Evaluation of quality indexes of bending performance and hardness for hardwoods

The mechanical properties of 613 small clear specimens of 35 species (11 ring-porous hardwoods, 19 diffuse-porous hardwoods, and 5 softwoods) were evaluated. The aim of the study was to examine indexes of wood quality that are easy to measure and that exhibit a high correlation with bending performance and hardness that are essential properties of hardwood products. The modulus of rigidity, dynamic modulus of elasticity, bending properties (modulus of elasticity, modulus of rupture, stress at the proportional limit, absorbed energy, Tetmajer’s modulus), dynamic energy absorption by an impact bending test, compressive strength parallel to the grain, shear strength, partial bearing strength, and Brinell’s hardness were measured. A high correlation was found between dynamic modulus of elasticity and static modulus of elasticity. Bending stress at the proportional limit was found to be approximately equivalent to the compressive strength parallel to the grain. Static energy absorption correlated with dynamic energy absorption. Tetmajer’s modulus was found to be closely related to the ratio of the initial stiffness within the elastic range to the secant modulus at the maximum load. A high correlation was observed between Brinell’s hardness and partial bearing strength. The difference in the regression coefficients obtained for these correlations between the species groups was small. Part of this study was presented at the All Division 5 Conference of IUFRO, Taipei, October 2007     

Effect of size and geometry on strength values and MOE of selected hardwood species

The total hardwood timber stock of German forests is fast growing. The lack of knowledge concerning test standards, product standards and sorting criteria makes it difficult to expand the processing and marketing of hardwoods into the field of construction usage. Strength and stiffness data derived from small, defect-free specimens do mostly exist, but in order to be able to insert hardwoods into building applications, data derived from real size specimens is needed. Subsequently, the results of these two different specimen categories need to be correlated and the so-called size effect needs to be quantified and qualified. This paper aims to analyze the size effect of defect-free compression, bending and tensile specimens for the six European hardwood species maple (Acer spp.), birch (Betula pendula), beech (Fagus sylvatica), ash (Fraxinus excelsior), oak (Quercus spp.) and lime (Tilia spp.). They are tested exclusively parallel to grain. Regarding the compression strength for maple, birch and ash, the specimen dimensions did not influence the compression strength value. For beech, oak and lime, it was observed that compression strength increased as the specimen volume was increased. The bending strength of all species decreased as the specimen dimensions increased. Concerning the tensile strength, a clear statement on whether dimensions influence the tensile strength value is not possible. Further research with adjusted specimen sizes, specimen shapes and machine set-ups is needed. Regarding the compression and bending MOE, in most cases, the dimensions did not influence the MOE values. In tensile testing, MOE values differed significantly for the different specimen sizes. Whether these differences were due to slightly different test set-ups in the different sizes or a true size effect could not be answered conclusively.     

测试方法对毛竹顺纹抗剪强度的影响

以4年生毛竹为对象,分别按照国家标准、行业标准和国际标准的方法,进行顺纹抗剪强度的测试.结果表明:测试方法、试件形状以及加载速度对抗剪强度均有显著影响;行业标准的试件形状能较准确测出竹材的纯抗剪强度,综合测试时间,加载速度以0.8～1.5 mm/min为宜.     

胶合木-钢夹板螺栓连接滞回性能试验

[目的]为探明胶合木-钢夹板螺栓连接的动力性能和抗震性能,确保连接件在车辆、机械振动等动力荷载下的可靠性.[方法]针对胶合木-钢夹板螺栓连接的构造特点,考虑胶合木厚度和螺栓直径之比(厚径比)、螺栓顺纹间距、螺栓并列和错列布置方式等参数的影响,设计制作了4类13组共39个胶合木-钢夹板螺栓连接件,在低周反复荷载作用下进行...     

