Shear resistance and failure modes of nailed joints loaded perpendicular to the grain

Shear tests were conducted on nailed joints in wood that were loaded perpendicular to the grain; these joints had 21 specifications depending on different combinations of wood species, nail dimensions, number of nails, and edge distances of the main members, and their effects on the shear resistance of the nailed joints were also investigated. The nailed joints with CN75 nails had higher initial stiffness than the joints with CN50 nails, provided the initial stiffness of nailed joints connected with 3 or 5 nails was not always a simple product of the number of nails and the initial stiffness of nailed joints connected with a nail, and instead depended on the combination of wood species of the main member and nail dimensions. When the edge distance decreased, the maximum load and energy capacity decreased, thereby affecting the energy capacity. The maximum load of the nailed joints with CN75 nails may be smaller than those with CN50 nails depending on the combination of wood species and nail dimensions. When the edge distance of the nailed joints was less than 26 mm, the energy capacity of the nailed joints with CN75 nails was less than or similar to those with CN50 nails.     

落叶松规格材的钉连接性能

钉连接是木结构中常用的连接方式之一。依据美国钉连接的试验方法及设计值的确定方法,对我国东北地区落叶松的钉连接性能进行了检测。结果表明,随着圆钉直径的增大,落叶松的抗拔出力和抗剪切力基本呈上升趋势。并对东北地区落叶松钉连接的参考设计值进行了分析。     

Analysis of load-slip characteristics of nailed wood joints: application of a two-dimensional geometric nonlinear analysis

We used a two-dimensional finite element method to analyze the load-slip characteristics of nailed wooden joints sheathed with a panel. We used tests of nail bending, nail shank embedment in a wood or a panel, nail-head embedment in a face of a panel, nail withdrawing from a wood, friction between a wood and a panel, and initial axial forces of the nails. The values of the material properties for the analytical models were determined with the above tests. With a conventional one-dimensional analysis it is impossible to evaluate the shearing performance accurately because the axial forces of the nail are not calculated. Therefore, we used two-dimensional geometric nonlinear analysis. The computed load-slip curves closely matched the original experimental results; and when the friction and initial axial forces were considered, the computed curves were more likely to match the original experimental results. Our results suggested that it is better to use geometric nonlinear analysis to analyze the shearing characteristics of nailed joints, and that friction and axial forces are important factors for accurate analysis.Parts of this report were presented at the 7th World Conference on Timber Engineering, Malaysia, August 2002     

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.     

Screw-nail withdrawal and bonding strength of paulownia (Paulownia tomentosa Steud.) wood

The effects of screw type, moisture content, and grain direction on the screw and nail withdrawal strength and bonding strength were investigated for paulownia (Paulownia tomentosa Steud.) wood grown in Turkey. The withdrawal strength was carried out according to the ASTM-D 143 and ASTM-D 1761 and Turkish Standard 6094 in three directions (tangential, radial, and longitudinal) on 60 samples. The moisture content of half of the samples was 12 % and that of the other half 28 %.The experiment of bonding strength (BS EN 205) was applied to both sanded surfaces jointed by poly-vinly acetate and Desmodur-VTKA adhesives. Results of the tests indicate that, the withdrawal strength values at 12 % moisture content were higher than the 28 % for screws whereas the withdrawal strength for 28 % moisture content was higher than 12 % for nails. The maximum withdrawal strength value was found in the chipboard screw. In the case of directions, the withdrawal strength values of radial direction were found to be higher than the others for all parameters. The lowest withdrawal strength values were found in the longitudinal directions for both nails and screws. For adhesive types, the highest bonding strength of D-VTKA was found to be 5.64 N mm^?2 and it was higher than the bonding strength with PVAc (5.33 N mm^?2). However, there were no significant statistical differences between the two adhesive types. The results show that paulownia wood can be used for different purposes such as house construction, roof systems, and box cases as it possesses enough strength.     

