Abstract
The use of large cross-section timber for structural purposes has increased in Spain, and knowledge of its properties is strategically necessary. The Spanish visual strength-grading standard UNE 56544 (2011) efficiency applied to large cross-section structural timber was analyzed using a sample of 363 specimens of radiata pine (Pinus radiata D. Don.) from the Basque Country and Catalonia, Spain. Different sizes were tested (80 × 120 × 2400 mm3, 150 × 250 × 5600 mm3, 150 × 250 × 4300 mm3, and 200 × 250 × 5000 mm3). Bending strength, modulus of elasticity, and density were obtained, and characteristic values were determined in order to assign strength class according to European standard EN 338 (2010). Knots and twists were the most relevant singularities for visual strength grading. It was concluded that large cross-section Spanish radiata pine timber was suitable for structures, and it was assigned to the C20 strength class.
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Visual Grading and Structural Properties Assessment of Large Cross-Section Pinus radiata D. Don Timber
Eva Hermoso,a,* Raquel Mateo,b Guillermo Íñiguez-González,c Joaquín Montón,d and Francisco Arriaga c
The use of large cross-section timber for structural purposes has increased in Spain, and knowledge of its properties is strategically necessary. The Spanish visual strength-grading standard UNE 56544 (2011) efficiency applied to large cross-section structural timber was analyzed using a sample of 363 specimens of radiata pine (Pinus radiata D. Don.) from the Basque Country and Catalonia, Spain. Different sizes were tested (80 × 120 × 2400 mm3, 150 × 250 × 5600 mm3, 150 × 250 × 4300 mm3, and 200 × 250 × 5000 mm3). Bending strength, modulus of elasticity, and density were obtained, and characteristic values were determined in order to assign strength class according to European standard EN 338 (2010). Knots and twists were the most relevant singularities for visual strength grading. It was concluded that large cross-section Spanish radiata pine timber was suitable for structures, and it was assigned to the C20 strength class.
Keywords: Characterization; Singularities; Bending strength; Modulus of elasticity; Density; Pinus radiata
Contact information: a: Department of Forest Products, Forest Research Centre INIA-CIFOR, Ctra. La Coruña km. 7,5, 28040 Madrid, Spain; b:Mixed unit INIA-AITIM, c/Flora 3, 28003 Madrid, Spain; c: Department of Forest and Environmental Engineering and Management. Universidad Politécnica de Madrid, Madrid, Spain; d: Department of Architectural Technology II, Universitat Politècnica de Catalunya, Escola Superior d´Edificació de Barcelona, Gregorio Marañón 44-50, 08028 Barcelona, Spain; *Corresponding author: hermoso@inia.es
INTRODUCTION
According to Spanish Forest Inventory data (IFN3 2007), the area occupied by radiata pine is 4.4% of the total of Spanish coniferous wood (287,771 ha), although it is the most widely used coniferous tree in forest plantations in this country. It is found mainly in Northern Spain in Basque Country (2.3%), Galicia, Cantabrian, and Catalonia. This species amounts to 25% (1,552,850 m3) of total lumber production. There is a strong industrial sector based on wood transformation, and particularly its use as a structural timber, which improves the economic profitability of sawmills and forest owners.
Most research on the structural properties of timber from Spanish species has been on specimens up to 200 × 70 mm2 in cross-section. These studies included Pinus pinaster (Aiton) (Ortiz et al.1990), Pinus radiata (D. Don) (López de Roma et al. 1991; Ortiz and Martínez 1991), Pinus sylvestris (L.) (Fernández-Golfín et al. 1997; Hermoso 2001; Hermoso et al. 2003), and Pinus nigra subsp. salzmannii (Dunal) Franco (Fernández-Golfín et al. 2000; 2001; Conde 2003). However, most structural uses of sawn timber in Spain use larger cross-sections than those studied.
The influence of specimen dimensions on mechanical properties has been covered in several studies (Rosowsky and Fridley 1997; Fernández-Golfín et al. 2002; Hermoso et al. 2002; Morel et al. 2002; Íñiguez 2007; Montón 2012; Hermoso et al. 2013), but it is unclear whether variation in mechanical properties is due to depth, width, or both (Newlin and Trayer 1924; Curry and Tory 1976; Madsen 1992; Böstrom 1994; Fernández-Golfín et al. 2002). Generally, strength decreases as the volume of a specimen increases, as a consequence of higher fragility (Morel and Valentin 1996). As a result, standards set reference values (a depth of 150 mm) to define timber properties. It is therefore necessary to characterize large cross-section timber of the species used the most in construction.