Load-carrying capacity of steel-to-timber joints with a pretensioned bolt

Previous experimental studies reported that bolt pretensioning greatly increases the initial stiffness and load-carrying capacity of bolted joints. It is also a matter of great importance to structural designers to understand the effect of pretension on the load-carrying capacities of bolted joints, and this study presents an extended yield model that considers the fastener’s pretension force. In the extended yield model, the load-carrying capacity was defined as the load at a slip of 15 mm. The ultimate fastener bending angle at the yielded cross section equivalent to this joint slip, which was affected by the fastener’s axial force, was iteratively evaluated in numerical analyses. The introduction of bolt pretensioning largely increased the joint slip resistance at initial loading, but it decreased the ultimate fastener bending angle. This decrease of fastener bending angle resulted in a relatively low stiffness hardening (or secondary stiffness), which is caused by secondary axial forces associated with embedment of steel plates into the wood member. Prediction was verified by the tests of 36 steel-to-timber joints under three different pretension forces and two loading directions relative to the grain. Some of the observed load-carrying capacities of the joints, particularly in loading perpendicular to the grain, however, were not as high as those expected by the numerical analyses considering the given pretension forces.     

Theoretical and experimental deflections of glued laminated timber beams made from a tropical hardwood

Abstract

Nondestructive testing (NDT) can play an important role in improving the quality and reliability of tropical hardwood as an engineering material. By means of these methods, the stiffness of the material can be determined and the information used to improve its structural performance. Although, it is a usual approach for qualifying the material used to manufacture engineered wood products made mainly from softwoods, it is not so common for tropical hardwoods. Additionally, the lack of information regarding properties of glulam beam made from these kinds of wood is evident. In this context, the paper aimed at evaluating the theoretical and experimental deflection of glulam beams made from the Brazilian hardwood louro-vermelho (Sextonia rubra). Initially, the stiffness of each lamina was determined nondestructively using transverse vibration method (E _dtv), which has been demonstrated to be the most suitable method for this wood species. Then, ten 5-lamina glulam beams were assembled according to descending lamina E _dtv values. The experimental evaluation was performed using a four-point bending schedule. In general, the theoretical values of deflection were 2% higher than the experimental ones. The transverse vibration showed to be a suitable method to both measure lamina stiffness and predict glulam beam deflection.     

Shear stiffness and its relation to the microstructure of 10 European and tropical hardwood species

In this study, shear stiffness properties of 10 different hardwood species and their relation to the corresponding species-specific microstructure are investigated. For this purpose, shear stiffness of 10 different hardwood species is experimentally measured by means of ultrasonic testing. In addition, a micromechanical model for hardwood is applied in order to illustrate the influence of certain microstructural characteristics such as mass density and volume fractions of vessels and ray cells on the shear stiffness. Comprehensive microstructural and mechanical data from previous investigations of the same hardwood material support the interpretation of the microstructure–shear stiffness relationships. Mass density was confirmed to be the dominant microstructural characteristic for shear stiffness. Also, ultrasound shear wave propagation velocity increases with density, particularly in the radial-tangential (RT) plane. In addition to density, comparably higher shear stiffness G_LR can be explained by comparably higher ray content and lower vessel content. As for G_LT, a ring porous structure seems to lead to higher shear stiffness as compared to a diffuse porous structure. For this shear stiffness, vessel and ray content were found to have a less impact. Also, the rolling shear stiffness G_RT was found to be higher for a diffuse porous structure than for a ring porous one. Moreover, the data supports that ray cells act as reinforcements in the RT plane and lead to higher G_RT.     

