Rotational performance of traditional Nuki joints with gap I: theory and verification

The Nuki joints in Taiwan and Japan are similar in appearance; however, due to lack of wedges used in Nuki joints in Taiwan, the gap between the column and beam increases the complexity of timber joints. In this article, the rotational performance of traditional timber joints is reported. A theoretical model considering Hook’s law and Hankinson’s formula was developed for predicting the rotational performance of Nuki joints with gaps. A total of 24 specimens was tested and used to verify the rotational performance of timber joints. The proposed model not only predicts the rotational stiffness of Nuki joints, but can also estimate the initial slip of these joints. Good agreement was found between predicted and experimental data.     

Effective lateral resistance of combined timber joints with nails and bolts

An experimental study on combined steel-to-timber joints with nails and bolts is conducted in this study. Principal results are as follows: The initial stiffness and effective allowable resistance of combined joints depend obviously on clearances in predrilled bolt-holes. The combined joints with nails and bolts have high potential of energy capacity to resist strong earthquake forces. There are upper limits of clearances in predrilled bolt-holes that allow advantages of considering the synthetic resistance of combined joints in practical structural design. Combining the joint components with appropriate design will give higher performance against strong earthquakes increasing the safety margin and energy capacity until the failure. The combined joints should be designed under the restrictions of particular specifications in closed design systems because the advantages of combining the joint components are influenced obviously by various actual conditions, which are too difficult to consider in detail in open design systems.     

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.     

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.     

Load-carrying capacity of steel-to-timber joints with a pretensioned bolt

Previous experimental studies reported that bolt pretensioning greatly increases the initial stiffness and load-carrying capacity of bolted joints. It is also a matter of great importance to structural designers to understand the effect of pretension on the load-carrying capacities of bolted joints, and this study presents an extended yield model that considers the fastener’s pretension force. In the extended yield model, the load-carrying capacity was defined as the load at a slip of 15 mm. The ultimate fastener bending angle at the yielded cross section equivalent to this joint slip, which was affected by the fastener’s axial force, was iteratively evaluated in numerical analyses. The introduction of bolt pretensioning largely increased the joint slip resistance at initial loading, but it decreased the ultimate fastener bending angle. This decrease of fastener bending angle resulted in a relatively low stiffness hardening (or secondary stiffness), which is caused by secondary axial forces associated with embedment of steel plates into the wood member. Prediction was verified by the tests of 36 steel-to-timber joints under three different pretension forces and two loading directions relative to the grain. Some of the observed load-carrying capacities of the joints, particularly in loading perpendicular to the grain, however, were not as high as those expected by the numerical analyses considering the given pretension forces.     

Bending moment resistance of corner joints constructed with spline under diagonal tension and compression

We determined the effects of the penetration depth and spline material and composite material type as well as joining method on bending moment resistance under diagonal compression and tension in common wood panel structures. Composite materials were laminated medium density fiberboard (MDF) and particle board. Joining methods were buttand miter types. Spline materials were high density fiber board (HDF).The penetration depths of plywood, wood (Carpinus betolus) and splinewere 8, 11 and 14 mm. The results showed that in both diagonal compression and tension, MDF joints are stronger than particle board joints,and the bending moment resistance under compression is higher compared with that in tension. The highest bending moment resistance under tension was shown in MDF, butt joined using plywood spline with 8 mm penetration depth, whereas under compression bending moment resistance was seen in MDF, miter joined with the HDF spline of 14 mm penetration depth.     

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.     

On mechanical behavior of traditional timber shear wall in Taiwan I: background and theory derivation

The objectives of this study were to explore the mechanical behavior of traditional timber shear walls in Taiwan and to propose a theoretical model to predict their lateral force resistance. An extensive field investigation was conducted, and the dimensions, tectonic detail, and materials used were recorded. The data collected were used as the reference for theoretical derivation and experimental design. In the theoretical model, the moment resistance of entire shear walls was derived from the contributions of the moment-resisting capacity supplied not only by embedment and friction action between board units and beams but also the dowel action of bamboo nails. Timber shear walls with various geometric conditions and material properties are considered. The theoretical model demonstrated in this study can be used to predict the mechanical behavior of timber shear walls and will be verified by experiments in our next article.     

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     

Lateral resistance of anchor-bolt joints between timber sills and foundations II. Effective lateral resistance of multiple anchor-bolt joints

Monte Carlo simulations were conducted to estimate the effective lateral resistance of multiple anchor-bolt joints with ordinary specifications of Japanese post and beam constructions. Basic lateral load-slip curves of single anchor-bolt joints required in the simulations were determined from the test results of our earlier report. The effective lateral resistance of multiple anchor-bolt joints was estimated for some combinations of loading direction, length/diameter ratio of anchor bolts, lead-hole clearance, and number of anchor bolts. The principal results of the simulations are: (1) anchor-bolt joints loaded perpendicular to lateral forces are not recommended to be counted as supplementary resisting elements because their supplementary shares are far less than those expected from their allowable lateral resistance; (2) multiple anchor-bolt joints with small length/diameter ratios have comparatively lower effective resistance ratios than multiple anchor-bolt joints with large length/diameter ratios; (3) the effective resistance of multiple anchor-bolt joints is affected not only by lead-hole clearance or number of bolts but also variance of load-slip characteristics of single anchor-bolt joints. Part of this work was presented at WCTE2004, Lahti, June 2004     

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.     

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.     

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     

Bending moment resistance of dowel corner joints in case-type furniture under diagonal compression load

We investigated bending moment resistance under diagonal compression load of corner doweled joints with plywood members. Joint members were made of 11-ply hardwood plywood of 19 mm thickness. Dowels were fabricated of Beech and Hornbeam species. Their diameters(6, 8 and 10 mm) and depths of penetration(9, 13 and 17 mm) in joint members were chosen variables in our experiment. By increasing the connector's diameter from 6 to 8 mm, the bending moment resistance under diagonal compressive load was increased, while it decreased when the diameter was increased from 8 to 10 mm. The bending moment resistance under diagonal compressive load was increased by increasing the dowel's depth of penetration. Joints made with dowels of Beech had higher resistance than dowels of Hornbeam. Highest resisting moment(45.18 N·m) was recorded for joints assembled with 8 mm Beech dowels penetrating 17 mm into joint members Lowest resisting moment(13.35 N·m) was recorded for joints assembled with 6 mm Hornbeam dowels and penetrating 9 mm into joint members.     

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.     

Mechanical behaviour of finger joints at elevated temperatures

Finger joints are commonly used to produce engineered wood products like glued laminated timber beams. Although comprehensive research has been conducted on the structural behaviour of finger joints at ambient temperature, there is very little information about the structural behaviour at elevated temperature. A comprehensive research project on the fire resistance of bonded timber elements is currently ongoing at the ETH Zurich. The aim of the research project is the development of simplified design models for the fire resistance of bonded structural timber elements taking into account the behaviour of the adhesive used at elevated temperature. The paper presents the results of a first series of tensile and bending tests on specimens with finger joints pre-heated in an oven. The tests were carried out with different adhesives that fulfil current approval criteria for the use in load-bearing timber components. The results showed substantial differences in temperature dependant strength reduction and failure between the different adhesives tested. Thus, the structural behaviour of finger joints at elevated temperature is strongly influenced by the behaviour of the adhesive used for bonding and may govern the fire design of engineered wood products like glued laminated timber beams.     

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。     

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.     

On mechanical behavior of traditional timber shear wall in Taiwan II: simplified calculation and experimental verification

In the previous report of this ongoing study, results of an extensive field survey were collated and a theoretical model was proposed to predict the mechanical behavior of timber shear walls of traditional design in Taiwan. The initial objective of the present report was to propose a simplified calculation method for estimating the initial stiffness and yield strength of traditional timber shear walls. Based on the results of the field survey, a total of 15 full-scale specimens were tested to verify the theoretical model and simplified calculation proposed previously. Good agreement was found from comparison of analytical and experimental results. The results of this study show that the friction behavior between board units and beams plays the major role in resisting the lateral force applied on the timber shear wall, followed by the resistance supplied by embedment. The resistance provided by bamboo nails is minor due to the small section. Another trend found was that for set dimensions of a timber shear wall, the board width can be increased to obtain higher stiffness and strength of the shear wall.     

Water vapour diffusion through historically relevant glutin-based wood adhesives with sorption measurements and neutron radiography

In this work, the sorption and moisture diffusion behaviour of historically relevant glutin-based adhesives (i.e. bone glue, hide glue, fish glue) is characterized. The adhesive’s sorption isotherms were assessed on thin film samples revealing fundamental differences between the glutin-based adhesives and the synthetic reference adhesive (polyurethane). Furthermore, the water vapour diffusion parallel to the fibre was examined by means of neutron imaging on bonded two-layer samples of Norway spruce wood. In contrast to previous studies using neutron imaging, a new evaluation approach is presented, which allows for nonzero initial moisture conditions and takes into account and compensates for the geometry changes in the sample caused by swelling and shrinkage, thus allowing for a characterization of the diffusion behaviour within the glue line. The diffusion coefficients determined with neutron imaging were interpreted in terms of a theoretical model which takes into account the glue line microstructure. Although the diffusion coefficients were on average larger values for the glutin-based adhesives compared to the reference polyurethane adhesive, the significant variation observed in the sorption measurement is not reflected. This can partially be ascribed to excessive penetration of the adhesives into the wood substrate in fibre direction, which impedes a continuous adhesive layer. Furthermore, deformation and densification of the wood structure was assessed in the vicinity of the adhesive joint. This effect can be ascribed to the surface roughness, which results in very high local stresses leading to buckling and deformation of the tracheids. This situation is similar to that found for adhesive joints in or close to the fibre direction such as finger or butt joints.