Estimation of yield and ultimate strengths of bolted timber joints by nonlinear analysis and yield theory

A finite element nonlinear analysis was conducted on bolted timber joints under lateral loads parallel and perpendicular to the grain. The results obtained from this analysis were compared with the experimental results and calculated values based on the yield theory. The analysis and experiment were performed on double shear bolted joints parallel and perpendicular to the grain with steel side plates and a slotted-in steel plate. It was found from the analysis that the yielding of wood and bolt occurred before the overall yielding of the bolted joint. Shear strength of bolted joints calculated from the yield theory using the embedding yield strength of wood and the yield moment of the bolt showed comparatively good agreement with the shear strength evaluated by 5% offset of the load–slip curve in the experiment and analysis. The shear strength of the bolted joint calculated from the yield theory using the embedding ultimate strength of wood and the ultimate moment of the bolt agreed quite well with the shear strength evaluated by the maximum load up to 15mm slip in the analysis. The former, parallel and perpendicular to the grain, were 11% and 34%, on average smaller than the latter in the experiment.Part of this paper was presented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002; the Annual Meeting of Architectural Institute of Japan 2002, Ishikawa, August 2002; and the World Conference on Timber Engineering 2002, Shah Alam, Malaysia, August 2002     

Effect of decay on shear performance of dowel-type timber joints

Monotonic and reversed cyclic loading tests were conducted on dowel-type timber joints with varying degrees of wood decay due to Fomitopsis palustris (Berk. et Curt.), a brown rot fungus, and the effect of decay on various shear performances of dowel-type joints was investigated. For joints affected by the brown rot fungus, the initial stiffness, yield load, and maximum load of dowel-type joints were significantly decreased, even with a small mass loss of wood. The reductions in shear performance were the largest for initial stiffness, followed by yield load and maximum load, in that order. For a 1% reduction of the yield load, initial stiffness and maximum load showed reductions of 1.15% and 0.77%, respectively. When dowel-type joints that had been exposed to the brown rot fungus were subjected to reversed cyclic loading, the gap between the dowel and the lead hole of the wood was increased and equivalent viscous damping was decreased. These results indicate that decay around the dowel lead hole especially affects the load-displacement behavior at small displacement level, and dowel-type joints under cyclic loading have very low resistance to forces acting on the wooden structure. Part of this report was presented at the 5th Symposium on Timber Bridges of the Japan Society of Civil Engineers, Tokyo, July 2006; the 56th Annual Meeting of the Japan Wood Research Society, Akita, August 2006; and the Annual Meeting of the Architectural Institute of Japan, Fukuoka, August 2007     

Evaluation of embedding strength on Uruguayan wood to apply the European yield theory for double shear bolted joint

The main objective of this study was to evaluate embedding strength on Uruguayan wood to apply the European yield theory (EYT) for double shear bolted joints’ on Eucalyptus grandis H.(EGH). To introduce the dowel-type connection performance, double shear tests were conducted. The embedding tests were conducted to calculate the yield strength of bolted joint by EYT and the compression test, to estimate the embedding strength. The yield strength obtained from the experiments showed a good agreement with the yield strength calculated by EYT method. The yield strength of double shear bolted joint evaluated from compressive strengths is a very close to the yield strength calculated by EYT. The average value and variability of the yield strength of double shear bolted joint calculated by EYT applying the embedding strengths of experimental results were very close to the yield strength or 5 % offset method of experimental results. The results from this study showed a good behavior to structural design with EGH in accordance to the Japanese standard code.     

Numerical simulation of embedding strength of glued laminated timber for dowel-type fasteners

A non-linear material model is proposed to describe the timber behavior. Anisotropic elasto-plastic constitutive law with hardening according to Hill yield criterion was used for the compressive behavior. Brittle behavior in tension and shear was modeled using the modified Hill failure criterion. The material model was implemented in a finite element code to simulate embedding strength of glued laminated timber for dowel-type fasteners. Reasonable agreement is found between numerical simulations and experimental measurements.     