Bearing properties of engineered wood products I: effects of dowel diameter and loading direction

The embedment tests of laminated veneer lumber (LVL) with two moduli of elasticity (MOE; 7.8 GPa and 9.8GPa), parallel strand lumber (PSL), and laminated strand lumber (LSL) were conducted in accordance with ASTM-D 5764. The load-embedment relation for each of these engineered wood products (EWPs) was established. The directional characteristics of bearing strength (_e), initial stiffness (k _e), and effective elastic foundation depth were obtained from the tested results. The effective elastic foundation depth (=E/k _e,E = MOE), based on the theory of a beam on elastic foundation, was obtained from thek _e and MOE. An of 90° (perpendicular to the grain) was calculated by dividingE ₉₀ [MOE of 90° from the compression test, but MOE of 0° (E ₀), parallel to the grain, obtained from the bending test] byk _e90, the initial stiffness of 90°. This study aimed to obtain the bearing characteristics of each EWP, taking into consideration their anisotropic structures, for estimating the fastening strength of a dowel-type fastener. The relations between the bearing coefficients ( _e,k _e,) on the loading direction and dowel diameter were established from the load-embedment curves. Based on the results of the embedment test, tested EWPs showed different tendencies in all directions from wood and glued laminated timber.Part of this study was presented at the 49th Annual Meeting of the Japan Wood Research Society, Tokyo, April 1999     

Mechanical properties and their interrelationships for medium-density European hardwoods,focusing on ash and beech

ABSTRACT

The usage of hardwoods for engineered wood products, such as glulam, requires defined mechanical properties reflecting the actual tensile strength of the material. Currently, the European strength class system EN 338 only covers profiles for hardwoods tested in bending. In this study, the material properties of medium-density hardwoods are analysed with the focus on a total of 3663 European ash (Fraxinus excelsior) and European beech (Fagus sylvatica) specimens tested in different loading modes (tension, compression, bending, and shear). The relationships between the material properties—tensile strength, stiffness, and density—are analysed on grouped data of both graded and ungraded specimens. As a result, a tailored ratio of tensile strength to tensile MOE and density is given, which allows to utilize a higher tensile strength of hardwoods (f_t,0,k over 30?N/mm²) compared to softwoods. Furthermore, the relationship of the test values and the derived values is checked. The equations for deriving the compression and bending strength from tensile strength are verified based on available data. For tensile and compression strength perpendicular to the grain and for shear strength of both beech and ash, higher strength values than the ones listed in EN 338 are possible. The relationship between the mechanical properties are combined to tensile strength profiles for hardwoods.     

三种阔叶树枝桠材应拉木和对应木的解剖特征及材性的研究

通过对３ 种阔叶树 （米槠、木荷、拟赤杨） 枝桠材的应拉木和对应木的解剖特征及材性的观察和试验,并与正常干材比较, 结果表明： 它们在纤维、导管的长度、宽度、纤维次生壁 Ｓ２ 层纤丝角、化学成分、纤维素相对结晶度、密度、吸水性、涨缩性以及顺纹抗压强度、抗弯强度及其弹性模量等都存在显著差异。     

Axially loaded glued-in hardwood dowels

The failure load of axially loaded hardwood dowels glued-in parallel to the grain direction of the jointed timber parts is considered. Two simple theoretical solutions using linear elastic fracture mechanics/ideal plasticity and linear elastic stress analysis, taking into account the finite shear stiffness of the bond line are, presented and compared with experimental results. Theory shows that bond line shear strength is the governing strength property for ductile joints and fracture energy is the governing strength property for brittle joints. Bond line shear strength and fracture energy are determined by means of curve-fitting. Received 24 April 1997     

山西省优良乡土阔叶树种研究初报

辽东栎、臭椿、皂荚等优良乡土阔叶树种在山西省分布广、适应性强,但在造林绿化中应用很少。课题组科技人员经过6 a时间,在山西省境内调查了主要优良乡土树种的分布情况,收集保存了14种优良乡土阔叶树种的优树共641株,并建立了优树档案;通过试验掌握了13种树种的无性繁殖方法,建设繁育圃15 hm2,采穗圃2.2 hm2,基因库4 hm2,测试林45 hm2,初步选择出优良无性系35个;审定了辽东栎1个新品种,申报了皂荚1个新品种;建立辽东栎母树林42 hm2,制定了《辽东栎育苗技术规程》和《栎类母树林建设技术规程》2个地方标准。     

