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.     

An application of the Tsaï criterion as a plastic flow law for timber bolted joint modelling

Summary This paper describes a modelling approach to predict the behaviour of an elementary thin timber bolted joint. The application concerns principally joints with steel side members; bolts have a constant 12 mm diameter with two ratios of end distance to bolt diameter and two bolt clearances. The behaviour of the bolted joints is characterized by a double non linearity; the first one is due to the contact area evolution between the bolt and the hole of the jointed elements. The second one is owing to the evolution of plasticity on the wood. A spring element compatible with isoparametric plane finite elements represents the contact evolution. The elastic-plastic wood is provided with a plastic flow rule according to the Tsa? criterion. This study allows an investigation on the parameters characterizing the Tsa? criterion, particularly F₁₂ which represents the interaction between the principal axis of orthotropy. A two-dimensional model is used. It permits the assessment of the clearance bolt, joint dimensions, wood plasticity and wood grain angle effect on the joint behaviour. The wood grain angle has a non negligible effect on the plastic strains distribution and it can create a parasite loading because the joint tends to rotate even for an axial loading. The results showed a good agreement between experimental values given by some authors and numerically-predicted stresses on the joint. So, the applications concern a two-dimensional joint with anisotropic plastic material. The generalization in the three-dimensional modelling is desirable to take into account the interaction between the wood and a metallic fastener in thick joints with different geometric characteristics.     

Nonlinear superposition model for the behavior of bolted joints

Summary A previously developed model to predict the load-slip relationship for mechanical joints using one bolt subjected to lateral loading (Part 1) was extended to incorporate the rotational resistance of joints containing two bolts. The rotation is about the long axis direction of the bolts, and considers the wood members oriented at arbitrary angles to the grain. The model utilizes nonlinear translational springs to represent the parallel and perpendicular to grain components of the reaction force present on each bolt resisting the applied moment. A series of experiments were conducted to determine the spring constants of bolted joints in axial loading and to verify the predictions of the mathematical model. Bolted joints subjected to a pure rotation were tested using combinations of steel plates and wood side members experiencing bolt reaction forces at various angles of load to grain. The results indicated an excellent agreement between theoretical predictions and experimentally obtained data.The authors wish to extend thanks to the Colorado State Agricultural Experiment Station for their financial support and to the Fulbright-Hayes Foundation for the educational scholarship and research funding provided to the project     

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     

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     

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.     

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。     

Nonlinear superposition model for the behavior of bolted joints

Summary A nonlinear superposition model was developed to assess the load-slip behavior of bolted joints consisting of a single bolt subjected to lateral loading at angles of load to grain. This model characterizes the bolted joint as a pair of orthogonal nonlinear springs aligned parallel and perpendicular to the grain of the wood members. The spring stiffnesses are quantified by a logarithmic or exponential function depending upon whether the connection softens or stiffens with increasing slip. The spring deformations are superimposed to determine the movement of each component of the connection. Deformations of connected members are added vectorially to determine their relative displacement. Spring constant were determined experimentally using metal-to-wood connections. Thick steel side plates were employed to limit the system deformation to the wood component. Wood members were evaluated at angles of load to grain ranging from zero to ninety degrees. Once the spring constants had been determined, the model was executed to predict the load-slip behavior of wood-to-wood connections. These predictions were compared to experimentally obtained load-slip values. The results indicate that the nonlinear superposition concept is a valid approach to predict joint deformation at angles of load to grain.The authors express their gratitude to the Fulbright-Hays Foundation and the Colorado State Agricultural Experiment Station for their financial support of this study     

Fracture of multiply-bolted joints under lateral force perpendicular to wood grain

The crack initiation and propagation of multiplybolted joints subjected to lateral forces perpendicular to the grain were analyzed. Two types of bolted joint were subjected to lateral loads perpendicular to the grain. One had joints of two bolts aligned with the wood grain (type H), and the other had joints of two or three bolts aligned perpendicular to the grain (type V). The crack initiation and propagation were analyzed by means of the average stress method (ASM) and linear elastic fracture mechanics (LEFM), respectively. The maximum loads calculated by LEFM agreed comparatively well with the experimental results, and it was proved that the LEFM was an appropriate tool to analyze the fracture of multiply-bolted joints subjected to a force perpendicular to the grain. It was also found that the multiply-bolted joints failed with the fracture of the wood before the joints yielded, and that it caused a considerable decrease of the maximum loads. The reduction of strength should be considered in the design of multiply-bolted joints subjected to lateral forces perpendicular to the grain.Part of this work was presented at the annual meeting of the Architectural Institute of Japan, Hikone, September 1996     

