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     

板式家具结构设计有限元法研究综述

文中总结、分析了有限元法在板式家具结构设计中的应用现状，并对该领域的未来研究趋势进行了阐述。国内外现有研究主要集中于板式家具在静载荷下的强度或刚度的有限元分析，研究对象多为板式家具构件或整体，而大多忽略了节点对板式家具整体强度的影响。未来可进一步从以下几方面进行深入研究:1）板式家具节点有限元模型的建立及其简化方法；2）板式家具结构耐久性有限元分析方法；3）板式家具的整体结构优化方法；4）板式家具结构设计有限元模块的二次开发；5）板式家具的智能设计及制造系统开发。     

Assessment of the bending strength of mortise-tenon and dovetail joints in leg-and-rail construction using Klainedoxa gabonensis Pierre Ex Engl. and Entandrophragma cylindricum (Sprague) Sprague

Mortise-tenon joints for working chair construction fail under bending stresses. Dovetail joints could offer an alternative due to their resistance to bending. However, furniture joint strength depends on the design of the parts and appropriateness of the timber for the construction. Information on timber-joint design combination that would improve joint strength is lacking for most secondary timbers with prospects for joinery-making. This study assessed the bending strengths of two joint designs (dovetail and mortise-tenon) for leg-and-rail construction from Klainedoxa gabonensis (a secondary timber) and Entandrophragma cylindricum (popularly used for furniture). Dovetail joints were stronger than those of mortise-tenon. For both joints, the design with longer, wider and thicker tails and tenons [large-sized (Type LS)] was stronger than its counterpart [small-sized (Type SS)]. Joints manufactured from K. gabonensis were also stronger than those from E. cylindricum. Thus, K. gabonensis could be an appropriate material for joinery/furniture production. This would broaden the raw material base for the furniture sector. However, its working chairs designed with Type LS dovetail joints would resist bending forces better and ensure stronger furniture than mortise-tenon. To offset frequent furniture breakdown, this study provides designers with reliable information regarding joint strength from different timbers to guide selection.     

A new method for estimating stiffness and strength in bolted timber-to-timber joints and its verification by experiments (II): bolted cross-lapped beam to column joints

Bolted cross-lapped joints (BCLJs) are one of the basic jointing methods used in Japan and European countries. There are, however, some problems in the design of BCLJs. With increasing use of large-scale wooden frame structures in Japan, it is necessary to establish proper estimating methods for predicting actual characteristics. A new approach was developed, using Saint Venant torsion theory, to estimate the performance of bolted timber joints in a more practical manner than using computer simulations. The calculated values were compared with the experimental results, indicating that the rotational stiffness and yield moment of BCLJs would be precisely predicted using the proposed theory. It was also found that the rotational stiffness calculated using the design method rooted on Coulombs torsion theory is about two times higher than the experimental results in the case of a rectangular arrangement of bolts.     

Finite element analysis of stress and strain distributions in mortise and loose tenon furniture joints

We studied the effect of loose tenon dimensions on stress and strain distributions in T-shaped mortise and loose tenon （M＆amp;LT） furni-ture joints under uniaxial bending loads, and determined the effects of loose tenon length （30, 45, 60, and 90 mm） and loose tenon thickness （6 and 8 mm） on bending moment capacity of M＆amp;LT joints constructed with polyvinyl acetate （PVAc） adhesive. Stress and strain distributions in joint elements were then estimated for each joint using ANSYS finite element （FE） software. The bending moment capacity of joints increased significantly with thickness and length of the tenon. Based on the FE analysis results, under uniaxial bending, the highest shear stress values were obtained in the middle parts of the tenon, while the highest shear elastic strain values were estimated in glue lines between the tenon sur-faces and walls of the mortise. Shear stress and shear elastic strain values in joint elements generally increased with tenon dimensions and corre-sponding bending moment capacities. There was consistency between predicted maximum shear stress values and failure modes of the joints.     

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。     

Testing and modeling of the behavior of wooden stairs and stair joints

Joints in traditional wooden stairs can be constructed in many different ways. Connections between tread and string are commonly made with tenons and screw joints. It is often hard to know which joint rigidity should be used in calculations. To study this type of stair, a total of 12 small straight stairs with three steps each were tested. The stairs were loaded and the deformation was measured. Modeling of the stairs with a finite element program was also performed, using linear elastic models in the analysis. The conclusion derived from the study is that the tested tread-to-string connections are not very rigid. Tolerances and accuracy of manufacture of the stair parts are important for the behavior of the joints. Comprehensive knowledge about stair joints can be used for development of new cost-effective and reliable design tools for stairs. In a computer program for stair design based on beam elements, the treads in this type of staircase should, according to this study, preferably be calculated as being hinged to the string, or else the rigidity of the joint should be a function of joint parameters.     

Cyclic behavior of Chinese ancient wooden frame with mortise–tenon joints: friction constitutive model and finite element modelling

The beams and columns in Chinese ancient wooden buildings were connected with mortise–tenon joints, which are semi-rigid due to the friction and squeezing deformation between mortise and tenon. In this paper, a friction constitutive model for the friction behavior between mortise and tenon was proposed based on the modern frictional theory. A series of surface topography and hardness tests were conducted to obtain some parameters used in the friction constitutive model. A finite element (FE) model of a Chinese ancient wooden frame with mortise–tenon joints under reversed cyclic loading was performed based on the proposed friction constitutive model, and a FE model using a constant friction coefficient was also carried out. Experimental results were used to validate the results modeled by the two models, and better agreement of the proposed model was observed.     

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。     

板式家具熔化喷射式角接合强度的研究

通过试验对柜类家具熔化喷射式角接合与常规圆榫接合的强度进行了比较分析，并用有限元法对各种角接合的柜体在受载时的变形进行了预测。     

One-year stress relaxation of timber joints assembled with pretensioned bolts

In our previous study, great increases of hysteretic damping and initial slip resistance of timber joints were attained by applying axial pretension to the steel fasteners. To evaluate the effectiveness of this method, 1-year stress-relaxation measurement was carried out. Nine prestressed joints were prepared and three of them were restressed after 3 and then 6 months after the initial prestressing. All joints were exposed to indoor conditions, and relaxation of the pretension was regularly measured from time-dependent decreases of axial strain of the bolts. After measurement, the joints were subjected to cyclic and monotonic loading tests until failure. The average ratio of residual stress to the initial prestress after 1 year was about 0.23 and 0.66, respectively, for joints without restressing and those with restressing. A simulated stress-relaxation curve developed from the four-element relaxation model predicted 3% of the initial stress after 5 years. Without a regular restressing program, the initial prestressing effect therefore must be considered negligible. However, about 20% of the pre-stress level can be reasonably assumed if restressing is carried out annually. This small residual stress was found to introduce suffi cient frictional damping to signifi cantly increase the equivalent viscous damping ratio of the joints. Part of this study was presented at the 10th World Conference on Timber Engineering, Miyazaki, June 2008     

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.     

Bending resistance of repaired column members and shear resistance of opening frames with repaired columns of conventional Japanese wooden houses

When it is necessary to repair conventional Japanese wooden houses, the decayed lower parts of columns should be replaced with new wood material. The bending resistance of columns repaired by four methods and the shear resistance of opening frames with those repaired columns were investigated in this study. Bending tests of the repaired columns showed differences in initial bending stiffness and maximum bending moment related to the repair methods and loading direction. Racking tests were conducted on door opening frames with conventional door head members or upper partial walls sheathed with 12-mmthick plywood. The conventional frame specimens broke at door head-column joints with no obvious bending deformation of the columns, resulting in little difference in load-shear deformation curves among the repair methods. The columns of plywood-sheathed specimens, on the other hand, clearly were bent after the nails at the plywood-to-wood frame joints started to pull off. The load-shear deformation curves of the plywood-sheathed specimens did not vary regardless of the repair methods when shear deformations were small but were affected by repair methods as shear deformation increased.     

Modelling the creep behaviour of chipboard: The rheological approach

Summary The process of applying the rheological approach to describe creep deformation has been investigated using the 5-parameter model developed by Pierce and Dinwoodie (1977).A method utilising exponential curve fitting techniques was examined by attempting to produce fits to the first 6 months' data from long-term experiments (12 years). The procedure included a means of assessing the variability occurring in the derived model parameters which showed that acceptable fits were obtainable for only 3 out of the 20 data sets available. Further analysis of one of these data sets (that for cement-bonded particleboard, subjected to a stress level of 30% of the short-term ultimate in an environment of 25 °C and 90% relative humidity) revealed that the model attributed most of the creep deformation to the viscous component; resulting in an over-estimate of the deformation at 12 years when the 6-month fit was projected.An alternative approach to deriving the value of the model parameters is proposed, based on the theory underlying the model. The applicability of this approach was investigated using the above mentioned data set. It was found that the model can be constrained to give a more realistic ratio between the time-dependent components of deformation (ie. viscoelastic and viscous creep) for the material considered, thereby improving the accuracy of the projected creep deformation over the 12-year period.     

