Duration-of-load and creep effects in strand-based wood composite: a creep-rupture model

In order to investigate the creep and duration-of-load (DOL) effects in thick strand-based wood composite products, a creep-rupture model is proposed linking the accumulated damage to creep deformation. Results from long-term constant load tests have been interpreted by means of this creep-rupture model, which is capable of representing the time-dependent deflection and time-to-failure data at different stress levels. The predictions of the model have been verified using results from ramp load test at different rates of loading. The creep-rupture model incorporates the short-term strength of the material, the load history and predicts the deflection history as well as the time-to-failure. As it is a probabilistic model, it allows its incorporation into a time-reliability study of wood composites’ applications.     

Evaluating rolling shear strength properties of cross-laminated timber by short-span bending tests and modified planar shear tests

This paper presents an experimental study on rolling shear (RS) strength properties of non-edge-glued cross-laminated timber (CLT) made out of New Zealand Radiata pine (Pinus radiata) structural timber. CLT specimens with 35 and 20 mm thick laminations were studied to evaluate the influence of lamination thickness on the RS strength of CLT. Short-span three-point bending tests were used to introduce high RS stresses in cross layers of CLT specimens and facilitate the RS failure mechanism. Modified planar shear tests from the conventional two-plate planar shear tests were also used to evaluate the RS strength properties. It was found that two test methods yielded comparable RS strength properties and the lamination thickness significantly affected RS strength of the CLT specimens. The test results also indicated that the recommended characteristic RS strength values of CLT products in Europe and Canada might be over conservative. Also, it might be more efficient to specify different RS strength values for CLT with different lamination thickness given the minimum width-to-depth ratio of laminations is satisfied.     

Duration-of-load and creep effects in strand-based wood composite: experimental research

The results from a duration-of-load and creep testing program on a thick strand-based composite product, using wood from forests attacked by the mountain pine beetle, are presented. The constant loading of experimental beams lasted for 1?year. The long-term deflection was monitored and recorded at a pre-set frequency. Time-to-failure data were also obtained for all the broken specimens. Ramp load test was carried out at different rates of loading to investigate its influence on short-term strength. Finally, the fatigue test was conducted with a triangular cyclic load history, and the strain history was obtained to elucidate the low-cycle fatigue behavior of the composite.     

Effect of loading frequency on fatigue life and dissipated energy of structural plywood under panel shear load

Wood-based panels are subjected to cyclic panel shear load caused by wind and seismic forces in such an application as the sheathing of bearing walls. The fatigue behavior of structural plywood under panel shear load with two different loading frequencies was examined. Pulsating panel shear load with a triangular waveform and loading frequency of 0.5 or 5 Hz was applied to the plywood specimens. Stress−strain hysteresis loops were measured throughout the fatigue tests. Fatigue life was highly dependent on loading frequency at more than 0.5 stress level. The deterioration of mechanical property and damage accumulation in plywood specimen was observed to be slower at higher loading frequency at more than 0.5 stress level. Analyses based on energy loss suggest that panel shear load with higher loading frequency causes less damage to the plywood specimen during one loading cycle at higher stress level, and that the fatigue damage accumulation causing failure might be dependent on stress level although it seems to be unaffected by loading frequency. Based on these results, a new fatigue failure model for plywood specimen was qualitatively developed by combining Weibull’s weakest link model and Daniels’ fiber bundle model.     

Shear behavior of cross-laminated timber wall consisting of small panels

A cross-laminated timber (CLT) wall plays the role of resisting shear stress induced by lateral forces as well as vertical load. Due to the press size, CLT panels have a limitation in size. To minimize the initial investment, some glulam manufactures wanted to make a shear wall element with small-size CLT panels and panel-to-panel connections and wanted to know whether the shear wall would have equivalent shear performance with the wall made of a single CLT panel. In this study, this was investigated by experiments and kinematic model analysis. Two shear walls made of small CLT panels were tested. The model showed a good agreement with test results in the envelope curve. Even though the shear walls were made of small panels, the global peak load did not decrease significantly compared with the wall made of a single CLT panel, but the global displacement showed a large increase. From this analysis, it was concluded that the shear wall can be designed with small CLT panels, but displacement should be designed carefully.     