Based on the research of Íñiguez (2007), the new edition of the Spanish visual strength grading standard incorporates new specifications for large cross-section timber, namely quality MEG (madera estructural de gran escuadría, i.e., large cross-section structural timber).
This work evaluated the efficiency of the current version of the Spanish visual grading standard (UNE 56544 2011) for radiata pine MEG and characterized mechanical properties to assign visual quality to a strength class according to EN 338 (2010).
EXPERIMENTAL
Materials
A total of 363 specimens of Pinus radiata D. Don large cross-section structural sawn timber were assessed. They were supplied, dried, and planed from several sawmills to represent the sources and variability of this Spanish species. Radiata pine timber is being supplied from managed forests, where usually it is logged every 32 to 35 years, with the application of silvicultural techniques.
Table 1 shows the number of specimens, cross-sections, average length, mean and coefficient of variation (COV) of moisture content, and the source of each sample. The final moisture content of the sample was 8% to 18% according to EN 384 (2010).
Table 1. Sample Characteristics
Methods
The Spanish standard UNE 56544 (2011) establishes three visual grades: ME-1, ME-2, and MEG. ME-1 and ME-2 are for small cross-section pieces, i.e., pieces with a thickness less than or equal to 70 mm, and the MEG grade is for pieces thicker than 70 mm (large cross-section pieces).
The Spanish standard sets a thickness of 70 mm as the threshold between small and large cross-section pieces because the mechanical properties of Spanish species were obtained, until approximately 2003, from tests carried out on pieces with a thickness equal to or less than 70 mm. Therefore, small cross-section mechanical properties could not be assigned to large cross-section pieces. Since 2004, large cross-section timber has been included in testing campaigns in Spain, due to the increasing use of these sawn timber sizes for structural purposes. The samples analyzed in this work were visually graded according to MEG specifications (thickness greater than 70 mm), as shown in Table 2.
Table 2. Specifications for MEG Visual Grading of Structural Coniferous Sawn Timber with Thickness Greater Than 70 mm in Accordance With UNE 56544 (2011)
According to EN 408 (2011), to determine modulus of elasticity and bending strength, pieces were tested simply supported and symmetrically loaded in bending at two points over a span of 18 times the depth. Load was applied at a constant rate, and deformation was measured at the centre of the span and from the centre of the tension edge. The density of the test pieces was determined on a full cross section slice, free from knots and resin pockets and it was cut as close as possible to the fracture zone.
The characteristic values of former properties were required to assign strength class according to EN 338 (2010). To determine these characteristic values, EN 384 (2010) was used to establish the following parameters, all measured in N/mm2: fm, bending strength; fmean, sample mean value of strength; fSD, standard deviation of strength; f05, sample 5‐percentile value of strength; and Adjusted f05, kh adjusted sample 5‐percentile value of strength.
Bending strength was adjusted to the 150 mm depth by dividing by the factor kh (Eq. 1),
where when h = 120, kh = 1.046, and when h = 250, kh = 0.903.
The span factor, kl, was determined when the bending test arrangement was not specified in EN 408 (2011) (i.e., span, l, is 18 h and the distance between inner load points, af, is 6 h). The bending strength was adjusted by dividing by the factor kl (Eq. 2),
where ; and have the respective values for the test. In this study kl = 1 y kl1 = 1.01 (where l1 = 16 h).
The characteristic value of strength fk (N/mm2) was obtained by applying also the ks factor according to the number and size of samples and kv factor according to type of grading used (mechanical or visual), where ks = 0.95 and kv = 1.0 in this case.
The bending modulus of elasticity of the sample was determined through the following parameters, all in N/mm2 and adjusted to 12% moisture content: Emean, mean value of modulus of elasticity; ESD, standard deviation of modulus of elasticity; and E0,mean, mean characteristic value weighted according to the number of specimens in each sample. The last parameter included an adjustment to pure bending modulus of elasticity, as determined by Eq. 3:
Finally, the characteristic value of global density ρk, in kg/m3 was determined and adjusted to 12% moisture content, weighted according to the number of specimens in each sample and using ρ05, 5-percentile value of global density distribution.
One of the more relevant singularities in visual grading is knottiness (see Fig. 2), which was evaluated by a simplified parameter known as concentrated knot diameter ratio (CKDR) (Divos et al. 2005).
