Báo cáo lâm nghiệp: "properties of Norway spruce timber. Comparison between fast- and slow-grown stands and influence of radial position of sawn timber"

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Original article Bending properties of Norway spruce timber. Comparison between fast- and slow-grown stands and influence of radial position of sawn timber Mikael Germund Johansson I. Robert Kliger* Perstorper Chalmers University of Technology, Department of Structural Engineering, Division of Steel and Timber Structures, SE-412 96 Göteborg, Sweden (Received 5 November 1996; accepted 25 November 1997) primary objective of this work was to study one aspect of improving timber Abstract - The quality. The aim of this paper is to supplement previously published results in Wood Science and Technology. Bending strength and stiffness of Norway spruce (Picea abies) from three stands in southern Sweden, two fast-grown and one slow-grown, were measured. Radial variations were studied using six studs (45 mm x 70 mm x 2 900 mm) per log cut along a diameter, with a total of 500 studs. The bending strength of studs from the slow-grown stand was 57 % higher and the modulus of elasticity 54 % higher than that of the fast-grown stands. The bending strength of studs from mature wood (near the bark) was 47 % higher and modulus of elasticity 30 % higher than that of the core studs. The improvement in mechanical properties from pith to bark was far more significant for the studs from the slow-grown stand than from the fast-grown ones. (© Inra/Elsevier, Paris.) Norway spruce / strength / stiffness / mechanical performance Résumé - Propriétés de flexion de l’épicéa commun. Comparaison entre sites à croissance rapide et lente et influence de la position radiale des sciages. L’objectif premier de ce travail est l’étude de paramètres importants contrôlant la qualité des sciages. Cette étude complète des résultats publiés précédemment. La résistance et la rigidité en flexion de l’épicéa commun (Picen abies) provenant de trois sites du sud de la Suède, deux à croissance rapide et un à croissance lente, été mesurées. Pour l’étude des variations radiales, six débits (45 x 70 x 2 900 mm) ont été ont effectués le long d’un diamètre pour chaque grume, avec un total de 500 débits. La résistance en flexion et le module d’élasticité étaient respectivement 57 et 54 % plus élevés pour les débits provenant d’un site à croissance lente que pour ceux provenant d’un site à croissance rapide, et respectivement 47 et 30 % plus élevés pour les débits près de la périphérie que pour ceux qui sont * Correspondence and reprints E-mail: robert.kliger@ate.chalmers.se
près du c&oelig;ur. L’amélioration des propriétés mécaniques du coeur à la périphérie était bien plus sig- nificative pour les débits du site à croissance lente que pour ceux des sites à croissance rapide. (© Inra/Elsevier, Paris.) épicéa commun / résistance / rigidité / propriétés mécaniques 1. INTRODUCTION forest and log utilization. In an ideal ’end- use-oriented’ system, each stand, each tree and each part of the stem should be given The position of timber products in the a destination for an end product in terms of competition with other load-carrying an optimum end use. However, forest building materials depends to a large management techniques, which optimize extent on a knowledge of their mechanical the volume of fibre which is produced, properties. Relationships between the raw have been implemented with little regard material parameters and strength and stiff- for the compatibility between the wood ness, as well as their variability within a properties that are produced and the end stand, a species, a tree or a log, are at pre- use. In order to make the most rational use unknown unclear. For the timber sent or of the timber from intensively managed production and construction industries, it forests in particular, appropriate informa- is highly beneficial to know which mate- tion on the properties of the material needs rial parameters are of importance to the to be available. structural performance of sawn timber when grading or selecting the raw mate- The fact that variations in conifer wood rial. The structure of the timber industry exist and are dependent on growth condi- with its predominance of small compa- tions has been established by many sci- nies has prevented the development of entists in the past ([1-3, 10, 15]; among methods for selecting the raw material to others). However, these variations are not produce high-quality products in terms of used to create advantages for timber prod- their structural performance, as these com- ucts and produce the ’right’ products with panies are often not aware of the needs of the ’right’ properties for the ’right’ end end-users. As a result, we should first try use in a positive manner. The variability of to understand how and why various raw wood properties can have both positive materials affect the structural performance and negative effects, depending on how before attempting to improve some prod- it is used [8]. The amount of work that is develop new ones. ucts or carried out on the mechanical properties of The most basic requirements for any timber from conifers is too voluminous to material used in engineered construction be included in this journal. However, sys- are that it should have sufficient strength tematic comparisons of mechanical prop- guarantee the desired level of structural erties and existing variations in material to safety and sufficient stiffness to meet the properties and sawing patterns are lack- ing. In recent years and in various parts and any desirable stability requirements serviceability criteria. The main disad- of the world, a great deal of work has been vantage of timber as an engineering mate- carried out to develop models in order to rial is that it does not have consistent, pre- predict various properties of sawn timber dictable, reproducible and uniform from known agricultural regimens [5, 12, properties. The great variability between 16-18]. Shivnaraine [14] and individual trees, as well as within and Kretschmann and Bendtsen [9] studied the between stands, indicates that there is large effect of juvenile wood on the bending potential for more efficient and optimized properties of structural size timber. The