空心胶合木-钢插板螺栓节点承载力试验研究

为研究空心胶合木-钢插板螺栓连接节点承载力及延性性能,探讨了不同胶合木宽度和螺栓直径对破坏模式的影响,分析了不同宽径比、承压宽度对节点承载力、延性等的影响。结果表明:增加宽径比及承压宽度,可有效提高节点延性。宽径比12.5~16.4的节点延性较佳;宽径比10.0~10.9是单、双塑性铰破坏模式的临界点,可作为工程设计参考值。     

Effects of pretension in bolts on hysteretic responses of moment-carrying timber joints

The adoption of a concept similar to the prestressing technique used in laminated wood decks of bridge structures might increase the initial stiffness or ultimate resistance of dowel-type timber joints by applying pretension to their bolts. This study investigated the effect of pretension in bolts on hysteretic responses and ultimate properties of moment-carrying timber joints with steel side plates. A pretension of 20 kN that yielded a prestress level of 1600 kPa or about 90% of the allowable long-term end-bearing strength of spruce species was applied to the bolts of prestressed joints. The superiority of the prestressed joint over the non-pre-stressed joint was proved by very high hysteretic damping, equivalent viscous damping ratio, and cyclic stiffness. At any given rotation level, hysteretic damping reduction and moment resistance decrement due to continuously reversed loads were found to be small because bolt pretensioning minimized the pinching effect. This study showed that the hysteresis loop of the prestressed joint can be obtained by adding the frictional hysteresis loop due to pretension force into the hysteresis loop of the non-pre-stressed joint. Despite a great increase of initial stiffness, only slight increments in ductility coefficient and ultimate moment resistance were found in the prestressed joint.     

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.     

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。     

Prediction of the load carrying capacity of bolted timber joints

Failure of bolted timber joints is analyzed experimentally and numerically. In this study, the prediction of the load-carrying capacity of dowel-type joints with one dowel under static loading is based on the analysis of fracture in wood contrarily to most engineering methods that are based on the yield theory. Mechanical joints consist of glued laminated spruce members and steel dowels. In the different analyzed tests, the bolt loads the wood parallel or perpendicular to the grain. The wood member thickness is chosen sufficiently thin to avoid the fastener from presenting plastic hinges. The influences of different structural parameters such as the dowel diameter, the edge- and end-distances are investigated. The fracture propagation analysis is carried out with the Finite Element (FE) method in the framework of Linear Elastic Fracture Mechanics (LEFM). The only identified parameter is the critical energy release rate in mode I (G_Ic). The comparison between experimental and numerical results shows that the fracture must be considered for a correct prediction of the ultimate load and that LEFM can help to improve design codes. Received 11 August 1997     

Thermal degradation modeling for single-shear nailed connections

The load-carrying capacity of a timber structure is highly dependent on the strength of its connections. Very limited information exists on the thermal degradation, or the properties at elevated temperatures, of such connections. This insufficient information is one of the major impediments for modeling elevated temperature performance for wood-frame structures. The objective of this study was to characterize thermal degradation of yield load capacity of single-shear nailed connections between wood and oriented strand board (OSB) as a function of time and temperature. The mechanism of degradation was explained using first order kinetics. Using the principles of time–temperature superposition (TTS), predictive models for connection capacity degradation were developed. A total of 450 wood-to-OSB connections were tested laterally after exposure to nine different elevated temperature regimes. The degradation of yield load strength over time occurred at a constant linear rate. Temperature dependence of this rate was modeled using the Arrhenius activation energy theory. Using Arrhenius activation energy theory and principles of TTS, a master curve was developed to predict the performance of connections after exposure to a temperature of 150°C. The predictions matched well with independent experimental observations. The master curve developed using TTS provides predictive estimates of residual capacity in a connection as well as its failure times.     