Fig. 2. Concentrated knot diameter ratio
The knot diameter ratio (KDR) is the knot diameter divided by the depth or width of the piece. The CKDR is the sum of the KDRs of the knots existing in any 15 cm length of timber without overlapping. The maximum CKDR, which includes all four faces, represents the quality of the piece (Fig. 2). This value of CKDR is obtained for the worst cross-section along the whole length of the piece.
RESULTS AND DISCUSSION
Results and Yield of Visual Strength Grading
Table 3 shows the results obtained from the application of the visual grading standard. The main causes of rejection were analyzed; Fig. 1 shows the percentage for each timber singularity considered in visual grading and for each sample.
Table 3. Specimen Number and Yield Resulting from the Application of the UNE 56544 Standard
Fig. 1. Percentage of rejected specimens for each singularity for sample A (dark grey), sample B (dotted), sample C (grey), and sample D (diagonal lines)
The presence of twists in the specimens was the main cause of rejection in three of the samples studied and mainly in the smallest cross-section sample (sample A with a percentage of 31%). This singularity used to be strongly associated with juvenile wood and its offset.
This result can be explained because in large cross-section specimens it may not be possible to avoid the presence of pith (and juvenile wood associated around it) because of the sawn pattern applied to obtain this cross-section size. If pith is more or less centered in the section, then the drying stresses due to the different behavior of juvenile wood are balanced, and then twist is minimized. When cross-sections are smaller (sample A), then the sawn pattern can provide specimens with pith close to the face of the section or without it, but with juvenile wood in high proportion of the cross-section, therefore the balance is not produced and the presence of twist increases. However, this phenomenon can be avoided through careful sawing.
Furthermore, deformations and defects, especially twists, have no relevant effect on mechanical properties (Montón et al. 2015).
Table 4 shows the average CKDR values for each sample. It can be seen that the CKDR of MEG grade was similar for all four samples (0.27 to 0.31). Additionally, the CKDR in the rejected pieces was not far from that of MEG grade pieces in samples C and D, although in the smaller cross-section sample (A) the difference was greater.
Table 4. Average CKDR Values
Mechanical Properties and Strength Class Assignment
Mechanical properties are shown in Table 5.
Applying visual strength grading according to standard UNE 56544 (2011), the MEG grade did not always present mechanical properties that were clearly differentiated from reject ones. The mean bending strength value of sample A was about 34% higher in the MEG grade compared with rejected specimens, while this value fell to less than 3% on average for samples B, C, and D. Similarly, the modulus of elasticity of MEG was 26% higher than the rejected pieces in sample A, and was less than 1% on average in samples B, C, and D.
These results agree with knottiness values shown in Table 4. Samples A and B showed notable differences of knottiness between MEG graded timber and rejected pieces (about 33% more in rejected pieces). On the other hand, samples C and D showed similar knottiness in MEG grade and rejected pieces (only 4% more in rejected pieces). Nevertheless, comparing mechanical properties of samples A and B, with similar knottiness differences between grades, a relevant difference can be observed: sample A showed a strong decreasing of properties in rejected pieces, but sample B showed no relevant differences between graded and rejected timber. These results are explained because the knottiness effect is tempered in bigger sections (sample B) in comparison to smaller ones (sample A).
According to the EN 338 (2010) standard, timber is assigned to a strength class if its characteristic values of bending strength and density equal or exceed the values for that strength class, and its characteristic mean modulus of elasticity in bending equals or exceeds 95% of the value for that strength class.
Table 5. Mechanical Properties
The strength class assigned to the specimens of Spanish radiata pine graded as MEG was C20. This assignment was determined by strength and stiffness, while the density requirement was easily met, as is usual in Spanish coniferous timber.
Table 6 shows the mechanical characterizations for each sample. All calculations and coefficients used to obtain the characteristic values were applied in accordance with the EN 384 (2010) standard.
Table 6. Characteristic Values According to EN 384 (2010) for Large Cross-Section Radiata Pine
CONCLUSIONS
- Large cross-section of Spanish radiata pine timber for structural purposes graded as MEG (UNE 56544:2011) was assigned to strength class C20 according to the EN 338 (2010) standard.
- Cross-section size had a relevant difference in the mechanical properties of timber pieces. Smaller cross-section pieces showed a difference in MEG grade mechanical properties compared with rejected pieces, while this difference was very small in large cross-section pieces.
- Twist was revealed as the key singularity for the visual grading result, mainly in the smaller cross-section size.