radial variation in mechanical properties based material from study was on one stand of was considerably less pronounced than slow-grown Norway spruce [6]. that found in studies of clear wood. The However, this second study was limited compared with the first one. Only studs grain distortions around knots appear to diminish the effects of juvenile wood from the butt logs, cf. figure 1, were included and fewer parameters were mea- found in clear wood. The linking of wood properties and grading rules for various sured (knot area ratio was not measured, end uses is fundamental for the future for example). development of the sawing simulation sys- In this paper, the results obtained in tem. both studies are combined and joint con- clusions have been drawn. In general, it The primary objective of this paper is to is shown how the modulus of elasticity, show how a radial position in a tree affects and the bending strength, f E m ) edge (E strength and stiffness. Furthermore, the (sometimes referred to as MOR in the lit- relationships between the strength and erature), in studs varies according to: stiffness and between some growth char- acteristics and strength and stiffness are a) position in the stem, i.e. in the radial shown. The results of these findings can be direction - the difference between studs applied first by the forest industry by allo- sawn close to the pith and further away cating the ’right’ raw material to the ’right’ from the pith of the butt log; based on industry and second by the sawmills to studs from both fast-grown and slow- obtain a better basis for choosing raw stands; grown materials and/or sawing patterns in order b) the variation in wood density to produce structural timber with the opti- (DENS), ring width (RW), grain angle mum mechanical properties for the (GA) and knot area ratio (KAR), where intended end use. Furthermore, this paper KAR is based studs from fast-grown on presents information and data related to stands alone. the effects of the raw material parameters on the mechanical bending properties of structural timber. 2. MATERIALS AND METHODS 2.1. Specimen preparation Scope of two studies 1.1. presented 2.1.1. Fast-grown stands in this paper description of the two detailed For a more This paper consists of some results Perstorper et al. [13]. fast-grown stands, see However, a brief summary of the most impor- obtained during two parallel studies con- tant issues related to this paper is presented ducted at Chalmers University of Tech- here. All the timber used in this study came nology in recent years on studs measur- from a relatively fast-grown stand, about 65 ing 45 x 70 x 2 900 (in mm) from Norway years old, which contained large trees (dbh = spruce (Picea abies) grown in southern 360 mm). These trees had been planted on land Sweden. The first study, already reported where animals had previously grazed. Log sam- by Perstorper et al. [13] and Kliger et al. pling, the number of logs from each stand, saw- ing patterns and notations are shown in figure [7], was based on material from two stands I. Two sets of logs were taken from the butt of fast-grown Norway spruce, see figure 1. end (lower part of the large diameter butt logs However, only the data from the first stand LBL, upper part of the large diameter butt - are used in this paper, as they are suffi- logs - UBL) and one set from near the top (TL, cient to make a comparison with the data not included in this paper) of the fast-grown obtained in the second study. The second trees (see figure I). Beams from these logs
2.2. Modulus of elasticity, E and [measuring 70 x 290 x 2 900 (in mm)] from the butt end were sawn from the central part bending strength of each log (containing the pith), dried and ripped prior to being equilibrated to12 % MC. Different methods were used for measur- Six studs were sawn from each beam from the modulus of elasticity in each study. It ing the butt. In all, 249 studs from butt logs (both LBL was not possible to comply with the test stan- and UBL) were used for evaluation from this dards for many reasons. In order to compare stand. Three studs from position1 and 6 different E-values for all the members, includ- (mature wood) were missing (failure due to ing some large-beam members (not included handling or during measurements of the mod- in both these studies mentioned in this paper), ulus of elasticity). it was necessary to measure the curvature over the same distance. This means that the length to depth ratio had to vary (in the first study) 2.1.2. Slow-grown stand between studs and some large beams used in parallel studies during this period. All the timber used in the second study from a slow-grown stand of large-diam- In the firststudy (studs from two fast-grown came eter trees (dbh ≈ 400 mm). This stand (proba- hydraulic jack was used to load all stands), an the specimens using the test set-up shown in bly self-seeded) was about 105 years old. For different reasons, it was only possible to take figure 2A. As a result, the load versus curvature two sets of logs from the butt end (lower part of was plotted continuously. The maximum load butt logs - LBL, upper part of the large diam- corresponded to a bending stress value of no eter butt logs - UBL) in the same manner as the more than 10 MPa. In the second study (studs logs from fast-grown trees, cf. figure 1. As a from the slow-grown stand), two different dead result, only the radial variation was studied weights were applied to the specimens using from the material obtained in the second study. the test set-up shown in figure 2B. These loads In the same way as for the butt logs from fast- corresponded to a bending stress value of 2 and 5 MPa. However, despite the two different grown trees, six studs were sawn from each beam and a total of 251 studs was obtained. ways of loading, the length over the constant One stud from position 4 was missing (failure moment area and the measurements of the cur- vature over a length of 1 m were the same for during measurements of the modulus of elas- specimens in both studies, see figure 2. all the ticity).