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滞回性能试验。【结果】试验结果表明:在单螺栓连接中,连接部位的破坏模式逐渐由"螺栓刚直"向"双铰"转化,胶合木销槽破坏模式逐渐由销槽整体承压破坏向两端部挤压破坏转变,试件滞回曲线基本都呈现饱满的弓形和棱形,具有良好的耗能能力和抗震性能,但其承载能力较低。在多螺栓连接中,螺栓和胶合木的破坏模式分别以"双铰"破坏和销槽端部挤压破坏为主,试件滞回曲线均呈现饱满棱形,该类试件在承载能力、抗震性能和耗能能力上均有大幅提升;随着螺栓顺纹间距的增大,试件的承载能力不断增大,但螺栓顺纹间距在200 mm时,极限荷载增幅趋于平缓,初始刚度增涨大幅放缓,且整体刚度退化与螺栓顺纹间距为250 mm时基本相同;螺栓并列布置滞回曲线饱满程度好于错列布置;螺栓双排布置承载能力比单排布置的承载能力更高,刚度退化更小。【结论】胶合木-钢夹板螺栓连接具有较好的耗能能力、抗震性能及延性性能;螺栓顺纹间距在200 mm时,抗震性能最佳;螺栓错列布置的抗震性能比并列要好,螺栓双排布置的抗震性能更优越。     

Numerical analysis on tensile performance of bolted glulam joints with initial local cracks

Under varying climate conditions, cracks are commonly observed in bolted joints, owing to the shrinkage of wood and confinement from slotted-in steel plates and bolts. A three-dimensional finite element model was developed to investigate the mechanical behavior of bolted glulam joints with initial cracks. Wood foundation was prescribed in the model to simulate the local crushing behavior of wood surrounding the bolts. The behavior of wood in compression and the foundation were defined as transversely isotropic plastic in the software package ANSYS. Cohesive zone model was applied in the numerical analysis to consider the propagation of initial cracks and brittle failure of wood in the bolted joints under tension load. The numerical model was validated by the experiments conducted on full-scale specimens and it is indicated that the numerical model has good ability in predicting the failure modes and capacity of tension joints with local cracks. To further investigate the influence of crack number, length and locations, a parametric study was conducted with the verified model. Moreover, to study the effects of cracks on the behavior of bolted joints with different failure modes, another bolted joint including bolts with different strength grades and diameters was designed and analyzed in the parametric study, which was expected to have bolt yielding failure mode. It was found that the initial cracks can decrease the capacity and initial stiffness of tension joints by up to 16.5 and 34.8%, respectively.     

Influence of crossing-beam shoulder and wood species on moment-carrying capacity in a Korean traditional dovetail joint

This study investigated the interaction effects of a crossing beam on the moment-carrying capacity of a Korean traditional dovetail joint. In particular, the length of the crossing-beam shoulder (B _s ) and the wood species were varied as important factors. Clearly, the B _s acts as a fastener that improves the performance of timber joints by preventing splitting failure parallel to the grain. All the specimens experienced tension failure by tension force in the direction perpendicular to the grain; therefore, the tension strength perpendicular to the grain could be considered an important property, and standard values could be determined to develop a formula for predicting the structural behavior of the joints or the structural design codes of the joints. The results of the tests indicated that the moment resistance of the joints increased as the length of the crossing B _s and the density of the wood species increased. Joint stiffness results also indicated that the joints became stiffer when the crossing beam had shoulders, but the results were not affected by the length of the B _s . In addition, the joint stiffness was proportional to the density of the wood species.     

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     

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.     

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.     

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 performance of a semi-rigid timber frame structure: theoretical analysis and experimental study

Semi-rigidness of the joint connections is one of the main characteristics of timber structures. The pin-joint assumption for the semi-rigid joint connections might be not conservative in the timber structural design. In this paper, structural analysis was conducted on a semi-rigid timber portal frame; the formulas were derived in terms of the internal force and the lateral stiffness, and the influence of the semi-rigid connections was discussed. Moreover, experimental tests were performed on three full-scale timber portal frames and five bolted timber connections to study the lateral performance of the frames and the moment resistance of the connections. For consistency, the connections from the portal frames and the connections for bending tests were of the same configuration. Finally, a calculation flowchart of the lateral performance on a semi-rigid frame was presented to verify the derived formulas and to show a framework of the lateral structural design process.     

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.     

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。     

Dynamic response of wall-floor joints of wooden light-frame constructions under forced harmonic vibrations

Shaking table tests of the wall-floor joints of wooden light-frame constructions under forced harmonic vibrations are conducted in this study so as to observe the dynamic responsive characteristics. The principal results are as follows: The responsive characteristics of timber constructions under strong earthquakes cannot be directly correlated with their resonant frequencies under free or forced vibrations with low input accelerations, because they behave as continuous bodies when the input accelerations are less than the apparent frictional limits of structural joints. The apparent frictional limits are reduced by periodic fluctuation of the effective vertical loads as a result of the vertical motion of the specimens. The characteristic dynamic responses of wall-floor joints depend clearly upon the frequency and input accelerations of forced vibrations. These dependencies arise from the nonlinear load-slip relationship of the wall-floor joints. The equivalent stiffness in their successive transient phases decreases as joint slip increases, which gradually changes the resonant frequencies of the wall-floor joints. This indicates that the frequency components dominant to ultimate or safety-limit resistance should be distinguished from those dominant to allowable or serviceability-limit resistance.