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Mechanical properties of Chinese larch (Larix gmelini) dimension lumber.

Abstract

A testing program was conducted to investigate mechanical properties such as bending, tensile, and compressive strengths of Chinese larch (Larix gmelini) dimension lumber. The species was sampled in three sizes: 40 by 65, 40 by 90, and 40 by 140 mm. These lumber test samples were conditioned to a standard moisture content of 12 percent. After grading and grouping, mechanical properties were measured in full-size testing. The data developed in this study will be used to make technical submissions for the assignment of design properties of Chinese larch in the Chinese timber design code.

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In China, sawn lumber is mainly used in decoration, furniture, and other nonstructural applications. With the fast development of timber construction industries in recent years, sawn lumber from domestic species may be used in the construction of homes and light commercial and industrial buildings. The Chinese larch forest area is 10.63 million hectares, which account for 6.83 percent of the total forest area in China. Chinese larch is an important species for producing structural lumber.

The use of lumber in structures requires that mechanical properties of dimension lumber be estimated so that design values can be assigned in the Chinese design codes. Establishing the structural performance of dimension lumber must consider the anisotropic nature of wood. In the past, the design properties of Chinese timber in the Chinese timber design code GB50005-2003 (Ministry of Housing and Urban-Rural Development [MOHURD] 2005) have been derived from test data of small clearwood specimens with adjustments for strength-reducing characteristics such as knots and slope of the grain (Editorial Committee 2005). Recent studies conducted by the Subcommittee 4 on Timber Structures, National Technical Committee 41 on Wood, of the Standardization Administration of the People's Republic of China (SAC) suggested that the design properties for Chinese larch (Larix gmelini) and Chinese fir (Cunninghamia lanceolata) dimension lumber may be too conservative (Zhou et al. 2012, Zhou and Xu 2012). Since the 1970s, full-size in-grade lumber testing has been used in North America and other countries to determine the mechanical properties of dimension lumber (Bodig 1977). As the full-size in-grade lumber test can more accurately estimate the mechanical properties of dimension lumber, lumber design values for limit states design based on a reliability approach can be developed (Madsen 1992).

In 2006, China began a 5-year, full-size in-grade test project. In this article, the test project on Chinese larch is described. Mechanical properties such as bending strength, modulus of elasticity (MOE), tensile strength, and compression strength of three lumber sizes (40 by 65, 40 by 90, and 40 by 140 mm) are presented as a whole, based on summarizing work performed by other researchers (Zhao et al. 2009, 2010). Moreover, the size effects on mechanical properties are also analyzed. The data developed in this study will be used to reassess the design properties of Chinese larch in the Chinese timber design code GB50005.

Materials and Methods

Sampling

Because there is currently no commercial production of Chinese larch dimension lumber, the sampling of dimension lumber could be conducted in sawmills. The larch logs in this study were collected from two regional forestry centers in Cuigang and Pangu of the Daxing'anling region in northeastern China. In these forestry centers, larch logs with diameters at breast height of above 240 mm were harvested from the secondary forest and cut into logs of 4,000 mm in length. The sampling plan focused on collecting representative logs with the small-end diameter ranging from 160 to 340 mm. The number of selected logs was roughly in proportion to the annual cut of each forestry center in order to provide a sample group that was representative of the whole growing region of the species. A total of 454 [m.sup.3] of Chinese larch logs was sampled, with 286 [m.sup.3] from Cuigang and 168 [m.sup.3] from Pangu.

Lumber was sawn from the logs following a cant sawing pattern typically used in China and then kiln-dried to a target moisture content (MC) of approximately 12 percent. After kiln-drying, the lumber pieces were planed to standard sizes of dimension lumber. The dimensions and numbers of the target sample sizes are shown in Table 1.

Grading

Lumber was visually graded according to the rules in the Chinese timber design code GB50005-2003 (MOHURD 2005). The grading rules were derived from the National Lumber Grades Authority (NLGA) Standard Grading Rules (NLGA 2005). In GB50005, the visual grade [I.sub.c] is equivalent to NLGA SS, visual grade [II.sub.c] is equivalent to NLGA No. 1, visual grade [III.sub.c] is equivalent to NLGA No. 2, and visual grade [IV.sub.c] is equivalent to NLGA No. 3.

During grading, the grade-controlling characteristic and maximum strength-reducing characteristic (MSRC) were identified and recorded for each specimen.

