Study on the estimation of the strength properties of structural glued laminated timber I: determination of optimum MOE as input variable

There have been many attempts to predict the performance of glulam beams. Several approaches have been taken, from early empirical techniques to more sophisticated stochastic methods. In recent years, more emphasis has been placed on the modeling of material properties. Generally, the modulus of elasticity (MOE) has been used as a criterion of laminar strength for the prediction of glulam performance in the traditional models. Most of the current models are based on MOE that was measured using the long span test; that is, they account only for variability between pieces of lumber. Therefore, these models do not account for the variation of material properties within a given piece of lumber. Five methods were considered to choose the appropriate one that could effectively predict the performance of glulam in this study. Prediction of glulam performance was done by the transformed section method. MOEs measured with the five methods were applied to a strength prediction program to compare the actual test results and the predicted results. MOEs used as input variables are as follows: long span MOE of the static bending test, localized MOE of the static bending test, long span MOE of the stress wave test, localized MOE of the stress wave test, and MOE of the machine stress rating (MSR) test. Results of the localized test showed excellent signification compared to those of the long span test. The MSR method, when used as input variable, obtained the most approximate result, so it is considered adequate for predicting the strength of glulam.An outline of this paper was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

Stochastic model for predicting the bending strength of glued-laminated timber based on the knot area ratio and localized MOE in lamina

The aim of this study was to develop a stochastic model for predicting the bending strength distribution of glued-laminated timber (GLT). The developed model required the localized modulus of elasticity (MOE) and tensile strengths of laminae as input properties. The tensile strength was estimated using a regression model based on the localized MOEs and knot area ratios (KAR) which were experimentally measured for lamina grades samples. The localized MOE was obtained using a machine stress-rated grader, and the localized KAR was determined using an image-processing system. The bending strength distributions in four types of GLTs were simulated using the developed GLT beam model; these four types included: (1) GLT beams without finger joints; (2) GLT beams with finger joints; (3) GLT beams with different lamina sizes; and (4) GLT beams with different combinations of lamina grades. The simulated bending strength distributions were compared with actual test data of 2.4 and 4.8 m-long GLTs. The Kolmogorov–Smirnov goodness-of-fit tests showed that all of the simulated bending strength distributions agreed well with the test data. Especially, good agreement was shown in the fifth percentile point estimate of bending strength with the difference of approximately 1%.     

Prediction of bending properties for structural glulam using optimized distributions of knot characteristics and laminar MOE

This study established a prediction model for bending properties of glued-laminated timber (glulam) using optimized knot and modulus of elasticity (MOE) distributions of lumber laminate as the main input variables. For this purpose, knot and MOE data were investigated for all pieces of lumber that were prepared for glulam manufacturing, and statistical distributions of knot size, knot number in one lumber, and MOE of each laminate were optimized as distribution functions. These knot and MOE data were used as input variables in the prediction model for bending properties, and were also used in generating virtual glulam using the inverse transform method. Prediction of bending properties for glulam was carried out using the transformed section method, which is partially provided in ASTM D 3737 (Annex A4). Predicted values were compared with those from full-scale four-point bending tests for 60 six-layered glulams with 10 different laminar combinations. Finally, the allowable bending properties of glulam for each specific laminate combination were determined by calculating the fifth percentile of the modulus of rupture and the average modulus of elasticity from virtual test results of more than 1000 virtual glulams. From the results of this study, predicted bending properties for glulam and their distributions could be used for structural design in both allowable stress design and limit state design.     

Fresh-stem bending of silver fir and Norway spruce

The bending and growth characteristics of large fresh stems from four silver fir (Abies alba Mill.) and three Norway spruce (Picea abies (L.) Karst.) trees were studied. Twenty logs taken from different stem heights were subjected to four-point bending tests. From the bending test records, we calculated stress-strain curves, which accounted for detailed log taper, shear deformation and self weight. From these curves we determined, among other parameters, the modulus of elasticity (MOE), the modulus of rupture (MOR) and the work absorbed in bending (W). No significant differences were found between species for the wood properties examined. Values of MOE, MOR and W generally decreased with stem height, with MOR in the range of 43 to 59 MPa and MOE ranging from 10.6 to 15.6 GPa. These MOE values are twice or more those reported for stems of young Sitka spruce (Picea sitchensis (Bong.) Carr.) trees. Based on the radial growth properties measured in discs from the logs, we calculated predicted values of MOE and MOR for the stem cross section. The predictions of MOE were precise, whereas those of MOR were approximate because of a complex combination of different failure mechanisms. Methods to test and calculate MOE, MOR and W for the stems of living trees are discussed with the aim of improving analyses of tree biomechanics and assessments of forest stability protection.     