木结构燕尾榫节点应力分析研究

以古建筑中燕尾榫节点为研究对象,依照《营造法式》“材分制”标准,设计三个不同模型比例的二等材燕尾榫节点和一个三等材燕尾榫节点,基于ABAQUS对其进行单调加载与低周循环加载模拟试验,得到燕尾榫节点的拔榫量转角关系与应力状态。试验结果表明:节点模型比例越大拔榫量越大,且同模型比例下二等材较三等材拔榫量更大;模拟中榫头顺纹方向、剪应力均未超过应力极限,而横纹径向除节点YS-3外均达到屈服,进入弱强化段;榫头等效塑性应变(PEEQ)值呈线性增长,在榫头端部的PEEQ值最小,榫颈处PEEQ值达到最大。     

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...     

Determination of embedding strength of wood for dowel-type fasteners

Embedding tests parallel and perpendicular to the grain were conducted to produce a database of embedding strength of wood for the design of dowel-type joints. Dowel diameters were 8,12,16, and 20mm. Embedding strength was evaluated by the 5% off-set method and a maximum load up to 5mm displacement according to EN383. The embedding strength parallel to the grain evaluated by the former method showed values close to those obtained with the latter method, but they showed a significant difference in tests conducted perpendicular to the grain. The embedding strength parallel to the grain was 0.9 times as large as the compressive strength parallel to the grain regardless of the evaluation method. The embedding strength perpendicular to the grain evaluated by the 5% off-set method was four times as large as the compressive strength perpendicular to the grain. When the embedding strength perpendicular to the grain was evaluated by a maximum load up to 5mm displacement according to EN383, the ratio of embedding strength perpendicular to the grain to the compressive strength perpendicular to the grain decreased as the dowel diameter increased.Part of this paper was presented at the annual meeting of the Architectural Institute of Japan 1999, Hiroshima, September 1999; the 50^th annual meeting of the Japan Wood Research Society, Kyoto, April 2000; and the World Conference on Timber Engineering 2000, Whistler, Canada, July–August, 2000     

A new proposal to reinforce planked timber shear walls

Timber is one of the most common materials used in traditional buildings worldwide. Our previous research has suggested that timber shear walls play an important role in resisting external loadings, such as earthquakes. Thus, improving the structural performance of in-filled shear walls can also improve that of the entire structure. In the traditional Taiwanese timber shear wall system, the embedment strength of beams and friction between wooden planks and beams significantly affect the strength of the shear wall. This article proposes a new method of reinforcing traditional timber shear walls in Taiwan by inserting teak and padauk strips into the grooves between wooden planks and beams to increase the embedment strength of beams and the friction between wooden planks and the hardwood strips. A total of 18 full-scale specimens were tested under reversed cyclic loading. The results revealed that the strength and energy dissipation capacities of a wooden shear wall can be significantly increased by inserting teak and padauk strips into the grooves between planks and beams. Furthermore, the simplified calculation method proposed in this study can be used to calculate the strength of both reinforced and unreinforced wooden shear walls with satisfactory agreement.     

Feasibility of improved slotted bolted connection for timber moment frames

This note examines the feasibility of an improved slotted bolted connection for timber moment frames. In the improved connection, steel tubes are inserted into drill holes in glulam and fixed to the glulam with resin injection. Aluminum splice plates with curved slots, or curved elongated holes, are fastened mechanically by using high-strength bolts that go through the steel tubes. Since the compression due to bolt tension is fully supported by the steel tubes, the reduction of bolt tension due to shrinkage of the glulam can be avoided. The use of slotted aluminum splice plates allows stable energy dissipation due to smooth sliding between the aluminum splice plates and the end surfaces of the steel tubes within the specified range of rotation angle. Through quasistatic cyclic loading tests of two connection specimens, it was demonstrated that stable and nearly rigid-plastic hysteresis loops were obtained whose equivalent viscous damping ratio was more than 30% in the range of rotation angle close to or greater than 1/50 radian. Although further improvement is necessary, the experimental results demonstrate the feasibility and potential of the present connection.     

