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.     

Measurement of the shear modulus of wood by asymmetric four-point bending tests

We conducted asymmetric four-point bending tests of wood and obtained the shear moduli on the basis of Timoshenko's theory of bending. Akamatsu (Japanese red pine,Pinus densiflora D. Don) and shioji (Japanese ash,Fraxinus spaethiana Lingelsh.) were used for the tests. Asymmetric four-point bending tests were undertaken by varying the depth/span ratios; and Young's modulus and the shear modulus were calculated by Timoshenko's bending theory. Independent of the asymmetric bending tests, we also conducted three-point bending tests, free-freeflexural vibration tests, and numerical calculations by the finite element method. Young's and shear moduli obtained by these methods were compared with those derived from the asymmetric bending tests. Based on these comparisons, we concluded that the shear modulus can be properly obtained by the asymmetric four-point bending tests when the span is 20 times larger than the depth.     

Study on half-scale model test for light-frame shear wall

A half-scale model of a light-frame shear wall was developed to evaluate the racking performance of a fullscale shear wall (prototype). The effect of nail size on the performance of the shear wall was also investigated using models constructed with three types of nail. Materials for the model were determined through experimental methods, which included nail-head push-through, stud-to-sheathing nail connection, and static bending tests. Materials with which the model was made to be in similarity to the prototype were three-layer 4.8-mm plywood, 39.72-mm long nails, and 1 × 2 lumber cut from 2 × 4 studs. In accordance with ASTM E 72 and ASTM E 564, racking resistance tests were conducted on 20 shear walls. The results showed that the maximum load capacities of the prototype walls could be evaluated by the model without significantly different failure modes. Tests on the effect of nail size revealed that increasing the nail head diameter may improve the performance of shear walls.Parts of this paper were presented at the International Conference on Effective Utilization of Plantation Timber (ICEUPT'99), Chi-Tou, Taiwan, May 1999; and the World Conference on Timber Engineering (WCTE2000), Whistler, Canada, July–August 2000     

Influence of knot area ratio on the bending strength of Canadian Douglas fir timber used in Japanese post and beam housing

A study was conducted to establish the engineering properties and the influence of knot area ratio (KAR)-based grading rules on the bending strength properties of full-size Canadian Douglas fir timber used in Japanese post and beam building construction. In-grade tests were conducted on lumber selected at random from coastal mills in British Columbia, Canada, that manufacture products for the Japanese post and beam housing market. Bending strength and modulus of elasticity test results and KAR-based out-turn information on the 105 × 105mm and 45 × 105mm specimens are presented in this article. The in-grade test results indicate that KAR-based grading rules can be successfully applied to Canadian Douglas fir timber to meet strength property requirements.     

Fatigue and creep of chipboard

Structural grade chipboard was subjected to fatigue and creep loads in four-point bending, the peak fatigue stress being equal to the constant creep stress. Peak fatigue stresses of 50, 60, 70 and 80% of the static bend strength were selected and an S-N (stress versus log₁₀ (cycles)) curve was generated. Stress versus strain hysteresis loops were captured automatically throughout fatigue tests so that underlying creep strain, dynamic modulus and energy dissipated per cycle were continuously monitored. The possibility of superimposing creep and fatigue data was investigated.The S-N curve generated at R = 0.01 demonstrates that for lives of less than 10⁷ cycles chipboard does not show a fatigue endurance limit. The 70 and 80% samples experience a gradual decrease in dynamic modulus and an increase in the area of the hysteresis loop during fatigue tests. Samples at the 50 and 60% levels show an initial increase in dynamic modulus before a decline to failure is observed.Creep samples never failed before fatigue samples at the same peak stress level, but until close to the point of failure, creep strains were nearly always greater than fatigue strains on elapsed time. It is concluded that the mechanism of fatigue damage accumulation differs from the mechanism of creep deformation.     

无损检测技术评估规格材的性质

采用两种应力波设备对北美黄杉规格材进行抗弯强度性质的预测,分析规格材破坏模式,并讨论木材节子对规格材强度的影响。测试结果表明,动态与静态测试结果之间的相关程度较高。结构材的破坏模式主要为纹理断裂和脆性断裂;在规格材中间部位节子较多,且节子在规格材横断面上投影面积较大时,易发生脆性断裂。     

Evaluation of the fire endurance of mechanically graded timber in bending

AbstractThis study examined the performance of mechanically graded timber in bending when exposed to fire at various load ratios. The test specimens were 150 pieces, each with the dimensions of 60 × 120 × 3500mm. The modulus of elasticity (MOE) of 150 specimens was measured, and 60 among them were selected to formulate the prediction equation for MOE and modulus of rupture (MOR), which was used to predict the remaining 90 specimens. These were tested under fire exposure in bending using three-point loading at 11.1%, 16.7%, 33.3%, 66.7%, and 83.3% of the ultimate load. Using mechanically graded timber, which means acknowledging the actual strength of the bending member, permits fairly precise application to the targeted design load. This research confirmed that mechanically graded timber under fire exposure has the following tendencies: under the same load ratio, time to failure is independent of strength class, and, at any load ratio, the critical strength is dependent on the timber strength class. The obtained design bending strength under fire exposure using the reduced cross section method and the reduced strength method conformed to those calculated based on Eurocode 5. Following those findings, mechanically graded timber can be applied to obtain the design bending strength when taking into account the fire attack.     