Grouping

For 40 by 65-mm specimens, the dynamic Young's modulus ([E.sub.f]) of the lumber was measured by the longitudinal vibration method. These values were calculated from the resonance frequency as determined from a fast Fourier transform spectrum analysis of the tap tone. The specimens were ranked according to their [E.sub.f] values in ascending order for each grade. Three groups of matched specimens were obtained by assigning the lumber to the three test modes, with lumber having the lowest [E.sub.f] value assigned to the bending group, lumber with the next-lowest [E.sub.f] value to the compression group, and lumber with the third-lowest [E.sub.f] value to the tension group. The specimens with the next three lowest [E.sub.f] values were then selected and assigned similarly. This process was repeated until all the lumber specimens for each grade and size were assigned to the three groups.

All samples were stored in a conditioning chamber maintained at 20[degrees]C and 65 percent relative humidity before testing.

Testing

Edge-wise bending testing was conducted in accordance with ASTM D198-09 (ASTM International 2009) to evaluate the MOE and modulus of rupture (MOR; Fig. 1). For each specimen, MOE was first measured using the Yoke deflectometer and then followed by loading the specimen to failure in order to determine the maximum load. Test procedures are summarized as follows: third-point loading conditions, 18:1 span-to-width ratio, tension edge of the bending specimen selected at random, MSRC located randomly within the total test span, loading rate to cause failure in about 10 minutes, and deflection of neutral axis at midspan measured at both sides using a full-span yoke.

Axial tensile testing according to ASTM D198 was conducted to evaluate the tensile strength parallel to the grain of each specimen. The tensile test machine Metriguard Model 401 was used in the tension test. The tension machine was equipped with serrated plates for gripping the specimen. Test procedures are summarized as follows: 2,500-mm test span, MSRC located randomly within the total test span, and tests conducted at a loading rate to cause failure in 1 to 5 minutes.

Compression testing according to ASTM D198 was conducted with a compression test machine (WE-1000B) to evaluate the compression strength parallel to the grain of each specimen. Test procedures are summarized as follows: short column method (without lateral support), 250-mm test span for 40 by 65-mm specimens, 350-mm test span for 40 by 90-mm specimens, 450-mm test span for 40 by 140-mm specimens, and tests conducted at a loading rate to cause failure within 10 minutes. Because the short-column method was used, two to three short-length pieces were cut from each full-length lumber specimen for testing. One of the samples was selected as the representative of the lumber in strength properties. Two samples were sawn from each [I.sub.c] grade specimen, one containing MSRC and the other with clearwood. The compression strength of each [I.sub.c] grade specimen was derived from the lower strength value of the two samples. For grades [II.sub.c], [III.sub.c], and [IV.sub.c], three short-length samples were taken for each full-length specimen, one containing MSRC, the second with minor defects, and the third with clearwood. The compression strength was determined from the lowest strength value of the three samples.

The annual ring width (ARW) and density for each specimen were measured near the rupture location according to Chinese national standards GB/ T1930-2009 (SAC 2009a) and GB/T1933-2009 (SAC 2009b).

Data processing

The size effect on the strength of lumber is based on the weakest link theory (Madsen 1992). The size effect on the strength using the brittle fracture theory is described as a relation between the volume and strength of two members. This relation is given by

[x.sub.1]/[x.sub.2] = [([V.sub.2]/[V.sub.1]).sup.s] = [([V.sub.1]/[V.sub.2]).sup.-s] (1)

where [x.sub.1] and [x.sub.2] are the strengths of members of volumes [V.sub.1] and [V.sub.2], respectively, and s is the size effect parameter. The change in strength for doubling the volume can be obtained by setting [V.sub.2]/[V.sub.1] = 0.5. If s becomes greater, the effect of doubling the volume becomes severe, and for s = 0.3, only 81 percent of the strength remains.

[FIGURE 6OMITTED]

In general, there are three methods to obtain estimates of the size effects from experiments: the slope method, the shape parameters, and the fracture position (Madsen 1992). The slope method was used in this study. With the slope method, Equation 1 can be rearranged to give a linear relation between the logarithm of strength and the logarithm of volume:

[1n [x.sub.2] - ln [x.sub.1]] / [1n [V.sub.2] - 1n [V.sub.1]] = -s (2)

where s is the size effect parameter, which is the slope of the regression line of x on V (disregarding the negative sign).

The ASTM 1990 (ASTM International 2007) standard indicates that the length effect parameter is zero for compression strength parallel to the grain and that the thickness effect parameter is zero for MOR, tension strength parallel to the grain, and compression strength parallel to the grain.