- Knottiness (CKDR) was high in general because of radiata pine is a whorled species, but it has different influence on mechanical properties depending on the cross-section size. For bending strength results it was indicated that in larger cross sections the influence is less than in smaller ones.
ACKNOWLEDGMENTS
The authors are grateful for financial support from the Ministerio de Ciencia e Innovación (Plan Nacional I+D+i 2000-2003, Proy.: AGL 2002-00813); the INIA (Proy.: CON09-070); the Mesa Intersectorial de la Madera Pais Vasco, and the Institut Català de la Fusta (INCAFUST).
REFERENCES CITED
Böstrom, L. (1994). “Machine strength grading: Comparison of four different systems,” Building Technology SP Report, Swedish National Testing and Research Institute.
Conde, M. (2003). Caracterización de la madera estructural de Pinus nigra subsp. salzmannii. (Structural timber characterization of Pinus nigra subsp. salzmannii), Ph.D. Dissertation, Universidad Politécnica de Madrid, Madrid, Spain.
Curry, W. T., and Tory, J. R. (1976). The Relation between the Modulus of Rupture (Ultimate Bending Stress) and Modulus of Elasticity of Timber, Princes Risborough Laboratory, Aylesbury, UK.
Divos, F., Denes, L., and Íñiguez, G. (2005). “Effect of cross-sectional change of a board specimen on stress wave velocity determination,” Holzforschung 59, 230-231. DOI: 10.1515/hf.2005.036
EN 338 (2010). “Structural timber. Strength classes,” European Committee for Standardization (CEN), Brussels, Belgium.
EN 384 (2010). “Structural timber. Determination of characteristic values of mechanical properties and density,” European Committee for Standardization (CEN), Brussels, Belgium.
EN 408 (2011). “Timber structures. Structural timber and glued laminated timber. Determination of some physical and mechanical properties,” European Committee for Standardization (CEN), Brussels, Belgium.
EN 1912 (2012). “Structural timber. Strength classes. Assignment of visual grades and species,” European Committee for Standardization (CEN), Brussels, Belgium.
Fernández-Golfín, J. I., Díez, M. R., and Gutiérrez, A. (1997). “Caracterización mecánica de la madera aserrada de pino silvestre de los sistemas Central e Ibérico mediante probetas de tamaño estructural (Mechanical characterization of silvestre pine structural timber from Central and Ibérico Mountains),” Revista de Investigación Agraria 6 (1 and 2), 183-215.
Fernández-Golfín, J. I., Díez, M. R., Conde, M., and Hermoso E. (2000). “Caracterización de la madera aserrada con destino estructural efectuada de acuerdo con las normas europeas: aplicación al caso del pino laricio español (Timber characterization with structural purpose according to European standards: case of Spanish laricio pine),” in: Proceedings of the I Congreso Iberoamericano de Investigación y Desarrollo de Productos Forestales, Concepción, Chile.
Fernández-Golfín, J. I., Díez, M. R., Baonza, M. V., Gutierrez, A., Hermoso, E., Conde, M., and Van Den Eynde, V. (2001). “Caracterización de la calidad y las propiedades de la madera de pino laricio (Pinus nigra) (Quality and properties characterization of laricio pine timber (Pinus nigra),” Revista de Investigación Agraria, Sistema Recursos Forestales 10(2), 311-332.
Fernández-Golfín, J. I., Hermoso, E., and Díez, M. R. (2002). “Análisis del efecto del volumen sobre la resistencia característica a flexión de la madera de los pinos silvestre y laricio de procedencia española (Analysis of the volumen effect on the characteristic bending strength of the silvestre and laricio pine from a Spanish source),” Materiales de Construcción 268(52), 43-55. DOI: 10.3989/mc.2002.v52.i268.316
Hermoso, E. (2001). “Caracterización mecánica de la madera estructural de Pinus sylvestris L. (Mechanical characterization of Pinus sylvestris L. structural timber),” Ph.D. Dissertation, Universidad Politécnica de Madrid, Madrid, Spain.
Hermoso, E., Fernández-Golfín, J. I., and Díez, M. R. (2002). “Madera estructural de pino silvestre caracterización mecánica (Mechanical characterization of silvestre pine structural timber,” Revista de Investigación Agraria, Sistema Recursos Forestales 11 (2), 425-440.
Hermoso, E., Fernández-Golfín, J. I., and Díez, M. R. (2003). “Mechanical characterization of timber according to European standards from Spanish provenances of Scots Pine,” Revista de Investigación Agraria, Sistema Recursos Forestales 12(3), 103-110.