Effective lateral resistance of timber–plywood–timber joints connected with nails

In this study, an experimental study was conducted on the nailed timber–plywood–timber joints extended from the standard wall–floor joints of wooden light frame constructions, where the bottom plates of shear walls are nailed to the floors consisting of joists and floor sheathings nailed to them. The principal conclusions are as follows: The allowable lateral resistance of the nailed timber–plywood–timber joints can roundly be estimated by neglecting the plywood panels if their densities are higher than those of the timber main-members and they are fastened effectively onto the timber main-members. The stiffness of the timber–plywood–timber joints is less than that of the control timber–timber joints, which is improved by increasing the number of nails used to fasten the plywood panels onto the timber main-members. The stiffness of the joints whose floor sheathings are glued onto the joists is equivalent to the control timber–timber joints. The timber–plywood–timber joints with appropriate specifications have greater energy capacity until the failure than that of the control timber–timber joints. This ensures their energy capacity, which is important in dynamic resistance, to be equivalent to the control timber–timber joints.     

Dynamic responsive characteristics of nailed plywood–timber joints under harmonic vibrations

Dynamic tests of nailed plywood–timber joints are conducted under harmonic vibrations from 2 to 7 Hz. The principal results are as follows: under dynamic loading, nailed plywood–timber joints may break in low-cyclic bending fatigue failure of nails besides the other failure modes typical under static loading. The dynamic response of nailed plywood–timber joints is clearly dependent upon both the input frequency and the acceleration. These responsive characteristics arise from the nonlinear load–slip relationships and the characteristic cyclic stiffness degradation of nailed joints; that is, the cyclic degradation of the equivalent linear stiffness decreases the resonant frequencies of the same joints, which results in a transition of dynamic responses. It indicates that frequency components of seismic waves resonant to the frequencies corresponding to safety-limit stiffness of nailed joints may lead them to critical failures, even if the accelerations do not exceed the accelerations equivalent to the static damage-limit resistance.     

Stiffening phenomena in annual rings of coniferous wood

Summary The structure of coniferous wood is characterized by pronounced differences in density between earlywood and latewood. This could result in disadvantages in wood strength especially against bending. A summary of the observations in this paper leads to the following hypothesis: Coniferous trees meet this danger by forming numerous stiffenings especially in broad annual rings with a high percentage of earlywood. Such stiffenings may be produced by more or less slight fluctuations of density or chemistry within annual rings in radial as well as in tangential directions. The question remains open which anatomical or chemical reasons, upon sandblasting, locally cause radial or radially branched ribs and craters in cross sections, waviness in radial sections and cloudy structures in tangential sections.Paper presented to the IUFRO-Division 5. Conference in Madison, Wisc. 1983.     

Yield load prediction of nailed timber joints using nail diameter and timber specific gravity

Prediction of yield load has nowadays been accepted as the basis for the limit state method of the design. Currently, there are many methods to predict the yield load of timber joints. These methods, i.e Foschi, Johansen, Smith and US 5% nail diameter offset, are reviewed in-depth in this paper. In this study, the authors presented a new approach which, hopefully, is simpler than the existing methods. The paper outlines the works carried out to arrive at the formula. About 300 Malaysian timber nailed joints were fabricated and tested to obtain the relationship between yield load and maximum load and to propose a new method to predict the yield load of a nailed joint. There seems to be a direct relationship between the yield load and the maximum load. From this relationship, a new formula, which depends only on nail diameter and specific gravity, was established.     

Effect of moisture content of members on mechanical properties of timber joints

To investigate the effect of moisture content (MC) of members on the mechanical properties of timber joints, bending tests of precut joints and shear tests of dowel-type joints were carried out using timbers of Japanese cedar (Cryptomeria japonica D. Don) with three moisture conditions: green, kiln-dried with a MC target of 15%, and over-kiln-dried with a MC target of 5%. For the bending test, timbers were processed with a precut processing machine into “koshikake-ari” (a kind of dovetail joint) and “koshikake-kama” (a kind of mortise and tenon joint). A pair of members was jointed together without mechanical fasteners. Bolts (diameter = 12 mm) and nails (diameter = 2.45 mm) were used as dowels in the shear test. Bolted joints were constructed with one bolt and two metal side plates. Two nails and two metal side plates were used for the nailed joint. For precut joints, no clear effect of MC was recognized on maximum moment and initial stiffness. The maximum strength of mechanical joints assembled with kiln-dried wood was changed by the degree of drying. Stiffness of the joints assembled with kiln-dried specimens was larger than that of the joints assembled with green specimens.Part of this study was presented at the 7th International IUFRO Wood Drying Conference, Tsukuba, July 2001     