Outdoor exposure tests of structural laminated veneer lumber (II): evaluation of the strength properties after nine years

To investigate the durability of structural laminated veneer lumber (LVL), outdoor exposure tests have been conducted since 1990 at a field-testing site at the Forestry and Forest Products Research Institute. This article is the second interim report on the results after 9 years of exposure. Seven kinds of structural LVL with no preservative treatment were subjected to the tests. Almost all the exposed specimens were decayed by a kind of brown rot fungi (Pseudomerulius aureus (Fr.) Julich). The degree of decay varied with wood species; grand fir and western hemlock LVL in particular showed weak resistance against the decay. All the specimens were stored for more than 1 year in a testing room conditioned at 20°C and 65% relative humidity. We then measured the ultrasonic velocity of the specimens by the Pundit method, penetration depth by the Pilodyn method, and bending strength by a conventional bending test. Correlation between nondestructive measurement factors and the density was strong even on LVL with many adhesive layers. The nondestructive testing method was found to be applicable to LVL as well as solid lumber. After the nondestructive measurements, each LVL was cut into three types of specimen (top: T, middle: M, and bottom: B) for the bending tests. The bending strength varied with the type of specimens. Correlation between modulus of elasticity and modulus of rupture was strong even in the decayed specimens.     

Duration of load revisited

A duration of load study representing 13 years of testing was recently terminated. Preliminary results have been published over the years. This paper represents the final account of the study, which was focused on the influence of moisture content on time to failure for structural timber subjected to bending under constant load conditions. Two constant moisture conditions (MC = 11 and 20%) and one condition of varying moisture (MC between 11 and 20%) were applied. A total of 816 Norway spruce boards of dimensions 44 × 95 × 1,800 mm³ were included. Eight groups of non-destructively matched samples were formed. Four groups were subjected to short-term strength tests, and four groups were subjected to long-term tests. Creep and time to failure were monitored. Time to failure as a function of stress level was established and the reliability of stress level assessment was discussed. A significant mechanosorptive effect was demonstrated both in terms of increased creep and shortening of time to failure. The test results were employed for the calibration of four existing duration of load models. The effect of long-term loading was expressed as the stress level SL₅₀ to cause failure after 50 years of loading. SL₅₀ was found to be of the order 0.60 for MC = 11%, 0.50 for MC = 20% and 0.44 for MC varying between 11 and 20%. The test results revealed no evidence of a threshold stress level. A reliability based calibration of load-duration factors was performed using probabilistic models of loads and of the short-term and long-term strengths. For permanent and imposed library loads, reliability-based estimation of the load duration factor gave almost the same results as direct, deterministic calibration.     

Prediction of bending stiffness and moment carrying capacity of sugi cross-laminated timber

Cross-laminated timber (CLT) panels consist of several layers of lumber stacked crosswise and glued together on their faces. Prototype sugi CLT floor panels were manufactured and bending tests were carried out under the different parameters of lumber modulus of elasticity (MOE), number of layers, thickness of lumber and thickness of CLT panels. On the basis of above tests, bending stiffness and moment carrying capacity were predicted by Monte Carlo method. MOE of lumber was measured by using grading machine and tensile strength of lumber was assumed to be 60 % of bending strength based on the obtained bending test. Bending stiffness EI of CLT panels could be estimated by adopting composite theory and equivalent section area. Experimental moment carrying capacity showed 12 % higher value than the calculated moment carrying capacity by average lumber failure method, and also showed 45 % higher value than the calculated moment carrying capacity by minimum lumber failure method due to the reinforcement of the outer layer by the neighboring cross layer.     