Prediction of oriented strand board properties from mat formation and compression operating conditions. Part 2: MOE prediction and process optimization

A model to predict the bending modulus of elasticity (MOE) of oriented strand board (OSB) panels produced by batch processing is presented. The approach developed herein is unique in its comprehensiveness since the MOE is determined from information on the panel structure, temperature and moisture profiles and vertical density profiles obtained from the mat formation and compression models presented in Part 1. Comparison of predicted MOE values with those measured from 24 commercially produced panels shows good agreement considering some of the uncertainties involved. Simulations show that the MOE can be increased by any of the following changes: reduced fines content, increased panel density, better flake alignment in each of the three layers within a panel, increased flake length and a larger difference between the density of the face and core layers. The model was also used in a genetic algorithm to carry out an optimization study of batch OSB manufacturing. This analysis showed that by combining the appropriate reduction in the amount of flakes used, increase in fines content, improvement in flake alignment within each of the face and core layers and shortening of the batch time, a significant theoretical profit increase from the base case scenario can be obtained.     

On the frequency dependence of the modulus of elasticity of wood

 This short note reviews the reasons for the frequency dependence of the Modulus of Elasticity, MOE, of wood. It has in fact been reported in several publications on wood that depending on the technique used in the test experiment, the value of the MOE depends to some degree on the frequency at which it is evaluated. The frequency ranges used are namely zero frequency in the case of static bending, audio frequencies when using mechanical vibrations or sound radiation and finally ultrasonics. The results from implementing these three different techniques show that the lowest value that may be obtained for the MOE occurs when using the static mode, and thereafter increases with increasing frequency. This property of increasing dynamic MOE with frequency is shared by all solid materials, and finds its theoretical explanation in the Kramers-Kronig relations. Dispersion in conjunction with the notion of complex MOE permit to establish the relation between the real and the imaginary components of the MOE, i.e. respectively the dynamic and loss moduli. Due to the mathematical difficulties encountered in using the exact expressions, approximations are necessary for applications in practical situations. Hence, an improved version of the Zener model for viscoelasticity, which has lately been proposed by Pritz (1999), is presented. With some assumptions, and under which excellent agreement has been obtained with the exact theory, this model is used for predicting the viscoelastic properties of wood. Received 2 October 2000     

竹材复合板的试件尺寸对静曲强度及弹性模量测试值的影响

采用二次正交旋转组合设计试验,研究了竹材复合板静曲试件的宽厚比、跨厚比对静曲强度及弹性模量测试值的影响。结果表明：在试验研究范围内,试件的宽度不影响静曲强度、弹性模量的检测结果;试件长度（或试验跨距）对静曲强度、弹性模量的测试值影响显著;合理的跨厚比为２５左右。为保证试件的代表性及在弯曲过程中的稳定性,建议试件宽度为５０ｍｍ左右。     

Modeling rot in wood by replacement of wood with sand: an experimental study

Rot is known to affect the strength properties of wood. At the same time, the damping properties of the attacked material have also been shown to change. This article presents the results of an experimental study in which rot in wood was modeled by the replacement of wood with sand. The procedure entailed the drilling of holes in the body of a wooden beam, filling the holes with sand, and monitoring the changes induced by the sand-filled holes on the values of the modulus of elasticity (MOE) and of the loss factor. The MOE was calculated from the resonance frequency of the first longitudinal mode of vibration, and the loss factor was obtained indirectly from the impulse response by means of a room acoustical technique. The results show that the MOE value, and hence the strength characteristic of the wood specimen, decreases at the same time as the loss factor increases.     

Effects of density profile of MDF on stiffness and strength of nailed joints

Nail-head pull-through, lateral nail resistance, and single shear nailed joint tests were conducted on medium density fiberboard (MDF) with different density profiles, and the relations between the results of these tests and the density profiles of MDF were investigated. The maximum load of nail-head pull-through and the maximum load of nailed joints were little affected by the density profile. However, the ultimate strength of lateral nail resistance, the stiffness, and the yield strength of nailed joints were affected by the density profile of MDF and showed high values when the surface layer of the MDF had high density. It is known that bending performance is also influenced by density profile. Therefore, the stiffness and the yield strength of nailed joints were compared with the bending performance of MDF. The stiffness of nailed joints was positively correlated with the modulus of elasticity (MOE); in the case of CN65 nails, the initial stiffness of joints changed little in response to changes in MOE. The yield strength of nailed joints had a high positive correlation with the modulus of rupture (MOR). The stiffness and the yield strength of nailed joints showed linear relationships with MOE and MOR, respectively.     