Radial variation in partial compression properties perpendicular to the grain of Japanese larch (Larix kaempferi)

This study investigated the densities, average width of annual rings, and partial compression stresses at 5 % strain perpendicular to the grain of air-dried wood specimens, which were continuous in the radial direction from the pith and were obtained from Japanese larch (Larix kaempferi) trees with different diameters at breast height in the same stand, to evaluate the radial variations in partial compression properties perpendicular to the grain. The air-dried densities of the wood increased with the distance from the pith. The average width of annual rings of the wood tended to decrease with increasing distance from the pith and those of medium- and large-diameter trees seemed to increase near the pith. The partial compression stresses at 5 % strain in the tangential loading direction tended to increase with the distance from the pith and with air-dried wood density. However, in the radial loading direction, this tendency was not observed. The partial compression stresses at 5 % strain in the radial loading direction tended to be low in wood with a small average width of annual rings. These results indicate that the factors affecting the radial variations in the partial compression stress at 5 % strain differ depending on the loading directions.     

Annual ring orientation effect on bending strength of subfossil elm wood

The effect of annual rings’ orientation on bending strength was examined on subfossil elm wood. Elm is extremely rarely found as subfossil wood, and during the last 50–60 years it has almost disappeared from natural forest stands of south-eastern Europe, due to the Elm Dutch disease. The samples were cut from approximately 670 years old subfossil elm trunk retrieved from the bed of the river Sava in the area between the villages Grebnice and Domaljevac in north Bosnia. The wood was identified to the genus level based on optical microscopy analysis of three wood sections—transverse, tangential and radial. Bending strength was determined by the three-point bending test. The load was applied to the longitudinal–tangential surface (LT) and to the longitudinal–radial surface (LR). The bending strength values of subfossil elm do not differ from the values of recent elm, despite the 700-year resting in anoxic river conditions. Bending strength in LT direction was slightly higher than bending strength in LR direction. The coefficient of variation and the standard deviation of the arithmetical mean were higher for LT direction. It was found that the measured bending strength for both directions follows Weibull distribution. Coefficient of determination of Weibull functions was close to 1 for both directions. The Weibull shape parameter was higher for LR direction.     

马尾松木材横向液体渗透性的研究

分析和讨论了影响马尾松木材横向液体渗透性及其各向异性的机理.对马尾松木材的径向液体渗透性和弦向液体渗透性进行了测量.结果表明,马尾松木材的径向液体渗透性大于弦向液体渗透性,并且,马尾松木材的径向液体渗透性和弦向液体渗透性都是从边材向心材逐渐减小,并随着含水率的下降而升高.其原因是径向液体流动的主要通道是木射线和水平树脂道,弦向主要是管胞壁和交叉场上的纹孔.     

兴安落叶松钉连接施工技术研究

对我国东北地区落叶松在木结构中钉连接施工技术进行的试验研究表明，用兴安落叶松进行钉连接时，预钻孔直径应是钉直径的85％左右，预钻孔深度应该超过钉入深度；用于端部钉连接的钉不易太粗，可通过具体试验确定或采取相应技术措施，保证连接性能。     

Theoretical models for creep slip of nailed joints between wood and wood-based materials

Summary Applying modern methods of analyzing floor, wall, and connection subsystems in light-frame wood buildings requires information on the stiffness of nailed joints under long-term loads. Because this information can best be derived by testing nail-joint specimens under constant loads, theoretical procedures were developed that use test data for constant loads to predict stiffness under variable in-service loads; five nonlinear, viscous-viscoelastic models were develpoed on the basis of existing formulations of creep and mechanisms of load transfer between nails and wood. The models incorporated the modified superposition and strain-hardening principles in describing responses to discrete or continuous loading functions. Tests have shown that the models closely predict creep slip of typical nailed joints.This research was jointly supported by the Forest Research Laboratory, Oregon State University, and the Cooperative State Research Service, U.S. Department of Agriculture, Special Grant 85-CRSR-2-2553. This is Paper 2288 of the Forest Research LaboratoryFormerly Graduate Research Assistant Forest Research Laboratory     