Evaluation of rolling shear strength of plywood by flexural vibration method

The rolling shear strength of plywood was evaluated using a flexural vibration test. Test specimens were lauan and Douglas fir three-ply plywoods made from thick veneers. The dynamic shear and Young's moduli were determined using the flexural vibration method, which involved in-plane and out-of-plane flexural vibration. The rolling shear strength was determined using the static destructive method, which is dependent on the direction of the lathe check in the core veneer. Before and after accelerated aging treatments were conducted, there were relations between out-of-plane dynamic properties (out-of-plane shear and Young's moduli) and its rolling shear strength. It was concluded that the rolling shear strength is related not only to the shear property of the core but the flexural stiffness of two faces when the deformation of out-of-plane plywood was not restrained.Part of this work was presented at the 47th Annual Meeting of The Japan Wood Research Society, Kochi, April 1997     

基于换算长细比的正交胶合木稳定系数计算方法

正交胶合木(CLT)纵横向层板交叉组坯,纵向层板强度和模量较高,横向层板强度和模量较低,其截面呈强弱相间的条带状结构.当轴心受压构件失稳时,截面剪切变形对CLT稳定承载力的影响不可忽略,故不能采用一般木产品的稳定承载力计算方法.目前我国相关标准中尚无CLT轴心受压构件稳定承载力计算方法.鉴于此,从CLT产品的截面构成特...     

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.     

Effect of load pulse shape on predicted damage accumulation in wood

Summary Load-duration (creep-rupture) effects in wood are often modeled using cumulative damage theory. In application, such as for duration of load reliability analyses, incremental damage owing to stochastic load pulse models is summed over an assumed reference period (e.g., 50 years), with failure defined in terms of the damage state variable. The effect of load pulse shape on cumulative damage is examined herein through the use of one such proposed damage accumulation model. An exponential damage rate model (EDRM) is used to calculate the damage owing to a rectangular load pulse (RECT), a triangular load pulse (TRI), and two trapezoidal load pulses (TRAP1 and TRAP2). The differences in the calculated damage predicted by the EDRM and resulting from the four load pulse shapes are presented and the effect of critical load parameters (e. g., load intensity and duration) are illustrated. Also, the effect of neglecting ramp loading and unloading is discussed; that is, the error in assuming RECT versus either TRAP1 or TRAP2 load pulses is assessed.     

不同胶黏剂对南方松CLT胶合性能的影响

为探索不同胶黏剂用于正交胶合木(CLT)生产的可能性,研究采用来自不同厂家的5种胶黏剂制备南方松CLT,以浸渍剥离率、剪切强度和木破率评价南方松CLT的胶合性能。结果表明:仅A-API和B-PUR1胶黏剂制造的南方松CLT胶层耐久性达到标准要求,木破率均未达到标准要求;5种胶黏剂对南方松CLT的浸渍剥离率、剪切强度和木破率有显著影响(P<0.005);从胶层耐久性、剪切性能和生产成本考虑,宜用A-PUR和A-API制造南方松CLT。     

Shear behaviour of laminated Douglas fir veneer

The rolling shear and longitudinal shear behaviour of laminated Douglas fir veneers was studied using specimens with 15 layers of 2.5 mm veneers. The rolling shear specimens were constructed such that the 3 central veneers were cross-plies with grain angle oriented perpendicular to the long axis of the specimen. The other layers were orthogonal to the cross-plies. The longitudinal shear specimens had a LVL lay-up with the exception of a reinforcement layer of fiber-glass attached to the bottom face veneer. The fiber-glass layer prevented specimens from failing in bending/tension mode and ensured longitudinal shear failures. Specimens were subjected to static and cyclic loads in a “flatwise” three point bending configuration. It was found that the rolling shear failure mode exhibited a higher fatigue resistance than the longitudinal shear failure mode. A damage model that took the stress history into account was calibrated to the experimental data of each specimen type. Good agreement between model predictions and experimental results were obtained for both failure modes. Received 5 November 1997     