Effect of mat structure on modulus of elasticity of oriented strandboard

A model to predict bending stiffness of oriented strandboard (OSB) was tested with pilot plant experimental data. The experimental procedure developed in this study is unique in that it allows the model to be tested for extensive vertical configurations of strand angle distribution. After validation, the model was used to simulate a typical three-layer cross-oriented OSB panel with a vertical density profile and strand angle distribution measured on industrial panels. Analysis of the simulated vertical distribution of modulus of elasticity (MOE) indicated that the layers near the panel surfaces contributed much more to the effective parallel panel MOE than those close to the panel thickness center, with 80% of parallel MOE coming from the top 41% of weight and 32% of thickness. The effectiveness of methods to increase parallel bending stiffness through improving mat structure was evaluated. Increasing face/core weight ratio from 54/46 to 66/34 resulted in a 3.7% increase in simulated parallel MOE. Alignment of strands in face layers was identified having a greater potential to increase parallel MOE. Simulations with three improved strand angle distributions showed gains of 5.7, 12.0 and 19.8% in parallel MOE compared with a typical strand angle distribution of industrial OSB panels.     

Properties of strand boards with uniform and conventional vertical density profiles

Randomly oriented strand boards with both uniform and conventional vertical density profiles (VDP) were manufactured, and their properties were evaluated and compared. The bending modulus of elasticity (MOE) of conventional strand boards was predicted using the laminated beam theory and the MOE-density regression equation from the uniform strand boards. The results showed that the predicted MOE of conventional strand boards was close to the measured MOE with a difference of less than 10%. The internal bond strength values of uniform strand boards were found to be higher than conventional strand boards while no significant difference was found in water-related properties. Compared with uniform strand boards, MOE values of conventional strand boards were improved only at higher density level. About 10% of improvement in MOE can be obtained for the strand boards investigated by manipulating the VDP. Steeper VDPs were predicted to be required for thinner boards than for thick boards in order to achieve the same improvement in MOE.     

Index selection for growth and construction wood properties in Pinus elliottii open-pollinated families in southern China

The initial introduction of Pinus elliottii (PEE) to China occurred in the 1930s, and the planting of this conifer species has now attained close to 3 million ha in the subtropical zone of southern China. A large-scale genetic improvement program for PEE was implemented in southern China to produce fast-growing trees with high wood quality to address the severe shortage of timber production over the last two decades. In this paper, selection for stem volume, basic wood density (DEN) and modulus of elasticity (MOE) was based on the Smith–Hazel index, and a total of approximately 2 000 individual trees from 158 PEE open-pollinated families were selected at 22 years of age. The DEN and MOE for each tree were determined by non-destructive evaluation techniques using the Pilodyn and Hitman Director ST300® acoustic velocity device. The heritabilities and genetic and phenotypic correlations for the traits that were measured were estimated using the residual maximum likelihood approach in the flexible mixed modelling program ASReml-R. The results showed that the heritability estimates for the wood properties were between 0.292 and 0.309, and the heritabilities of the growth traits ranged from 0.129 to 0.216. The genetic correlation between the DEN_P and acoustic velocity (V?) with MOE_P was 0.45 and 0.95, respectively. An indirect selection based on V was observed to be highly effective for determination of MOE. It indicated that V can be integrated into tree improvement programs as a useful index of MOE by ranking candidate families or individuals within the selection population. The genetic correlations between the growth traits and wood properties were not significant. By contrast, the phenotypic correlations between them were significantly positive, but the correlation coefficients were very low. The appropriate selection index (I₄), which placed 10 times as much weight on DEN and MOE as the equal emphsis method, was determined as the appropriate selection index.     

新型轻质家具材料-木质材料强化蜂窝纸芯复合板的研制

选用木质材料强化蜂窝纸芯中蜂窝单元的边长、纤维板支撑条的厚度以及强化蜂窝纸芯的拉伸率作为试验因子,探讨这三个因子对复合板力学性能的影响。试验结果表明,蜂窝单元的边长对板材MOR、MOE和平面抗压强度有显著性影响,支撑条厚度对MOE有显著性影响,拉伸率对板材力学性能无显著性影响。     

慈竹竹帘胶合板力学性能研究

采用慈竹为原料制造竹帘胶合板,以三种不同的方式进行组坯,研究组坯方式对慈竹竹帘胶合板纵横方向静曲强度、弹性模量、压缩强度与水平剪切强度的影响。结果表明:组坯方式对胶合板的弹性模量与静曲强度影响较为显著。Ⅲ型板纵向各项力学性能最优,Ⅲ型板横向各项力学性能最弱。Ⅰ型板和Ⅱ型板的静曲强度和弹性模量均达到了汽车车厢用竹篾胶合板的A类标准。三种方式组坯板件的主要力学性能均达到了结构用竹木复合板国家A级标准与混凝土模板用胶合板主要物理力学性能指标。     