木结构连接用圆钉性能研究

钉连接是木结构中常用的一种连接方式，圆钉是木结构钉连接的主要材料，本文对国内和美国不同厂家生产的圆钉力学性能和尺寸进行了检测。试验证明，国产圆钉与进口圆钉的尺寸及力学性能相近，国产圆钉可以代替进口圆钉用于现代木结构建造中。     

落叶松木材API胶粘剂弦径面胶接强度的差异

对落叶松木材水性高分子异氰酸酯 (API)胶粘剂弦径面胶接强度进行了研究。结果表明 :落叶松木材API胶粘剂弦径面胶接强度存在着差异 ,落叶松木材API胶粘剂径切板胶合强度试件的常态压缩剪切强度是弦切板的 1 4 1倍 ,而反复煮沸压缩剪切强度径切板的却比弦切板的低。落叶松木材本身弦径面顺纹抗剪强度和剪切强度率的试验结果表明 :造成落叶松胶合强度试件弦径面常态压缩剪切强度存在差异的根本原因在于落叶松木材本身弦径向的强度存在差异 ,木材径向的强度比弦向的大 ,文中对这造成这种差异的原因进行了分析     

A three-dimensional paradigm of fiber orientation in timber

Knowledge of the three-dimensional orthogonal directions of wood material at any position within a tree is necessary for the understanding of strength reducing effects of knots and essential for the continuation of research in areas which relate small clear wood specimen behavior to the behavior of full size structural timber. A complete three-dimensional paradigm describing the geometry of knots and related fiber distortion, initially derived to predict the strength-reducing behavior of knots in structural timber of Norway Spruce with the finite element method, is presented in this article. Besides strength prediction analyses, it is believed that the paradigm may be useful in other areas of research on structural timber that are effected by fiber orientation, such as drying and form change of structural timber. The paradigm generates fiber orientation in any position within a log or lumber from assumed fiber patterns in planes parallel to the longitudinal direction of the original tree. Fiber patterns in the radial and tangential directions are derived from physical restraints related to fiber production within the annual increase surfaces of the tree and from theories of knot formation. The adaptability of the paradigm allows practically any softwood knot to be modeled with an accuracy that is limited only by input-data. The knot-axis may be non-linear, and the knot cross-section oval with its vertical and horizontal axis increasing from the pith of the stem at chosen rates. Spiral grain may also be included in the paradigm and vary with the annual growth layers. Investigations presented in this article showed that generated fiber orientations for Picea abies complied well with measured fiber distortions, and that the general trends of fiber orientation, explained by the applied knot formation theory, is reflected in the measured specimens. Received 12 May 1999     

长岭岗林场日本落叶松幼龄材干缩性能研究

测定了建始县长岭岗林场引种栽培的12年生日本落叶松标准木干缩性能指标，并分析了来自两个样地的20株样木株同差异，其结果表明：干缩性能指标问差异不显著；径向全干干缩率为4．64，弦向全干干缩率为6．81，体积全干干缩率为10．81。全干差畀干缩为1．47；径向气干干缩率为2．41，弦向气干干缩率为3．93，体积气干干缩率为6．59，气干差异干缩为1．63。在此基础上，讨论了日本落叶松的栽培与木材利用中的有关问题和新旧标准下测定的干缩性能指标的换算与利用。     

Microstructure,strength and structural integrity of heat-treated beech and spruce wood

Abstract

Heat treatment of wood is an effective method by which to improve the dimensional stability and biological durability, but the mechanical strength is decreased at the same time. Besides chemical modification of cell-wall constituents, physical weakening of the microstructure owing to heat-induced defects may also contribute to strength loss. Therefore, anatomical properties of heat-treated beech (Fagus sylvatica L.) and spruce (Picea abies Karst.), studied by light microscopic and scanning electron microscopic analysis, and their interrelation with strength properties and structural integrity were investigated. For determination of structural integrity, the high-energy–multiple-impact (HEMI) test was applied. Microscopic analyses showed frequent formation of radial cracks in heat-treated beech close to the rays as well as tangential cracks in the latewood of spruce. In addition, the modulus of rupture was more affected by the heat treatments than the resistance to impact milling (RIM) determined by the HEMI test, because RIM is based on multiple fractures on the microlevel that are not affected by the formation of intercellular cracks or other defects due to the heat treatment. It was concluded that heat-induced defects in the wood microstructure contribute to the substantial strength loss of thermally modified timber.