Near-infrared spectroscopy prediction of southern pine No. 2 lumber physical and mechanical properties

This study investigated near-infrared spectroscopy (NIRS) to rapidly estimate physical and mechanical properties of No. 2 2 × 4 southern pine lumber. A total of 718 lumber samples were acquired from six mills across the Southeast and destructively tested in bending. From each piece of lumber, a 25-mm-length block was cut and diffuse reflectance NIR spectra were collected from the transverse face using a FOSS 5000 scanning spectrometer. Calibrations were created using partial least squares (PLS) regression and their performance checked with a prediction set. Overall moderate predictive ability was found between NIRS and the properties for the calibration and prediction sets: block specific gravity (SG) (R ² = 0.66 and R _p ²  = 0.63), lumber SG (0.54 and 0.53), modulus of elasticity (MOE) (0.54 and 0.58), and modulus of rupture (MOR) (0.5 and 0.4). Model performance for MOE (R _p ²  = 0.70) and MOR (R _p ²  = 0.50) improved when performing PLS regression on a matrix containing lumber SG and NIR spectra. Overall NIRS predicted MOE better than linear models using lumber SG (R ² = 0.46), whereas lumber SG (R ² = 0.51) predicted MOR better than NIRS. Overall NIRS has reasonably good predictive ability considering the small volume of wood that is scanned with the instrument.     

Effect of load type on failure mechanisms of spruce in compression parallel to grain

Failure mechanisms of small clear specimens (6×6×24 mm) of air-dried black spruce (Picea mariana) under parallel-to-grain compression were investigated by polarised-light microscopy. Fatigue load was used with a peak stress level of 90% static strength, a load frequency of 0.5 Hz, and a square waveform with a duty ratio of 0.50. Matched pure creep and static load tests were carried out. Damage was quantified in terms of the permanent microstructural changes (kinks) in tracheid walls. In static load tests, kinks develop quickly with any increase in stress beyond the limit of proportionality. In creep tests, damage develops mainly from kinks formed during the initial load application. In fatigue tests, damage develops both from kinks formed during the initial load cycle, and kinks formed during subsequent cycles. The number of kinks exhibits a strong relationship with relative cyclic creep or relative creep.     

Estimating Strength of Finger-jointed Dimension Lumber by Nondestructive Methods

Structural finger-jointed (FJ) lumber was used mainly in structural applications including glue-laminated beams and wooden I-joists and more recently in parallel chord wood trusses. The paper evaluated strength properties of structural FJ lumber by three nondestructive methods (edge-wise bending, longitudinal and transversal vibration) in order to find an alternative to traditional evaluation methods. Lumber was sawn from the logs following a pattern typically used in China to maximize the volume of recover...     

Measurement of bending properties of wood by compression bending tests

We measured Young's modulus, proportional limit stress, and bending strength by the compression bending test and examined the applicability of the testing method by comparing it with conventional bending test methods. Long columns of todomatsu (Japanese fir,Abies sachalinensis Fr. Schmidt) with various length/thickness ratios were the specimens. A compressive load was axially applied to the specimen supported with pin ends. Young's modulus, the proportional limit stress, and the bending strength were obtained from the load-loading point displacement and load-strains at the outer surfaces until the occurrence of bending failure. Four-point bending tests were also conducted, and the bending properties obtained were compared with the corresponding properties obtained by the compression bending tests. Based on the experimental results, we believe that when the stress-strain relation is measured by the load-loading point displacement relation using specimens whose length/thickness ratio is large enough, the bending properties can be obtained properly using the compression bending test.     

Feasibility of near-infrared spectroscopy for online multiple trait assessment of sawn lumber

Near-infrared (NIR) spectroscopy coupled with multivariate analysis was applied to estimate multiple traits of sawn lumber. The effects of the lumber conveying speed (LCS) and measurement resolution of spectra (MRS) on the calibrations were examined. NIR spectra ranging from 1300 to 2300 nm were acquired at LCSs of 10, 20, and 30 m/min and at MRSs of 2, 4, and 16 nm. Prediction models of bending strength (F _b), modulus of elasticity in bending tests (E _b), dynamic modulus of elasticity (E _fr), and wood density (DEN) were developed using partial least-squares (PLS) analysis. LCS and MRS did not significantly influence the calibration performance for any wood property. The regression coefficients also showed no clear differences for any of the conditions. This indicates that the important explanatory variables included in the models are not greatly influenced by these measurement conditions. PLS2 analysis results, when presented graphically, allowed easy interpretation of the relationships between wood mechanical properties and chemical components, e.g., bending strength and stiffness were mainly related to polysaccharides cellulose and hemicellulose. NIR spectroscopy has considerable potential for online grading of sawn lumber, despite the harsh measurement conditions.     

Accuracy of shear properties of wood obtained by simplified Iosipescu shear test

We examined the accuracy of the shear properties of wood by the Iosipescu shear test using specimens whose shape was simplified. Quartersawn boards of sitka spruce (Picea sitchensis Carr.) and shioji (Japanese ash,Fraxinus spaethiana Lingelsh.) were used. Two types of specimen for the Iosipescu shear test were compared: a standard specimen whose notch angle is 90° and a keyhole type specimen, which is more easily prepared than the standard type. The shear modulus, yield shear stress, and failure shear stress of the keyhole-type specimen were compared to those of the standard specimen. Shear stress analysis was conducted using the finite element method (FEM). The results obtained were as follows: (1) The failure pattern obtained by the simplified Iosipescu shear test was similar to that seen with the standard Iosipescu shear test. (2) The shear modulus, yield stress, and failure stress obtained by the simplified Iosipescu shear test coincided with those by the standard Iosipescu shear test. (3) The principal strain angle and principal stress angle of the simplified Iosipescu shear test were about 45°. (4) It is recognized that pure stress is applied to the strain-gauge regions in the simplified Iosipescu shear test, and it is expected that the shear properties are independent of the notch angle.     