Crack influence on load-bearing capacity of glued laminated timber using extended finite element modelling

Abstract

Most of the cracks are caused by changes in temperature and relative humidity which lead to shrinkage and swelling of the wood and thereby induce stresses in the structure. How these cracks influence the strength of the wooden structure, especially the shear strength, is not well understood. However, it is reasonable to expect that cracks have an impact on the shear strength as they preferably run along the beams in the direction of grain and bond lines. The purpose of this study was to investigate the load-bearing capacity of cracked glulam beams and to find a model that could predict the failure load of the beams due to the cracks. Three-point bending tests were used on glulam beams of different sizes with pre-manufactured cracks. An orthotropic elastic model and extended finite element method was used to model the behaviour of the cracked beams and to estimate the load-bearing capacity. The conclusions were validated by numerical simulations of the mechanical behaviour of three-point bending of glulam beams with different crack locations. The crack initiation load was recorded as the failure load and compared to the experimental failure load. The results of the compaction simulations agree well with the experimental results.     

Fatigue life of structural plywood under two-stage panel shear load: a new cumulative fatigue damage theory

The fatigue life of structural plywood under two-stage panel shear load was experimentally examined. Two experimental conditions were determined for two-stage fatigue of plywood specimen: one used variable applied stress and the other used variable stress, loading waveform, and loading frequency, because fatigue life of wood composite under constant load depended on loading waveform and loading frequency as well as stress level. The most famous cumulative fatigue damage theory is the Palmgren-Miner rule, which is the summation of the ratio of the applied loading cycle to the fatigue life under each loading stage. However, the applicability of this rule to the two-stage fatigue of wood composites has not been investigated. It was first demonstrated in this study that the fatigue life of the plywood specimen reached in the two-stage fatigue test did not obey the Palmgren-Miner rule. Here, we propose the new cumulative fatigue damage model by modification of the Palmgren-Miner rule on the basis of the assumption that fatigue damage accumulates with loading cycle on a logarithmic scale. The newly proposed model was in good agreement with the fatigue life reached in the two-stage fatigue test.     

正交胶合木横纹承压性能研究现状

正交胶合木（CLT）在木结构建筑中应用越来越广泛,特别是在中高层建筑物中。CLT作为楼盖构件使用时,承受柱或墙体施加的垂直荷载,因此CLT产品的设计及应用需要其横纹承压弹性模量和强度方面的参数;改善CLT横纹承压性能,也可以提高整体结构强度和建筑安全系数。文中主要介绍目前CLT横纹承压性能测试方法,总结CLT横纹承压性能影响因素方面的研究进展,以期为国内CLT研究提供参考。     

An experimentally validated fatigue model for wood subjected to tension perpendicular to the grain

This study presents an experimental investigation of fatigue in wood subjected to tension perpendicular to the grain. The study has been designed with special reference to the influence of the frequency of loading. The investigation reveals an interaction between number of load oscillations and accumulated time under load to failure. This interaction corresponds to frequency dependent fatigue. Current models for damage accumulation and failure modelling are screened with respect to their ability to account for such “two-source” damage. The Damaged cracked viscoelastic material model proved to give a good basis.     

Load sharing and weakest lamina effects on the compressive resistance of cross-laminated timber under in-plane loading

A new approach was developed to predict the compressive resistance of cross-laminated timber (CLT) using the compressive strength of small samples of different grade lamina from E12 and E8 larch, and E10 and E6 nut pine. CLT of three different thicknesses was manufactured using different grades of laminas from each species. To evaluate the compressive resistance of CLT, three different methods were employed. The first method was used to determine the compressive resistance, which was predicted by multiplying the compressive strength of lamina aligned with the loading direction, and the cross-sectional areas of the lamina. The second method is similar to the first method, but additionally considering the stiffness ratio of the laminas. The third method developed from the current study accounts for load sharing and weakest lamina effects in the prediction of compressive resistance. When the lower 5th percentile compressive resistance in a major direction was predicted using the first two methods, the difference between the experimental test and the predicted value ranged from 2.5 to 43.4%. However, when the compressive resistance in a major direction was predicted using the developed method from the current study, the difference between the experimental test and predicted value ranged from ??8.7 to 10.8%.     