Maximum entropy modeling of mature hardwood forest distribution in four U.S. states

In the eastern United States, mature hardwood forest provides habitat for many species of native flora and fauna, but is much less common now than historically. This study examined the utility of maximum entropy modeling and spatial application to identify ecosystem types like mature hardwood forest. I performed pilot modeling in Charles County, Maryland, where I compared fine-scale geographic data available locally to coarse-scale data available nationally. As expected, a model constructed with the best locally available data, including LiDAR-derived canopy height and fine-scale soil maps, outperformed a model constructed with nationally consistent data. However, the model using national data nevertheless accurately identified most mature hardwood forest sites and excluded most young forest. I then applied the coarse-scale approach to four states: Pennsylvania, Ohio, Kentucky, and Tennessee. Average test AUC (area under the receiver operating curve) based on 10 replicates varied from 0.76 to 0.80 when comparing mature hardwood forest locations to general forest locations. The maximum training or test sensitivity plus specificity threshold, depending on the state, captured 78-79% of positive locations while rejecting 74-81% of negative locations. The maximum entropy approach is versatile, and can be applied to other ecosystems and species.     

Goodness-of-fit analysis on wood properties data from six Brazilian tropical hardwoods

As part of a larger effort to develop design stresses for six hardwood species indigenous to tropical Brazil, a study was conducted to evaluate the relative goodness-of-fit of four distributions (normal, lognormal, Weibull, and S_B) that are used in wood related applications to the characterization of the modulus of elasticity (MOE) and modulus of rupture (MOR) data obtained from tests on lumber and clear wood specimens. Three criteria [maximum likelihood, Kimball, and Kolmogorov-Smirnov (K-S)] were used to evaluate the distributions over their entire domain. The K-S test was also employed at the lower tail of the distributions to evaluate goodness-of-fit at this crucial location for design stress development. The results showed that with respect to both MOR and MOE, the S_B distribution was as good or better than the other distributions. This was especially noticeable with the MOR data, since this data is often more skewed than MOE and the S_B is particularly well suited to describe positively and negatively skewed data. The lognormal and the Weibull were both found to be useful under certain circumstances; the normal distribution, due to its lack of ability to characterize anything but symmetrical distributions, was found to be of virtually no value in this application. Significant distinctions were found between the structure of MOR and MOE data sets obtained from the tropical and temperate zone species. For reasons believed to be related to gross and anatomical structure, more skewness was found in the two temperate zone species studied than in the six tropical species. As such, the S_B distribution, with its greater ability to quantify skewed data, performed overwhelmingly better than the other distributions. In describing the six tropical species, the advantage of the S_B distribution was less pronounced. Received 16 November 1998     

Effect of grain angle on shear strength of glued end grain to flat grain joints of defect-free softwood timber

To evaluate the effect of grain orientation on the adhesive bond strength, three-layered Norway spruce wood specimens were tested in shear. The two axial-oriented outer layers were jointed with the middle layer using three typical glues for load bearing constructions, i.e. one-component polyurethane (PUR), melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF). The grain orientation of the middle layers was varied from 0° (parallel to grain of the surface layer) to 90° (perpendicular to grain of the surface layer) in incremental steps of 10°. Samples with middle layers oriented parallel to the outer layers showed shear values in the range of solid spruce wood. Decreased shear strength values were expected for increased grain angles of the middle layer. However, no explicit tendency was observed for the shear strength in dependence of the grain direction. In general, MUF-bonded samples showed slightly higher shear strength values compared with PUR- and PRF-bonded specimens.     

