Strength and modulus of elasticity of pine structural round timber

Transcription

1 Annals of Warsaw University of Life Sciences - SGGW Forestry and Wood Technology 92, 2015: (Ann. WULS - SGGW, For. and Wood Technol. 92, 2015) Strength and modulus of elasticity of pine structural round timber ANDRZEJ NOSKOWIAK, GRZEGORZ PAJCHROWSKI, GRZEGORZ SZUMIŃSKI, LECHOSŁAW JABŁOŃSKI Wood Technology Institute, Winiarska 1, Poznań, Poland Abstract: Strength and modulus of elasticity of pine structural round timber. The article presents the results of the basic physical and mechanical properties of pine (Pinus sylvestris L.) structural round timber originated from Lesser Poland natural-forest region. Based on the tests and determination of characteristic values according to the European Standards for structural timber with rectangular cross-section, it was found that tested population could be assigned to a strength class C30. Determined bending strength dependence on modulus of elasticity is characterised by coefficient of determination at similar level as for timber with rectangular cross-section. Keywords: structural round timber, physical and mechanical properties, bending strength, modulus of elasticity INTRODUCTION Modern architectural solutions of wood and wood-based materials are based partly on the centuries-old tradition, and mainly on a comprehensive, prepared according to strict rules, strength characteristics of materials. Wood as a building material is characterised by great diversity. Strength characteristics of wooden structural components are a function of many variables such as wood species, habitat, age of the tree, moisture content, density, specific arrangement of anatomical features, dimensions of the cross-sections. In construction wood is most often used in the form of sawn timber, much less as round or semi-round timber. Round timber is used as columns supporting floors and roofs or for log walls. Relatively large quantities of round timber is still used for the construction of overhead power and telecommunication lines [1-2]. Performing the tests of mechanical properties of beams of pine round timber with diameter of not less than 150 mm as commonly used in the construction of overhead power lines was considered justified in the context of continuous need for broadening of knowledge on the rational and technically correct use of wood in modern structural applications. Interest in the use of structural round timber is shown in research of various centres [3-6]. The construction of the pavilion of the largest German manufacturer of wooden prefabricated houses is an example of interesting practical use of round timber (Fig. 1). Figure 1. Pavilion of WEBERHAUS company 300

2 MATERIALS AND METHODS The tests of physical and mechanical properties were performed on 40 beams of pine (Pinus sylvestris L.) structural round timber with a nominal length of 4 m and diameter greater than 150 mm. Timber was originated from Lesser Poland natural-forest region. Before destructive tests dimensions, mass and moisture content by electrical resistance method were determined for each beam. Bending strength and modulus of elasticity (global and local) were tested according to the standard EN 408:2010+A1:2012 in four-point scheme (1000 mm 1000 mm 1000 mm). The beams were placed in the machine in such a way that the so-called worst cross-section was in the pure bending zone. Measurements of deflection for the local modulus of elasticity were made on the distance of 800 mm. (Fig. 2 4) Figure 2. Four-point bending of the beams Figure 3. Deflection measurement for determination of the local modulus of elasticity 301

3 Figure 4. Exemplary destruction of the beam After the above tests appropriate samples of the entire cross-section were cut out near the place of destruction in order to determine the following properties: moisture content by ovendry method, density, width of annual growth rings, share of heartwood and sapwood, total knot area ratio (tkar). RESULTS On the basis of the measurements the values of selected physical and mechanical properties of tested beams were determined. The results of these calculations are summarised in Table 1. Table 1. Statistical parameters of selected physical and mechanical properties of tested round timber (* values adjusted to moisture content of 12%, X mean value, SD standard deviation, COV coefficient of variance) Statistical parameter Bending strength [N/mm 2 ] Modulus of elasticity* [N/mm 2 ] local global Density* [kg/m 3 ] tkar Average width of annual growth ring [mm] Share of heartwood [%] X min max SD COV [%] Next, characteristic values of bending strength, modulus of elasticity and density were calculated in accordance with the requirements of EN 384:2010. These three values are the basis for strength class determination according to EN 338:2009. The results are presented in Table