Hermoso Prieto, E., Diez Barra, M. R., Fernández-Golfín, J. I., and Íñiguez- González, G. (2013). “Efecto del tamaño de la sección y la médula sobre la resistencia y rigidez de la madera aserrada estructural de pino radiata (Pinus radiata D. Don.) Section size and pith effect on strength and stiffness of radiata pine timber),” in: Proceedings of 6º Spanish Forestal Congress, Pontevedra (Spain).
IFN3 (2007). Tercer Inventario Forestal Nacional (Spanish Forest Inventory). Ministry of Agriculture, Food and Environment, Spain, (www.magrama.gob.es/es/biodiversidad/servicios/banco-datos-naturaleza/informacion-disponible/ifn3.aspx).
Íñiguez, G. (2007). Clasificación mediante técnicas no destructivas y evaluación de las propiedades mecánicas de la madera aserrada de coníferas de gran escuadría para uso estructural (Grading by non destructive techniques and assessment of the mechanical properties of large cross-section coniferous sawn timber for structural use), Ph.D. Dissertation, Universidad Politécnica de Madrid, Madrid, Spain.
López De Roma, A., Gutierrez, A., Álvarez, H., Baonza, M. V., Díez, M. R., Fernández-Golfín, J. I., González, F., Jiménez, F. J., Navarrete, M. A., Ortiz, J., Seoane, I., Arana, M. C., Guijarro, A., López De Roma, M. T., and García De Cela, J. L. (1991). Propiedades y Tecnología de la madera de pino radiata del País Vasco (Radiata pine properties and wood technology from the Basque Country, Monografías Instituto Nacional de Investigación y Tecnología Agraria y AlimentariaINIA nº 80, Ministerio de Agricultura, Pesca y Alimentación MAPA (Ministry of Agriculture, Fishing and Food MAPA), Madrid, Spain.
Madsen, B. (1992). Structural Behavior of Timber, Timber Engineering LTD, North Vancouver, BC, Canada.
Montón, J. (2012). “Clasificación estructural de la madera de Pinus radiata D. Don procedente de Cataluña mediante métodos no destructivos y su aplicabilidad en la diagnosis estructural (Structural grading of Pinus radiata D. Don timber from Catalonia using nondestructive methods and their applicability in structural diagnosis),” Ph.D. Dissertation, Universitat Politécnica de Catalunya. Escola Tècnica Superior D’Arquitectura de Barcelona, Barcelona, Spain.
Montón, J., Arriaga, F., Íñiguez-González, G., and Segués, E. (2015). “Warp requirements and yield efficiency in the visual grading of sawn Radiata pine timber,” BioResources 10(1), 1115-1126. DOI: 10.15376/biores.10.1.1115-1126.
Morel, S. and Valentin, G. (1996). “Size effect in crack shear strength of wood,” Journal of Physique IV 6 (C6), 385-394. DOI: 10.1051/jp4:1996638
Morel, S., Bouchaud, E., and Valentin, G. (2002). “Size effect in fracture: Roughening of crack surface and asymptotic analysis,” Physical Review B 65(10), Article ID 104101. DOI:10.1103/PhysRevB.65.104101
Newlin, J. A., and Trayer, G. W. (1924). Form Factors of Beams Subjected to Transverse Loading Only, Report No. 181, United States National Advisory Committee for Aeronautics, Washington, D.C., USA.
Ortiz, J., Curz, H., and Blanchón J. L. (1990). Standard Quality of Pinus pinaster (Final Internal Report of the Project MA1B-0129, FP1-RAWMAT 3C), Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria INIA (Food and Agriculture National Research Centre INIA), Madrid, Spain.
Ortiz, J., and Martínez, J. J. (1991). “Características mecánicas de la madera de pino gallego, obtenidas a partir de ensayos con piezas de tamaño estructural,” AITIM Journal 150, 95-101.
Rosowsky, D. V., and Fridley, K. J. (1997). “Effect of discrete member size on reliability of wood beams,” Journal of Structural Engineering 123(6). DOI: 10.1061/(ASCE)0733-9445(1997)123:6(831)
UNE 56544 (2011). “Clasificación visual de la madera aserrada para uso estructural,” Asociación Española de Normalización (AENOR), Madrid, Spain.
Article submitted: January 26, 2016; Peer review completed: March 18, 2016; Revised versión received: April 18, 2016; Accepted: April 22, 2106; Published: April 28, 2016.
DOI: 10.15376/biores.11.2.5312-5321