Results and Discussion

Mechanical properties of Chinese larch dimension lumber

The ARW and the density at 12 percent MC were 1.47 mm and 0.64 g/[cm.sup.3], respectively. The nonparametric 5th percentile of strength distributions was obtained according to ASTM D2915-10 (ASTM International 2010). Summaries of the mechanical properties for Chinese larch are shown in Tables 2 through 4. Figures 2 through 10 show cumulative probability distributions of mechanical strength of the test specimens.

It is noted that [I.sub.c] grade lumber has higher mechanical properties than [II.sub.c], [III.sub.c], and [IV.sub.c] grade lumber. In some cases, [III.sub.c] grade lumber is stronger than [II.sub.c] grade lumber, even though the latter should have smaller knot sizes according to grading rules in Chinese national standard GB50005-2003. This is also found in test data of mechanical properties of Canadian coastal Douglas-fir (Pseudotsuga menziesii) and hem-fir (Tsuga spp. and Abies amabilis) (Chen et al. 2009).

Size effect on strength properties of Chinese larch dimension lumber

The ASTM D1990-07 standard (ASTM International

2007) indicates that the property values of all lumber test data should be adjusted to the characteristic size. The values of strength properties in the characteristic size should also be adjusted to the actual size for the application of lumber in buildings. However, the size adjustment on strength properties has not been specified in the Chinese standards. Madsen and Buchanan (1986) showed species dependence of the size effect. Therefore, the size factor in ASTM D1990, which was derived from North American--grown species of Douglas-fir, hem-fir, southern pine (Pinus palustris Mill., Pinus echinata Mill., Pinus taeda L., Pinus elliottii Engelm., Pinus rigida Mill, and Pinus serotine Michx.), and sprucepine--fir (Picea spp., Pinus spp. and A lies spp.), may not be appropriate for adjusting strength properties of Chinese larch lumber.

Based on all test data, the size effects on the bending, tensile, and compression strengths of visually graded Chinese larch lumber were analyzed. For each lumber size, the test data of [I.sub.c] grade lumber was the high-grade (FI) group, and the test data of [II.sub.c] and [III.sub.c] grade lumber was the low-grade (L) group. The size effects of 5th percentile of strength properties for each sample group were estimated by the slope method (Zhou et al. 2010). The effect parameters were calculated by the least-squares method as shown in Figures 11 through 13. The parameters and the [R.sup.2] values are summarized in Table 5. With the increase of the effect parameters, the shape of these curves becomes steeper. The combined length and width size effect parameters for the H and L groups were 0.21 and 0.23, respectively, for the bending strength. The estimated width effect parameters were 0.29 for the FI group and 0.33 for the L group for the ultimate tensile parallel-to-grain strength. The estimated width effect parameters for the H and L groups were 0.12 and 0.20, respectively, for the ultimate compression parallel-to-grain strength. The size effect parameters for the H groups were close to the recommended size effects in ASTM D1990, whereas those for the L groups were consistently larger. The results also showed the difference in lumber characteristics between North American--grown softwood species and Chinese-grown softwood species. The size effect parameters of the L group were found to be larger than those for the H group. The observed differences in size effects between grades may reflect that the character, frequency, and size of characteristics are different for each grade. The H group has fewer strength-reducing characteristics, such as knots and other growth characteristics, than the L group.

Conclusions

The properties of interest are the mean MOE and the 5th percentiles of bending strength, tensile strength, and compression strength. The characteristic values of mechanical properties of 40 by 65-mm, 40 by 90-mm, and 40 by 140-mm Chinese larch dimension lumber with four visual grades are presented. The [I.sub.c] grade lumber always shows higher mechanical properties than the [II.sub.c], [III.sub.c], or [IV.sub.c] grade lumber. Some strength properties of IIC grade lumber are not consistently higher than those of [III.sub.c] or [IV.sub.c] grade lumber. These characteristic values may be further modified for a standard size in order to be converted into the allowable design stresses for normal loading conditions.

The data developed in this study will be used to make technical submissions to national building code committees for determination of the design properties assigned to Chinese larch. Code submissions prepared for MOHURD. will include consideration of size factors appropriate for 40-mm-thick structural lumber and design properties appropriate for limit states design codes.

Acknowledgment

The authors are grateful for the support of the Chinese Academy of Forestry (Research Grants 2015BAD14B05 and CAFINT2012C01).