4 Table 2. Characteristic values for strength class determination according to EN 338:2009 Number of beams 40 Value of the 5 th percentile of bending strength f 05 [N/mm 2 ] 39.4 Value of k s 0.77 Characteristic value of bending strength f m,k = f 05 k s [N/mm 2 ] 30.4 Mean value of local modulus of elasticity adjusted to moisture content of 12% E 0,mean [kn/mm 2 ] 13.2 Mean value of density adjusted to moisture content of 12% mean [kg/m 3 ] 474 Characteristic value of density ρ k [kg/m 3 ] 426 Strength class according to EN 338:2009 Bending strength dependence on various properties and characteristics of wood is essential for structural timber. Understanding these relationships is a key issue for machine strength grading. Figures 5-10 present bending strength dependence on several parameters. There are given values of coefficient of determination R 2 of the linear regression for each relationship in the Figures. C30 Figure 5. Relationship between bending strength and local modulus of elasticity Figure 6. Relationship between bending strength and global modulus of elasticity 303

5 Figure 7. Relationship between bending strength and tkar Figure 8. Relationship between bending strength and density of small samples Figure 9. Relationship between bending strength and average width of annual growth ring 304

6 Figure 10. Relationship between bending strength and share of heartwood Coefficients of determination were also calculated for bending strength dependence on two parameters at the same time by using multiple linear regression. In case of bending strength dependence on modulus of elasticity (local or global) and tkar, R 2 value was 0.65 (z = x 37.7y for local and z = x 57.5y + 22 for global), while for bending strength depending on density of small samples and tkar, R 2 value was 0.53 (z = 0.126x 76.6y ). The values of R 2 obtained for all relationships are summarised in Table 3. Table 3. Coefficient of determination R 2 for selected relationships of bending strength Parameter R 2 Local modulus of elasticity 0.61 Global modulus of elasticity 0.56 tkar 0.32 Density 0.36 Width of annual growth ring Share of heartwood Local modulus of elasticity + tkar 0.65 Global modulus of elasticity + tkar 0.65 Density + tkar 0.53 CONCLUSIONS 1. Following the rules of the system of European Standards for structural timber with rectangular cross-section, the population of pine round timber could be assigned to the strength class C Correlation for bending strength dependence on modulus of elasticity is at similar level as for the timber with rectangular cross-section. 3. Relatively strong correlation between bending strength and tkar indicates the possibility of using this parameter in determining the rules of visual grading for structural round timber. 4. Application of multiple regression allows to increase the value of R 2 coefficient, in particular for bending strength dependence on density and tkar, justifying strength class determination on the basis of more than one parameter. 5. Test results enable the design of timber structures based on specific characteristic values of pine round timber. 305

7 REFERENCES 1. NOSKOWIAK A., 2013: Badania typu zgodnie z normą EN słupów drewnianych do linii napowietrznych, maszyn. ITD Poznań 2. CERDA G., Wolfe R.W., 2003: Bending strength of Chilean radiata pine poles, Forest Products Journal 53(4); MORGADO T. F. M., SAPORITI MACHADO J., DIAS A. M. P. G., CRUZ H., RODRIGES J. N. A., 2010: Grading and testing of maritime pine roundwood, Proceedings of World Conference on Timber Engineering, Riva del Garda 4. MALO K. A., ELLINGSBØ P., 2010: Roof structure in round timber, Proceedings of World Conference on Timber Engineering, Riva del Garda 5. FALLER R. K., KRETSCHMANN D. E., REID J. D., HASCALL J. A., SICKING D. L., 2010: Midwest guardrail system with round timber posts, Proceedings of World Conference on Timber Engineering, Riva del Garda 6. RANTA-MAUNUS A., 1999: Round small-diameter timber for construction, Final report project FAIR CT , Technical Research Centre of Finland, VTT Publications 383, Espoo Streszczenie: Wytrzymałość i moduł sprężystości sosnowego drewna konstrukcyjnego okrągłego. W artykule przedstawiono wyniki badań podstawowych właściwości fizycznych i mechanicznych sosnowego (Pinus sylvestris L.) drewna konstrukcyjnego o przekroju okrągłym pozyskanego w Małopolskiej Krainie Przyrodniczo-Leśnej. Po przebadaniu i wyznaczeniu wartości charakterystycznych zgodnie z normami europejskimi dla tarcicy konstrukcyjnej o przekroju prostokątnym stwierdzono, że dla zbadanej populacji można przyporządkować klasę wytrzymałościową C30. Wyznaczone zależności wytrzymałości na zginanie od modułu sprężystości cechują się współczynnikiem determinacji na podobnym poziomie jak dla tarcicy o przekroju prostokątnym. Corresponding author: Grzegorz Pajchrowski, Winiarska 1, Poznań, Poland g_pajchrowski@itd.poznan.pl phone: