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.     

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.     

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.     

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.     

Deformation analysis of timber-framed panel dome structure I: simulation of a dome model connected by elastic springs

This study develops an analytical method that enables the simulation of the deformation of timber-framed plywood panel dome structures, of which strength is largely governed by the rigidity of joints. A hybrid truss structure model was employed to analyze this structure. In this model, we aimed to incorporate the mechanical properties of bolted and nailed joints, which were employed to build the structures, although the present investigation focused on the deformation characteristics of a dome in which the panel elements were connected mutually by elastic springs. The results of the theoretical analysis are compared with those obtained by experiments. The simulated results were found to be in good agreement with the results of the experiments under similar loading conditions.     

Dynamic responses of nailed plywood-timber joints under a band-limited white-noise wave

In this study, dynamic tests of nailed plywood-timber joints are conducted under a band-limited white-noise wave using a uniaxial hydraulic shaking table. The principal results are as follows: dynamic responses of nailed plywood-timber joints under a band-limited white-noise wave are reasonably related to the static load-slip relationships and the dynamic responses under harmonic waves of them. When nailed plywood-timber joints are shaken under a composite wave of wide frequency range, they resonate transitionally with one of the frequency components one after another decreasing their equivalent resonant frequencies along their static load-slip curves. Nailed plywood-timber joints are in danger of ultimate failures even though the input maximum accelerations do not exceed their damage limits, if they are shaken for long times with or without intermittences. In this case, nailed plywood-timber joints are prone to fail in low-cyclic bending fatigue failure of nails. Nailed plywood-timber joints, on the other hand, are prone to fail in typical static failure modes when they are shaken under strong input maximum accelerations equivalent to their ultimate limits.     

Withdrawal strength of nailed joints with decay degradation of wood and nail corrosion

Nailed timber joints are widely used in timber structures, and their deterioration may cause significant damage. We investigated the withdrawal strength of joints using steel wire nails in specimens exposed to a brown-rot fungus. We also examined the effects of nail corrosion on withdrawal strength, because high humidity conditions accelerate not only wood decay but also the corrosion of nails. We found that nail corrosion increased the withdrawal strength. The ratios of withdrawal strength of nailed joints with rusted nails to that of joints with a minimally rusted nails were 1.47 and 1.56 in joints nailed in radial and tangential directions to annual rings, respectively. Withdrawal strength, excluding the effects of nail corrosion, had a negative correlation with mass loss and Pilodyn-pin-penetration-depth-ratio. We estimated the withdrawal strength of the nailed joint with decayed wood and rusted nails by multiplying the values from the empirical formula (obtained from mass loss and Pilodyn-pin-penetration-depth-ratio) by 1.47 and 1.56 for joints nailed in radial and tangential direction to annual rings, respectively.     

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     

Lateral strength of nailed timber connections with decay

Loading tests were conducted on nailed connections with decay due to a brown rot fungus. The effect of the decay on the lateral strength of nailed connections was investigated. After loading tests, the sound and decayed regions of a nailed connection were observed in the cross section, which was cut parallel to the grain through the nailed point. The nailed connections with decay showed a low load during initial deformation when the main and side members had a decayed region in the boundary between them. The nailed connections showed low load after yielding when the sound region in the main member decreased. The yield load of nailed connections with decay was calculated based on the yield theory. The model of calculations had sound and decay regions within a member. The yield load of nailed connections obtained by the calculation based on the yield theory agreed with the results obtained by experiments when significant decay in a direction parallel to the grain was observed in the main and side members. This result indicates that the yield theory can estimate the yield load of nailed connections not only with a sound member, but also a member that is partly or wholly decayed.     

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。     

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.     

Development of test methods for wooden furniture joints

Abstract

Joints are generally the weakest part of furniture and they are primary cause of failure. To ensure durability and performance of furniture, it is important for a designer to understand the stresses acting on the joints for preparing suitable design and specification of a furniture. Since each type of joint is unique in construction, it is important to know their strength, when subjected to various stresses namely shear, bending, and tensile, by testing the joints. Since there is no established test method for joints, this study was carried out to categorize joints based on their shape, identify the stresses acting over them and develop test methods for testing furniture joints in three modes namely shear, bending, and tension, which are required for designing joints of any furniture and also provide means of comparing strength of different types of joints. Different types of furniture joints were taken for the study and were classified based on their shapes such as L-shaped, T-shaped, straight, and corner joints. Six types of jigs (one type for bending, two types for shear, and three types for tension) were designed and developed for testing the joints, in such a way that any type of joint can be tested in three modes, i.e., bending, shear, and tension using a universal testing machine. The criteria for section of jigs for performing tests on joints, based on their shape and stresses acting on a particular joint, were reported. Tests were carried out using all the six type of jigs to check the suitability of jigs and test methods developed in the study.     

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     

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     

Mechanism of partial fixation of compressed wood based on a matrix non-softening method

Abstract

Three different mechanisms to explain the partial fixation of the compressive deformation of wood are postulated: non-softening, cross-linking and stress relaxation. This study attempted to fix the compressive deformation of wood by the non-softening mechanism of the cell-wall matrix using acetylation of the cell wall making it more hydrophobic. In this method, partial recovery of compressive deformation by wetting decreased at room temperature as the acetyl content increased. However, almost complete recovery occurred by boiling the compressed wood in water or soaking in acetone. This is due to the ability of boiling water or acetone to soften the cell-wall matrix of acetylated wood enough to enable recovery from compression. It is, therefore, possible to partially fix the compressive deformation of wood, preventing the resoftening of the cell-wall matrix in water.     