Fatigue of structural plywood under cyclic shear through thickness III: energy dissipation performance

Wood and wood composites have viscoelasticity, and show a hysteresis loop in the stress-strain relationship during cyclic loading such that part of the mechanical work applied is dissipated in the materials. In this study, the energy dissipation performance of plywood specimens under cyclic shear through thickness was investigated. Fatigue testing was conducted under three loading conditions: a square waveform at a loading frequency of 0.5 Hz, a triangular waveform at 0.5 Hz, and a triangular waveform at 5.0 Hz. The stress level was determined to be 0.5, 0.7, and 0.9 of the static strength in shear through thickness. The energy dissipation ratio was defined as the ratio of energy loss per cycle to the strain energy per cycle, and was evaluated throughout the fatigue test. It was found that the energy dissipation ratio of a plywood specimen was kept constant during most of the fatigue process for a given stress level and loading condition. The energy dissipation performance was significantly dependent on stress level and loading condition, and became higher according to the damage intensity of cyclic load even if the same strain energy was applied.     

正交胶合木结构抗震研究进展

正交胶合木(CLT)结构板材物理特性良好,与连接件有效配合,在地震条件下抗震性能表现优越。通过分析欧洲和加拿大最新修订的CLT结构标准,对减震系数q和超强度系数γRd进行论述。同时,针对部分CLT结构试验测试结果,按连接件、剪力墙和整体结构分类展开讨论,并阐述了抗震技术领域最新研究成果,以期为正交胶合木结构抗震性能研究提供参考。     

Study of mechanical properties of wooden bolt-nut connector I: effect of size and shape of thread on withdrawal strength

The withdrawal strength of a bolt-nut connector made from wood-based material was evaluated. The thread strength of the wooden bolt-nut connector was tested to select various parameters of the connector and the type of wood material; the wood materials tested were hard maple, white oak, ebony, glue-laminated bamboo, and densified Japanese cedar. A plane model of wooden threads with various thread angles was also evaluated. The results showed that the maximum failure load of the thread increased with increasing bolt density and connection area, which was calculated from the diameter of the bolt and the thickness of the nut. The withdrawal resistance after reaching the maximum load underwent a graded decrease because the bolt threads were broken one by one. In addition, the thread strength depended on the thread angle. In the model with a thread angle of 90°, compressive deformation in the transverse direction occurred prior to shear deformation along the root of the threads; the model with this thread angle thus had higher strength than those with other angles.     

A determination method of compressive design value of dimensional lumber

The objective of this study was to develop a determination method of compressive design value of dimensional lumber for its safety design and rational application as a green building material. A total of 1049 full-size 2?×?4 samples of Chinese larch (Larix gmelinii) dimensional lumber, including visual grades Ic, IIc, IIIc, and IVc, were tested by static compressive tests. Compression strength parallel to grain (UCS) of different grades were summarized. By the least square method, the fitted parameters and results for UCS were obtained under different probability distribution models (normal, lognormal and 2-P-Weibull) and test data points (100, 75, 50, 25, and 15%). Based on reliability analysis method, relationships between the reliability index and partial factor were investigated under different probability distribution models, fitting data points, load combinations and load ratios. Finally, this study suggested that the lognormal distribution, 25% data points, load combination of dead load plus residential live load, and load ratio of 1.0 be selected for the determination of compressive design value of Chinese larch dimensional lumber. For Chinese larch dimensional lumber, the compressive design values of grade Ic, IIc, IIIc and IVc were 22.9, 18.3, 14.6 and 13.8 MPa, respectively.