Literature Cited

ASTM International. 2007. Standard practice for establishing allowable properties for visually-graded dimension lumber from in-grade tests of full-size specimens. ASTM D1990-07. ASTM International, West Conshohocken, Pennsylvania.

ASTM International. 2009. Standard test methods of static tests of lumber in structural sizes. ASTM D198-09. ASTM International, West Conshohocken, Pennsylvania.

ASTM International. 2010. Standard practice for evaluating allowable properties for grades of structural lumber. ASTM D2915-10. ASTM International, West Conshohocken, Pennsylvania.

Bodig, J. 1977. Bending properties of Douglas-fir-larch and hem-fir dimension lumber. Special Report 6888. Department of Forestry and Wood Science, Colorado State University, Fort Collins. 59 pp.

Chen, Y., J. D. Barrett, and F. Lam. 2009. Mechanical properties of Canadian coastal Douglas-fir and hem-fir. Forest Prod. J. 59(6):44-54.

Editorial Committee. 2005. The Design Manual for Timber Structures. China Architecture & Building Press, Beijing. (In Chinese.)

Madsen, B. 1992. Structural behavior of timber. Timber Engineering Ltd., North Vancouver, Canada.

Madsen, B. and A. H. Buchanan. 1986. Size effects in timber explained by a modified weakest link theory. Can. J. Civil Eng. 13(2):218-232.

Ministry of Housing and Urban-Rural Development (MOFIURD). 2005. Chinese design code for timber construction. GB50005-2003. MOHURD, Beijing. (In Chinese.)

National Lumber Grades Authority (NLGA). 2005. NLGA standard grading rules for Canadian lumber. NLGA, Burnaby, Canada.

Standardization Administration of the People's Republic of China (SAC). 2009a. Test method for ring width of small clear wood. GB/ T1930-2009. SAC, Beijing. (In Chinese.)

Standardization Administration of the People's Republic of China (SAC). 2009b. Test method for density of small clear wood. GB/ T1933-2009. SAC, Beijing. (In Chinese.)

Zhao, X., J. X. Lu, and J. H. Jiang. 2009. Study on characteristic values of Chinese larch dimension lumber bending properties. China Wood Ind. 23(6): 1-4. (In Chinese.)

Zhao, X., J. X. Lu, and J. H. Jiang. 2010. Relationship between modulus of rupture and ultimate tension strength of dimensional larch lumber. China Wood Ind. 24(4): 1-4. (In Chinese.)

Zhou, H. B., J. X. Lu, and W. T. Xu. 2012. Establishing a standard system for structural wood products in China. China Wood Ind. 26(3):44-47. (In Chinese.)

Zhou, H. B., H. Q. Ren, L. X. Lu, J. H. Jiang, and X. S Wang. 2010. Length effect on the tension strength between mechanically graded high- and low-grade Chinese fir lumber. Forest Prod. J. 60(2): 144-149.

Zhou, H. B. and W. T. Xu. 2012. The industry status and standardization of engineered wood products. Constr. Sci. Technol. 2012(3):37-39. (In Chinese.)

The authors are, respectively, Associate Professor, Professor, and Professor, Research Inst, of Wood Industry, Chinese Academy of Forestry, Beijing, People's Republic of China (zhouhb@caf.ac.cn [corresponding author], renhq@caf.ac.cn, jianxiong@caf.ac.cn). This paper was received for publication in October 2012. Article no. 12-00110.

doi: 10.13073/FPJ-D-12-00110

Table 1.--Chinese larch lumber sample sizes.

Dimension (mm) (a)   No. of specimens

40 by 65 by 4,000         2,705
40 by 90 by 4.000         3,357
40 by 140 by 4,000        1,484

(a) Thickness by width by length.

Table 2.--Summary of mechanical properties of Chinese larch obtained
using third-point bending test. (a)

                           40 by 65 by 4,000 mm  40 by 90 by 4,000 mm

                           MOE (GPa)  MOR (MPa)  MOE (GPa)  MOR (MPa)

