 Fichtenkantho Èlzern (Picea abies). Mehrere Materialeigenschaften wurden ebenfalls gemessen, und zwar: Faserwinkel, Schwinden in drei Richtungen, Jahrringbreite und Dichte. Die Verdrehung der Kantho Èlzer wurde zu vier verschiedenen Zeitpunkten bei unterschiedlichen Feuchtegraden gemessen. Das Ausmaû der Verdrehung war gut korreliert mit der Feuchte. Mit abnehmender Feuchte stieg die Verdrehung und umgekehrt. Rund 50% der Verdrehungswerte sind durch einen einzigen Parameter erkla Èrt, na Èmlich die Jahrringkru Èmmung. Bei allen Kantho Èlzern mit starker Verdrehung lag die Mittelachse innerhalb eines Abstandes von 75 mm von der Markro Èhre. Die statistische Analyse ergab, daû Jahrringkru Èmmung und Faserwinkel zusammen ca. 70% der Variation der Verdrehung erkla Èren. Andere Parameter wie Schwindspannungen, Dichte und Jahrringbreite erho Èhten die Vorhersagbarkeit nicht. Mit Hilfe des Modells von Stevens und Johnson (1960) konnten rund 66% der Verdrehung erkla Èrt werden. Dieses Modell lieferte auch zufriedenstellende quantitative Ergebnisse. Beru Ècksichtigt werden dabei Jahrringkru Èmmung, Faserwinkel und tangentiale Schwindspannung.

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Strength and stiffness together with some properties characterizing the stand and the growth of trees were studied. Specimens (45 • 70 x 2900 mm 3) were cut from different radial and longitudinal positions, from fast-grown trees from two stands in southern Sweden. These trees had relatively large annual rings (4-6 mm) and were not representative of Norway spruce in Sweden but are an example of the intensivelymanaged stands which will probably constitute a substantial part of the raw material supply in the future.The results indicate that the mean values for strength and stiffness were lowest for the core studs and increased further away from the pith. This radial variation in strength and stiffness appears to be associated with the variation in ring width. Density alone, on the other hand, does not explain the radial variation but should be used together with either ring width or knot area ratio to explain the stiffness and strength respectively. The increase in the strength and stiffness of the core studs from the butt logs to the top logs was significant. Density alone was found to be the best variable to explain the longitudinal variation between the butt logs and the top logs. The heartwood formation in the butt log juvenile core appeared not to have a positive effect on strength and stiffness. The occurrence of compression wood, the magnitude of grain angle and the margin knot area ratio had only a minor effect on strength and stiffness.

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The overall aim of this study and series of papers is to increase the understanding of the mechanisms that govern moisture-related distortion of Norway spruce timber. In this ®rst paper the experimental study is described and the variation of wood properties presented. The study comprises 40 Norway spruce (Picea abies) trees from one fast-grown and one slow-grown stand in southern Sweden. From the trees 240 studs (45´70´2500 mm) were taken for measurement of distortion. Wood properties were measured on small specimens (13´13´200 mm) cut from the studs. Spiral grain angle was found to vary from approximately +3°( left-handed) close to pith to zero 150 mm from pith with a strong individual variation. The material from the fast-grown stand had a larger spiral grain angle compared with the slow-grown material. Spiral grain was poorly correlated to other parameters. Presence of knots had a substantial in¯uence on longitudinal shrinkage (a l ) measurements. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. It should be pointed out, however, that measuring shrinkage in small specimens containing even small knots can create a problem with regards to the obtained results, especially results of a l . It was found that presence of compression wood in several growth rings more than doubled the longitudinal shrinkage. For the radial and tangential direction the presence of compression wood decreased shrinkage with about 30%. The ratio between tangential and longitudinal shrinkage was 49 for normal wood whereas for compression wood the ratio was 13. These results con®rm the theory that the micro®bril angle governs shrinkage. Longitudinal shrinkage decreased slightly with increased distance from pith whereas radial and tangential shrinkage did not display any substantial radial variation. The fast-grown material had generally a higher longitudinal shrinkage and lower transverse shrinkage than the material from the slow-grown stand. About 50% of the variation in longitudinal shrinkage was explained by radial position, density and ring width. Density and ring width did explain 60% of the variation in radial shrinkage but only 30% of the variation in tangential shrinkage. Verwerfung von Fichtenschnittholz Teil 1. Variation der relevanten HolzeigenschaftenZiel dieser Arbeit ist ein tieferes Versta Èndnis der Mechanismen, die das Verwerfen von Fichtenschnittholz verursachen. In diesem ersten Teil wird die Variation der Holzeigenschaften vorgestellt. Vierzig Ba Èume (Picea abies) von einem schnell-und einem langsamwachsenden Standort wurden verwendet. Daraus wurden 240 Kantho Èlzer der Abmessung 45´70´2500 mm geschnitten und daran die Verwerfung bestimmt. Holzeigenschaften wurden an kleinen Proben (13´13´200 mm) gemessen, die aus diesen Kantho Èlzern hergestellt wurden. Der Faserwinkel variierte zwischen +3°in der Na Èhe der Markro Èhre bis 0°bei 150 mm Abstand vom Mark mit groûer individueller Streubreite. Die Proben vom schnellwachsenden Standort hatten einen gro Èûer...