Dynamic modulus of elasticity and bending properties of young Taiwania trees grown with different thinning and pruning treatments

The effects of different thinning and pruning methods on the bending strength and dynamic modulus of elasticity (DMOE) of young Taiwania (Taiwania cryptomerioides Hay) were investigated. The average DMOE, modulus of elasticity (MOE), and modulus of rupture (MOR) in the thinning treatments showed the following trend: no thinning > medium thinning > heavy thinning. This indicates that thinning reduces average bending properties. The average DMOE, MOE, and MOR in the pruning treatments showed the following trend: medium pruning > no pruning > heavy pruning. According to this tendency, better average qualities of lumber and specimens were from wood subjected to no-thinning and medium-pruning treatments according to an ultrasonic wave technique and static bending tests. However, most results showed no statistically significant differences among thinning, pruning, and thinning and pruning treatments. The average values of DMOE, MOE, and MOR of visually graded construction-grade lumber were significantly greater than those of below-grade lumber. Moreover, there were very significant positive relationships between density, ultrasonic velocity, DMOE, MOE, and MOR, although the determination coefficients were small.     

Strength properties of glued laminated timber made from edge-glued laminae I: strength properties of edge-glued karamatsu (Larix kaempferi) laminae

The object of this study was to investigate the strength properties of edge-glued laminae and to propose a suitable grading method based on the lamina modulus of elasticity (MOE). Edge-glued laminae composed of lumber with similar MOEs (uniform laminae) and edge-glued laminae produced by randomly gluing lumber independent of MOE (random laminae) were made from karamatsu (Larix kaempferi) lumber having the same thickness and length, but various widths. For both the uniform and random laminae, there was a strong correlation between MOE values measured using the longitudinal vibration technique, the static bending test, and a grading machine. The average values of bending, tensile, and compressive strengths of the uniform laminae were similar to those of the random laminae. On the other hand, the average strength of laminae without end joints was significantly higher than that of finger-jointed laminae for both uniform and random laminae. Finger-joints and knots played a significant role in the failure of specimens, but the edge-gluing and the difference in MOE within an edge-glued lamina did not appear to affect the strength properties. The bending, tensile, and compressive strengths of edge-glued laminae were strongly correlated to the lamina MOE.     

Properties of parallel strand lumber from Calcutta bamboo (<Emphasis Type="Italic">Dendrocalamus strictus)</Emphasis>

The objectives of this work were to analyze the physical and mechanical properties of parallel strand lumber (PSL) made from Calcutta bamboo. Based on the surface characteristics (Ahmad and Kamke 2003) and physical and mechanical properties (Ahmad and Kamke 2005) observed in previous work, a prototype PSL from Calcutta bamboo was manufactured and tested in the laboratory. Physical properties determined were dimensional stability and water absorption. The mechanical tests carried out were in compression and bending. Ultimate stress, stress at proportional limit, and modulus of elasticity were determined and compared to structural composite lumber (SCL) from several timber species produced by other researchers and manufacturers in the United States. The PSL produced was also exposed to accelerated aging process in order to assess its durability under extreme condition. PSL produced in the laboratory was stable in dimension. The mechanical characteristics compare favorably to SCL produced in other studies and SCL products available in the United States. The accelerated aging process was found to reduce the bending strength but no significant difference was detected in bending stiffness, and compression strength and stiffness. This is a promising indication of the suitability of Calcutta bamboo as raw material for structural composite products.     

Effect of size on the bending strength of laminated veneer lumber

Summary The effect of size on bending strength has been experimentally determined for laminated veneer lumber. Width was found to have no effect on bending strength. The effect of depth times length on bending strength obtained by the slope method was about 0.075, which is in good agreement with the results obtained by the shape parameter method. The effect of length is somewhat more severe than the effect of depth. Size was found to have no effect on modulus of elasticity or modulus of rigidity.In addition, the relationships between bending strength, modulus of elasticity and density of laminated veneer lumber were experimentally modelled.The author is pleased to acknowledge Research and Development Manager Matti Kairi, who represents the Kerto laminated veneer lumber manufacturer of Finnforest Oy, for fruitful co-operation over several years. The patience of senior research scientist Markku Kortesmaa, who calculated and several times explained the details of Appendix A for the author, is also acknowledged