[I.sub.c] visual grade
  Count                     212        212        458        458
  Mean                       16.19      83.48      15.12      73.10
  SD                          2.85      28.66       2.88      23.82
  5th percentile             11.65      39.55      10.16      38.38
[II.sub.c] visual grade
  Count                     109        109        205        205
  Mean                       14.43      63.98      12.92      51.75
  SD                          2.57      24.74       2.40      20.28
  5th percentile             10.28      29.51       9.09      25.85
[III.sub.c] visual grade
  Count                     261        261        290        290
  Mean                       14.16      65.72      13.49      57.11
  SD                          2.84      25.90       2.96      21.58
  5th percentile              9.69      29.44       8.78      26.57
[IV.sub.c] visual grade
  Count                     280        280        168        168
  Mean                       13.47      58.97      13.00      55.18
  SD                          3.14      26.04       3.07      24.55
  5th percentile              8.38      22.38       8.45      19.57

                           40 by 140 by 4,000 mm

                           MOE (GPa)  MOR (MPa)

[I.sub.c] visual grade
  Count                     261        261
  Mean                       15.00      61.43
  SD                          2.67      22.12
  5th percentile             10.77      28.93
[II.sub.c] visual grade
  Count                      28         28
  Mean                       13.54      40.72
  SD                          2.34      18.62
  5th percentile             10.03      14.22
[III.sub.c] visual grade
  Count                     136        136
  Mean                       14.99      54.31
  SD                          3.03      23.22
  5th percentile             10.67      24.17
[IV.sub.c] visual grade
  Count                      64         64
  Mean                       15.43      55.00
  SD                          2.98      21.21
  5th percentile             10.56      22.62

(a) MOE = modulus of elasticity; MOR = modulus of rupture.

Table 3.--Summary of mechanical properties of Chinese larch
obtained using tension parallel to the grain test.

                                   Tensile strength (MPa)

                            40 by 65      40 by 90      40 by 140
                           by 4,000 mm   by 4,000 mm   by 4,000 mm

[I.sub.c] visual grade
  Count                      212           501           260
  Mean                        46.66         42.66         35.65
  SD                          17.62         16.01         13.53
  5th percentile              20.85         20.76         16.80
[II.sub.c] visual grade
  Count                      102           212            26
  Mean                        30.60         26.48         24.81
  SD                          12.21          8.60          7.43
  5th percentile              12.49         14.35         14.11
[III.sub.c] visual grade
  Count                      271           319           139
  Mean                        32.92         33.41         31.69
  SD                          16.06         13.08         14.79
  5th percentile              13.91         13.88         12.64
[IV.sub.c] visual grade
  Count                      283           171            64
  Mean                        27.60         30.63         34.41
  SD                          13.65         17.81         14.53
  5th percentile              11.39          9.07         13.01

Table 4.--Summary of mechanical properties of Chinese larch
obtained using compression parallel to the grain test.

                                  Compression strength (MPa)

                             40 by 65      40 by 90      40 by 140
                            by 4,000 mm   by 4,000 mm   by 4,000 mm

[I.sub.c] visual grade
  No. of samples              209           416           262
  Mean                         55.53         53.66         46.01
  SD                           10.45         11.34          9.05
  5th percentile               37.01         37.40         31.73
[II.sub.c] visual grade
  No. of samples              209           205            29
  Mean                         44.16         42.40         38.05
  SD                            6.89         11.83          6.48
  5th percentile               33.18         28.95         24.19
[III.sub.c] visual grade
  No. of samples              272           266           142
  Mean                         45.92         45.73         41.86
  SD                           10.57         13.53         10.50
  5th percentile               30.89         25.89         24.77
[IV.sub.c] visual grade
  No. of samples              285           146            73
  Mean                         40.71         43.33         42.04
  SD                           12.19         13.60         11.41
  5th percentile               23.78         24.53         22.17

Table 5.--Summary of size effect parameters and [R.sup.2] values.

                                              Longitudinal
                                              section area

                              Visual       Size
Strength properties          grade (a)   parameter   [R.sup.2]

Bending strength                 H         0.21        0.88
                                 L         0.23        0.99
Tension strength parallel        H          --          --
  to the grain                   L          --          --
Compression strength             H          --          --
parallel to the grain            L          --          --

                                                 Width

                              Visual       Size
Strength properties          grade (a)   parameter   [R.sup.2]

Bending strength                 H          --          --
                                 L          --          --
Tension strength parallel        H         0.29        0.83
  to the grain                   L         0.33        0.82
Compression strength             H         0.12        0.55
parallel to the grain            L         0.20        0.99

(a) H = high; L = low.
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Author:Zhou, Haibin; Ren, Haiqing; Lu, Jianxiong
Publication:Forest Products Journal
Geographic Code:9CHIN
Date:Jan 1, 2016
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