Extent and causes of site damage due to forestry traffic

Increased mechanization in the forestry means usually increased traffic on a ground which is also used as a growth substrate. In this state‐of‐the‐art article it is found that the traffic could have an areal extent of 10–30% of the site area mainly due to yarding at clear‐felling and thinning. In the future there could be up to 6 machine entries on the site till a stand age of 50 year.

Skidding is often recognized as the major cause of soil disturbance with 50–75% of the skid trails considered as apparently compacted. Rut formation after forwarders can also be a problem but harvesters seems to be forgotten or give minor problems.

In order to reduce soil compaction smaller machines must be considered on many sites. Maybe 5–10 tonnes machines with a real mean ground pressure of 60–70 kPa could give, thanks to good tyres, an acceptable level of soil disturbance.

High tractive forces and slip may contribute to the soil disturbance as much as double the ground pressure. Calculations show it is therefore better to carry the load than to skid. However, the change in weight balance of the machine due to the load is still a problem and maybe anti‐slip control could give the machine a better torque distribution.     

IMPRESSIONS OF FORESTRY IN SOUTHERN EUROPE

The bonding of wood by means of glue has been practised for many centuries.

Adhesion between an adhesive and wood is the result of unbalanced secondary valency forces (Van der Waal's forces) present on the interfaces. It is fundamental to good adhesion that the adhesive must (a) wet the surface it is required to adhere to and (b) penetrate the wood capillaries. The phenomenon of “wetting” is indicated by the contact angle the adhesive forms on the wood surface as well as its ability to penetrate the wood capillaries. Maximum penetration of the capillaries is inhibited in practice due to air becoming trapped in “inkpot” type capillaries caused by the sawblade tearing the wood fibres over in the direction of the cut. Several ways to increase capillary penetration are suggested.

The anisotropic chemical reactivity of wood is theorized in so far that a unit area of wood substance (excluding lumen openings) cut on the cross-sectional plane cannot be as effectively glued as a unit area of wood substance on the radial and tangential plane. This is due to the positioning of the chemically reactive groups on the cellulose chains which are predominantly oriented parallel to the fibre axis.

The engineered design of joints is briefly discussed and mathematical expression given as to how incorrect joint design can be detrimental to the ultimate joint strength.

The general character of the better-known synthetic adhesives is briefly discussed. Little detail is given as excellent hand books exist on this specific subject.     

Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Abstract

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.     

Motion analysis of a semi-legged vehicle with soil deformation taken into account

A semi-legged vehicle was designed for forestry use. The equation of motion for the machine coupled with the equation of motion for soil was derived. Furthermore, the motion of the machine was analyzed taking into account soil deformation. The Extended Distinct Element Method, which can analyze both continuous and non-continuous materials, was used as a soil model. The effects of foot area and spike length were simulated by using two kinds of uniform soil. The specific power of a foot area of 3,200 cm² was smaller by 0.025 than that of a foot area of 1,600 cm² on soft soil. This was equal to the consumption energy for moving 2.5% of the machine weight, about 140 kgf. The maximum values of the forces acting on the second hydraulic cylinder were 300 kN and 500 kN, and the weights of the hydraulic cylinder generating these forces were 121 kgf and 229 kgf with spikes that were 0 cm and 30 cm long on hard soil, respectively. In a walking motion, such as lowering the boom to the ground, raising the stabilizers, and advancing the machine, the machine with a larger foot area and shorter spikes was more suitable for lightening the total weight and improving energy efficiency.     

Analysis and design of furniture frames

Summary The engineering design of furniture frames was investigated. The stiffness method of matrix structural analysis was applied to furniture frame design, and modifications of it which are needed to treat frames with semi-rigid, elastically non-linear joints of finite size were developed. A method of designing two-pin moment-resisting dowel joints was also developed. An experimental furniture frame was designed using the theories formulated. Results of tests performed on this frame agreed satisfactorily with those predicted theoretically.

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Zusammenfassung Die technische Konstruktion von Möbelrahmen wurde untersucht. Ein Rechenverfahren zur Bestimmung der Steifigkeit wurde mit Hilfe der Matrizen-Analyse auf Möbelrahmen-Konstruktionen angewendet und entsprechende Abwandlungen, die zur Behandlung von Rahmen mit halbstarren, elastisch nicht-linearen Verbindungen bestimmter Größe benötigt werden, wurden entwickelt. Gleichzeitig wurde ein Verfahren zur Konstruktion von biegesteifen Doppel-Dübelverbindungen entwickelt. Ein Versuchs-Möbelrahmen wurde aufgrund der erarbeiteten Theorie gebaut. Die Ergebnisse aus den Versuchen mit diesem Rahmen stimmten weitgehend mit den hierfür theoretisch berechneten Werten überein.

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Journal Paper No. 3575 of Purdue University Agr. Expt. Station.