云杉胶合木钢板斜螺钉连接的剪切性能研究

为准确评价斜螺钉连接钢 木节点的剪切性能,探明其受力机理,以云杉胶合木、钢板和自攻螺钉作为研究材料,测试不同荷载方向与受力情况下斜螺钉连接节点的承载性能,将试验数据与国外规范中的计算模型进行对比,提高了侧边钢板 胶合木(钢 木)斜螺钉连接节点承载性能的预测能力。结果表明:自攻螺钉与剪切面之间的角度变化对其在钢 木节点承受剪 压复合应力的承载力影响不明显,当偏转为剪 拉复合应力时,节点承载力明显增大,并在30°~45°获得最大值;剪 压复合应力时,现行EC5公式计算剪 压节点的极限承载力非常不安全;EC5的刚度预测结果在剪 压复合应力区和垂直剪切面钉入时,与试验值吻合度很高,但对剪 拉区节点的滑移模量没有预测性;将Tomasi模型应用于斜螺钉连接钢 木节点滑移模量理论计算时,在45°~90°时与试验值吻合度极高。单颗自攻螺钉的抗拔刚度计算节点滑移模量的方法极为有效,具有较高的借鉴意义。     

Plywood frame corner joints with glued-in hardwood dowels

This paper presents research on plywood frame corners jointed to glulam beams and columns by means of glued-in hardwood dowels. The frame corner was made of a solid block of ordinary plywood of the same width as the glulam beams and with plies parallel to the plane of the frame to avoid splitting due to stress perpendicular to the grain. Hardwood dowels with a diameter of 12mm and a maximum glued-in length of 120mm were glued into drilled holes in the plywood corner and glulam beam ends parallel to the grain direction of the beams to form a momentresisting joint. Static bending tests were conducted of frame corners with 100 X 200mm² and 120 X 420mm² beam cross sections. Bending capacities of the joints corresponding to a modulus of rupture of the jointed glulam beams of about 30MPa were obtained for both closing and opening moments for the small cross sections, and about 22MPa was obtained for the large cross sections. Simple design models for calculation of joint strength and rotational stiffness are also presented.     

空心胶合木梁柱式结构抗侧力性能研究

为研究空心胶合木梁柱式结构体系抗侧力性能,对纯框架及框架-人字撑两种单跨结构试件进行了单调及低周反复加载试验,考察了结构破坏模式、耗能能力、刚度等抗震性能。结果表明:纯框架与框架-人字撑结构均具有一定抗侧力性能,但纯框架结构承载能力较弱,不建议单独用于工程实践;框架-人字撑结构具较大抗侧刚度及承载力,分别为纯框架的6.3倍及2.68倍,但其延性仅为纯框架的67%。两种结构失效前强度退化小于30%,具足够剩余承载力。     

Armillaria species distribution on symptomatic hosts in northern hardwood and mixed oak forests in western Massachusetts

Armillaria spp. are some of the most important forest pathogens in mixed hardwood forests of southern New England, yet their role as prominent disturbance agents is still not fully appreciated. We investigated the distribution of Armillaria species across eight separate stands of northern hardwood and mixed oak forests in western Massachusetts. We were specifically interested in the Armillaria species distribution from live, symptomatic hosts and not in determining overall incidence in the forest. From 32 plots (16 within each forest type), 320 isolates were collected. Armillaria was routinely encountered causing disease of live trees. In total, 89% (286/320) of all isolations came from live hosts exhibiting symptoms of root and butt rot. Overall, A. gallica was the dominant species in each forest type, making up 88/160 (55%) isolates from northern hardwood and 153/160 (96%) of all isolations from mixed oak stands. However, northern hardwood forests showed much greater species diversity, as A. calvescens, A. gemina, A. ostoyae, and A. sinapina were all found. At one site, a northern hardwood forest surrounding a high elevation spruce-fir forest, A. ostoyae was the most abundant species encountered. All five Armillaria species were found causing disease of live hosts, including A. gemina, a species considered by some as weakly virulent. Armillaria gallica was found on 22/23 tree species’ sampled, and was found most often causing butt rot.