杉木材性株内变异的研究Ⅰ.木材力学性质和木材密度

对１５株浙江产杉木株内不同高度和圆周不同方位上木材的抗弯强度、抗弯弹性模量、顺纹抗压强度和木材密度的差异，木材密度的径向变异模式和木材力学性质与木材密度的相关关系进行了测定和分析。主要结果是：抗弯强度和抗弯弹性模量在株内不同高度上差异特别显著；顺纹抗压强度和木材密度未表现出显著差异；在圆周不同方位上，三项力学性质和木材密度均为南北向高于东西向，差异不显著；木材密度径向变异模式在不同高度和不同方位上均为接近水平有一定波动的直线；三项力学性质与木材密度的相关关系在不同高度和不同方位上均特别显著，但不同力学性质与木材密度的相关系数有明显差异，不同高度上和圆周不同方位上，亦有差异。     

Prediction of oriented strand board properties from mat formation and compression operating conditions. Part 1. Horizontal density distribution and vertical density profile

A model is presented to determine the horizontal density distribution (HDD) and vertical density profile (VDP) of oriented strand board (OSB) panels produced by batch pressing. The HDD is simulated using input distributions of flake dimensions and orientation from plant measurements. Many previous HDD models rely on assumed distributions, which may not accurately characterize current manufacturing processes. The model predicts the VDP based on the compression behaviour of cellular materials in combination with temperature and moisture profiles calculated using a previously published heat and mass transport model. A novel empirical approach is applied rather than the time–temperature–moisture superposition method commonly used. The model predictions compare favourably with plant data and exhibit trends similar to previously reported experimental results. This work is the first of a two-part publication. The second part is concerned with stiffness property prediction and an optimization of the OSB manufacturing process. This work is novel in that no comprehensive model including HDD, VDP, stiffness property prediction and optimization has been reported in the literature.     

A comparison of deconvolution techniques to improve MOR estimation from stress grading machine output

Summary Both time and frequency domain techniques to calculate lengthwise local MOE variation in timber, from stress grading machine output, have been examined. Three variants of a frequency domain method were selected for further evaluation with the objective of improving the timber stress grading process. MOR-local MOE correlation coefficients have been obtained by each method for over 50 structural timber specimens. These showed that due to the ill-conditioned nature of the deconvolution process and the inability to differentiate between noise and defect detail, only marginal improvements were possible over the existing technique that uses apparent MOE values calculated directly from suitably filtered machine load readings.This work was supported from contracts EMC 132/88 and EMC 3/91 awarded by the Building Research Establishment, Garston, Watford WD2 7JR.Appreciation is also expressed to R. O. Foschi and F. Lam (University of British Columbia, Canada) who supplied, through A. R. Fewell (Building Research Establishment, U.K.), Fortran routines of development versions of their methods     

Characterization of major,unused, and unvalued Indonesian wood species I. Dependencies of mechanical properties in transverse direction on the changes of moisture content and/or temperature

Mechanical property changes due to the moisture content (MC) and/or temperature changes were examined for 15 Indonesian wood species. A static bending test was carried out at 20°C, 65% relative humidity (air-dry), and water-saturated at 20°C (wet-20) and 80°C (wet-80). For individual test conditions, modulus of elasticity (MOE) and modulus of rupture (MOR) increased linearly with specific gravity regardless of wood species; however, maximum deflection did not correlate with specific gravity for any MC or temperature conditions. The relative values of MOE and MOR measured in wet-20 to air-dry conditions were variously affected from slightly to strongly depending on the wood species. However, the relative values always decreased markedly when saturated in water at 80°C, regardless of wood species. The relative MOE, MOR, and maximum deflection values due to the change in MC or MC and temperature combined were independent of specific gravity but may be dependent on wood type: softwood or hardwood.     

利用FAKOPP评价树干杨氏弹性模量可行性研究

杨氏弹性模量是反映树干或板材抗弯、抗压强度的重要指标,是开展林木材质育种研究的重要内容。笔者通过对多株树分别采用FAKOPP(应力波速度测定机)和敲打法2种不同方法测量同一树干的杨氏弹性模量,并对2种测定数据进行相关性分析,检验用FAKOPP测量立木状态下树干弹性模量的准确性。通过对杨树试验证明:二者具有很强相关性,说明利用FAKOPP的方法能准确评价树干杨氏弹性模量大小。     

结构胶合板几种力学检测方法的探讨

在不同加荷、不同跨距和不同加荷头数量的检测方法下,对杨木结构胶合板的静曲强度和弹性模量进行检测,比较3种方法对应测试结果的可互换性。对结果的分析表明,3种条件下测试的静曲强度值存在明显的相关性,说明试验设定的3种检测方法可以互换;而弹性模量的对比结果则差异显著,相关性较低,表明3种检测方法不存在替代性。