result, this difference in the experimental As It should also be pointed out that no grading a has not effected the comparison of whatsoever was performed prior to testing. All procedure studs from fast-grown and slow-grown stands. the studs were included in the analysis, irre- spective of severe cracks, slope of grain, com- pression wood, large knots and so on. The strength properties of graded material would 2.3. Measurements of the short-term probably be different, especially for the lower bending strength, fm tails of the distributions. This is also the reason why the 5th percentile for bending strength was not evaluated. The distance between the concentrated loads kept the same as that used when the mea- was surements of the modulus of elasticity were 3. RESULTS made. However, the total span was shortened for bending strength measurements in com- 3.1. General parison with measurements of the modulus of elasticity for studs from fast-grown stands to There was no significant difference avoid overly large deformation and possible second-order effects, figure 2C. No studs failed when comparing the respective values for in shear. As a result of the so-called length the lower and upper butt logs from the effect [11], the strength values were most prob- same spatial position. Consequently, when ably slightly lower than they would have been the variation in the radial direction is con- if the standard test set-up had been used. How- sidered, the lower and upper butt logs were ever, all the material was tested in the same treated statistically as the same type of way. Both mechanical properties in this study butt log (BL). This was valid for studs were obtained by applying a constant moment to a length (figure 2) which is much longer from both fast-grown (FG) and slow- than that recommended in standard procedure, grown stands (SG). i.e. 17 times the depth compared with 6 times The linear regression for all studs the depth (equal to one-third of the total span of specimens). (n 500) is shown in figure 3. The regres- = sion coefficient, R 0.83, is almost the = Moisture content, density, position of the same as that previously reported for the pith and mean ring width values were obtained same species by Johansson et al. [4], for prior to the tests to failure for all studs. The example. The relationship between bend- average moisture content was 12.2 and 11.8 % for studs from fast-grown and slow- ing strength and modulus of elasticity was grown stands, respectively. As a result, den- found by Johansson et al. [4] to be sity and bending strength were not adjusted f -2.4 + 3.8 E and is the basis for set- m m = owing to these small deviations from a 12 % ting values for grading machines in Swe- moisture content. Four studs (three fast-grown den. Some general results distinguishing and one slow-grown) failed owing to handling each stand [including butt logs (BL), top or during measurements of E.