汽车运材板式轨道的研究

作者根据运材岔线和装车场的装车线具有临时性的特点,研制了可拆卸的汽车运材钢筋混凝土板式轨道。其设计是把水泥混凝土板块作为纯受弯构件,在荷载作用下,根据破坏的不同阶段,并考虑到平截面、弹性体和受拉区不参加工作等假设条件,用许用应力法和极限状态法分别进行设计和比较,确定出适用于不同车型和运材方式的板块。     

现浇混凝土结构中临时留缝的分析

探讨了混凝土结构中各种临时留缝的种类及留缝原因,着重讨论了后浇带的设计与施工中存在的一些问题与解决方法,分析了由于施工工艺而在墙、柱构件中形成的水平施工缝对墙、柱构件抗剪承载力与节点强度的影响,认为应从设计的高度去处理工程结构中的临时留缝,并提出了解决的建议.     

Quasi-non-linear fracture mechanics analysis of the splitting failure of single dowel joints loaded perpendicular to grain

A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of dowel joints with a single dowel loaded perpendicular to grain. The properties of the elastic foundation are chosen so that the perpendicular-to-grain tensile strength and fracture energy properties of the wood are correctly represented. It is shown that this particular choice of foundation stiffness makes a conventional maximum stress failure criterion lead to the same solution as the compliance method of fracture mechanics. Results of linear elastic fracture mechanics are obtained as a special case by assuming an infinitely large value of the foundation stiffness. Results of tests on so-called plate joints are compared with theoretical predictions, showing good agreement for variations in initial crack length as well as edge distance.     

Lateral resistance of anchor-bolt joints between timber sills and foundations III: numerical simulations of the effect of sill thickness on the effective lateral resistance of multiple anchor-bolt joints

Effective lateral resistance of multiple anchorbolt joints was estimated by considering sill thickness or length/diameter ratios of anchor bolts. Load-slip relationships of single anchor bolt joints were analyzed by the stepwise linear approximation based on the generalized theory of a beam on an elastic foundation and the criterion of “fracture bearing displacement” for several sill thicknesses or length/diameter ratios of anchor bolts. Monte Carlo simulations of the effective lateral resistance of multiple anchor-bolt joints were conducted using the analyzed load-slip curves of single anchor-bolt joints. Effective resistance ratios of multiple anchor-bolt joints were simulated for some combinations of length/diameter ratios of anchor bolts, lead-hole clearances, and number of anchor bolts. The simulated results are: (1) the influence of lead-hole clearance becomes more apparent as length/diameter ratios of single anchor-bolt joints decrease; (2) there is no obvious effect of number of anchor-bolts over the range of 5 to 15; (3) average effective resistance ratios can be adopted for allowable stress design; and (4) reduction of effective resistance ratios should be considered particularly for small length/diameter ratios of anchor-bolt joints.     

Probabilistic strength prediction of adhesively bonded timber joints

The strength prediction of adhesively bonded timber joints is difficult due to the anisotropic and brittle nature of the adherends, the complex stress distribution as well as the uncertainties regarding the associated material resistance. This paper describes a probabilistic method for the strength prediction of balanced double lap timber joints. The method considers the statistical variation and the size effect in the strength of timber using a Weibull statistical function. The design method presents an explanation for the increased resistance of local zones subjected to high stress peaks as it takes into account not only the magnitude of the stress distributions, but also the volume over which they act. The predicted joint strengths are slightly underestimated compared with the experimental results due to inaccurate upper tail modelling of the material strength by the Weibull statistical distribution. The probabilistic method provides reasonable results for brittle joint failure and has immediate application in the design of adhesively bonded timber joints.     

Weeds in the South Western Cape Vegetation

The joint-slip in nailed joints in the form of standard block shear specimens was measured with the aid of a small apparatus which can be fixed directly to the specimen.

The method of measuring joint-slip of nailed joints as described in this paper yielded very accurate and satisfactory results. The load-displacement curves obtained are very typical for nailed joints.

The deformation registered by the Instron testing machine did not represent the true joint-slip, but rather the total deformation which took place in the wood as well as the machine parts.