Estimation of failure lifetime in plywood-to-timber joints with nails and screws under cyclic loading

The performance of plywood-sheathed shear walls is determined at the plywood-to-timber joints. In joints with dowel-type fasteners, such as nails and screws, the fastener is fractured under reversed cyclic loading (e.g., seismic force), reducing the ductility of the joint. The fracture is caused by low-cycle fatigue due to the reversed cyclic bending of the fastener. Therefore, evaluating the fatigue life is important for estimating the ultimate displacement. The main objective of this study is to estimate the ultimate displacement of the joints and to enable load–displacement calculation of single shear joints under reversed cyclic displacement when bending fatigue failure of the fastener occurs. Single shear tests were conducted under different loading protocols, and the damage performances of the fasteners were determined by subjecting them to reversed cyclic bending tests. Based on the results, the failure lifetimes of joints with dowel-type fasteners were estimated. In addition, the fracture mechanism of these dowel-type fasteners was elucidated. CN50-type nails and wood screws with dimensions of 4.1?×?38 and 4.5?×?50 mm were used as fasteners. The single shear tests showed that the smaller the displacements per cycle, the lower are the ultimate displacement and ductilities of the joints. Moreover, load–displacement relationship up to fastener failure can be approximately estimated by combining the yield model and failure lifetime.     

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     

Fracture analysis of perpendicular to grain loaded dowel-type connections using a 3D cohesive zone model

The strength and fracture behavior of dowel-type connections with stiff dowels loaded perpendicular to grain was studied by nonlinear 3D finite element (FE)-analysis. A cohesive zone model was used to model the perpendicular to grain fracture of the wood, i.e., failure by wood splitting along the grain. The influence of load eccentricity and dowel-to-loaded-edge distance was studied for a plate type of geometry loaded in tension and for a simply supported beam loaded in bending. The strength found from the FE-analysis is compared to strength from experimental tests with centric loading, showing overall good agreement. Numerical results for centric loading are further compared to strength predictions according to the linear elastic fracture mechanics (LEFM)-based design criterion present in Eurocode 5 (EN 1995-1-1:2004). The comparison showed good agreement regarding the relative influence of connection geometry, but the design criterion appears, however, to yield unconservative strength predictions. The results of the FE-analyses regarding dowel load eccentricity showed that such loading conditions may yield significantly lower strengths compared to centric loading. An approximate engineering method to account for the strength reduction due to load eccentricity is, furthermore, presented.     

Development of LVL frame structures using glued metal plate joints II: strength properties and failure behavior under lateral loading

In past years high priority was given to developing a seismic design for wood structures, including research on the response of wood structures to earthquakes. In this study a new type of portal frame with relatively large span was developed for the traditional Japanese wooden houses with large openings at the front to strengthen the structure. Stainless steel plates coated with zinc and glued with epoxy adhesives on laminated veneer lumber (LVL) members, composed of Douglas fir veneer and bonded with phenolformaldehyde resin, were used. The connection between the frame's beam and columns and between the columns and groundsills was mechanical, with bolts. The subject of this research was to analyze strength properties and failure behavior of glued LVL metal joints used as structural components and to evaluate the response of LVL portal frames under cyclic lateral loading. The results show that portal frames using glued LVL metal plates have a good multiplier for the shear walls and may be applied to traditional Japanese structures. The equivalent viscous damping provided good energy dissipation in the frames. The joints displayed good mechanical behavior during tests; moreover, the structures demonstrated high strength, stiffness, and ductility, which are necessary for a seismic design.Part of this paper was presented at the 47th annual meeting of the Japan Wood Research Society, Kouchi, April 1997; and at the 5th world conference on timber engineering. Montreux, Switzerland, August 1998