from the others when it came to bend- logs (TL) from fast-grown stands (FG) ent and stiffness, cf. table II. and thinning stand (ThL)] in terms of the ing strength measured mean values for strength (f However, it appears that the difference ), m between stud groups 25 and 34 from the modulus of elasticity (E), density (DENS) and ring width (RW) are shown in table I. fast-grown stand was not statistically sig- nificant when it comes to strength. Fur- thermore, the standard deviation for both f and E appears to be smaller nearest to m 3.2. Variations in the radial direction the pith; see figures 5 and 6, where the according to stud groups cumulative distribution in per cent clearly demonstrates that there is no difference The radial position in these studies is between groups 25 and 34 up to the 80 % expressed in three stud groups, i.e. core percentile for the fast-grown material. studs (34), intermediate studs (25) and mature studs (16), cf. figure 1. These stud The radial variation in bending strength groups are compared for butt logs only, and the modulus of elasticity (E), ) m (f i.e from the fast-grown stands (FG-BL) based on the division into stud groups, is and from the slow-grown stand (SG-BL). shown in figure 4. The corresponding vari- Both stands are represented by about 250 ation in density and ring width is shown in studs and each stud group (i.e. core, inter- figure 7. The mean value for the modulus mediate and mature) in each stand is rep- of elasticity was significantly higher in resented by about 84 studs. A summary the core studs from the slow-grown stand of these variations is shown in figures 4-6. than in the studs from all groups (includ- The mean values for each group and the ing those from the mature wood) from the statistical significance (unpaired t-test) fast-grown stand. The same thing applies when comparing these groups are shown to bending strength, but the difference in table II. between the core studs from the slow- grown stand (34) and the studs from the It was found that the mean values for mature wood (16) from the fast-grown both f and E were lowest for the core m stand was not statistically significant. studs (group 34) and increased further away from the pith (groups 25 and 16). The distribution of strength (f for the ) m studs from the butt logs divided into Each stud group was statistically differ-
well for core, intermediate and groups 16, 25 and 34 reveals that there is fairly studs. no difference between the 5th percentile mature values for each group, see figure 5. This result indicates that some very ’poor qual- ity’ studs, which would normally be 4. CONCLUSIONS rejected, influenced the 5th percentile for each group. In general, the knot area ratio There was a highly statistically signif- (KAR) decreases from the pith to the bark icant difference between studs from the [13]. However, the higher tail of the KAR slow-grown and fast-grown stands when it distribution is very similar for all stud came to both the modulus of elasticity and groups (16, 25, 34). It is therefore. ratio- bending strength. In terms of mean val- nal to suppose that the lower tails of the ues, the bending strength of studs from the slow-grown stand was 57 % higher bending strength distributions coincide
Approaches and Simulation Software’, Kruger and the modulus of elasticity 54 % higher National Park, South Africa, August 1996. than that of studs from the fast-grown stand. A clear radial variation in both the mod- REFERENCES ulus of elasticity and bending strength was observed in the studs from the two stands Bendtsen B.A., Properties of wood from [1] divided into three different groups. In and intensively managed trees, improved Forest. Prod. J. 28(10) (1978) 61-72. mean terms, the bending strength of the Bendtsen B.A., Senft J., Mechanical and studs from mature wood (near the bark) [2] anatomical properties in individual growth was 47 % higher and the modulus of elas- rings of plantation-grown eastern cottonwood ticity 30 % higher than that of the core and loblolly pine, Wood Fiber Sci. 18(1) studs. This increase in mechanical prop- (1986) 23-28. erties from the pith to the bark was far Büsgen M., Münch E., The Structure and Life [3] of Forest Trees, 3rd ed., John Wilcy & Sons, more significant for studs from the slow- New York, 1929. grown stand than for studs from the fast- Johansson C.J., Brundin J., Gruber R., Stress [4] grown one. grading of Swedish and German timber. A comparison of machine stress grading and three visual grading systems, Swedish National Testing and Research Institute Build- ACKNOWLEDGEMENT ing Technology, SP Report 1992, 23. Kellogg R.M., Second growth Douglas fir: [5] The authors gratefully acknowledge the Its management and conversion for value, financial support received from the EC forest Forintek Canada Corp., special publication, research programme, Contract No. MA2B- SP-32, Vancouver, Canada, 1989. CT91-0024, Nils and Dorthi Troëdsson’s Foun- Kliger R., Johansson G., Perstorper M., Struc- [6] dation, the Sawmills Research Foundation, the tural timber from large-dimension Norway Swedish Sawmills’ Association (Så bi), the spruce. Part 3: Stiffness and strength of wall studs (in Swedish). Chalmers University of Swedish National Board for Industrial and Technology, Division of Steel and Timber Technical Development (NUTEK) and, finally, Structures, Göteborg, Sweden, Publ. S 94:10, Södra Timber AB. 1994. The present paper was presented at the sec- Kliger I.R., Perstorper M., Johansson G., Pel- [7] ond workshop of the IUFRO Working Party licane P.J., Quality of timber products from S5.01-04: ’Connection between Silviculture Norway spruce. Part 3: Influence of spatial and Wood Quality through Modelling position and growth characteristics on bend-
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