Withdrawal capacity of washers in bolted timber connections

Abstract

The European yield model (EYM) has been accepted to determine the load-carrying capacity of structural timber connections. However, experiments of bolted connections are still not in agreement with the EYM unless the additional bearing capacity offered by the washers is taken into account. This bearing capacity is depending on the compressive strength perpendicular to grain of the structural timber. Tests results carried out with M16 and M24 washers on solid and glued laminated timber are used to verify the reliability of three strength capacity predicting models, one of which is an analytical model while the other two are empirical. It was concluded that the analytical model is the superior one. This model should be incorporated in all new structural timber design code revisions.     

Critical discussion of perpendicular to grain tension testing of structural timber – case study on the European hardwoods ash,beech and maple

ABSTRACT

Knowledge about perpendicular to grain tension behavior of wood is essential, since in construction tension stresses perpendicular to grain cannot be avoided completely. Especially for hardwoods, the data basis is scarce. EN 338 design values are with 0.6 N/mm² characteristic strength set very low. The US-American National Design Specifications even set this value to zero and make local reinforcements mandatory. This paper compares strength and stiffness values attained with newly-designed, little, prismatic specimens and EN 408 structural timber specimens to evaluate the current European design values. Little specimen’s characteristic strength values range from 7.2 to 10.6?N/mm² and are assumed to be real material properties. EN 408 specimen values are with approximately 4.0?N/mm² lower. These lower values are mainly due to stress peaks introduced by the force introduction. Strength values attained for the medium-dense European hardwoods beech, ash and maple exceed EN 338 design values by a factor of six to seven. Adaptation of the EN 338 design value is not recommended, though. The abundance of influencing factors makes clear that the design value and the ensuing design code have to be synchronized carefully by tedious testing in order to make use of the perpendicular to grain tension strength potential of the selected hardwoods.     

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.     

正交胶合木横纹承压性能研究现状

正交胶合木（CLT）在木结构建筑中应用越来越广泛,特别是在中高层建筑物中。CLT作为楼盖构件使用时,承受柱或墙体施加的垂直荷载,因此CLT产品的设计及应用需要其横纹承压弹性模量和强度方面的参数;改善CLT横纹承压性能,也可以提高整体结构强度和建筑安全系数。文中主要介绍目前CLT横纹承压性能测试方法,总结CLT横纹承压性能影响因素方面的研究进展,以期为国内CLT研究提供参考。     

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.     

Spatial correlation of tensile perpendicular to grain properties in Norway spruce timber

Tensile strength perpendicular to grain constitutes one of the most vulnerable properties of timber. Due to versatile influencing parameters this property exhibits a high amount of uncertainty. Thus, progress in modeling, in particular by considering stochastics, is seen as worthwhile. This increases the reliability estimates of timber constructions but also their economic efficiency. Test data of tensile properties determined on consecutive board segments of Norway spruce are analyzed. The data consists of four subgroups, classified in regard to segment length and radial position within the log. The correlation in longitudinal direction of perpendicular to grain tensile strength and elastic modulus as well as of density is examined. This is done depending on the radial position of structural timber within the log. A second-order hierarchical model together with equicorrelation is used. The results outline the applicability of the model and allow the quantification of equicorrelation coefficients of all three properties. The outcome provides a valuable and necessary input for state-of-the-art mechanics-stochastic modeling of the resistance perpendicular to grain tensile strength and elastic modulus of unjointed and jointed structural timber, but in particular of products available in large dimensions, like glued and cross-laminated timber. Additionally, the spatial correlation of density is discussed which is seen as worthwhile for the estimation of group action of fasteners. The necessity to differentiate between the variability within and between segments of structural timber is clearly demonstrated.     

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.     

Varying moisture content and eigen-stresses in timber elements

Abstract

The load-bearing capacity of timber elements and systems is affected by moisture exposure. Varying moisture content in the ambient air and resulting non-uniform moisture profiles on cross-sections generate stresses perpendicular to grain because of restraint of hygroexpansion. This paper presents effects of indoor moisture diffusion in timber elements; moisture profiles within member and corresponding stresses are determined using finite element analysis and a relevant constitutive model. The stress variability is larger near the surface compared with the middle of a cross-section. No major differences are noticed between different climatic locations, but the induced stresses reach high levels above the characteristic strength in tension perpendicular to grain. It appears that indoor moisture effects differ insignificantly between the different climatic locations investigated in this study.     

An experimental approach to determine pull-out strength of single and multiple axially loaded steel rods bonded in glulam parallel to the grain

ABSTRACT

A glued-in rods' connection is generally constituted by a group of steel bars bonded by an adhesive into timber elements. In the past, most of the research focused on single-rod connections, in order to exploit the maximum resistance of the connection without accounting for interaction among bars or splitting failure in the timber member due to close edge distances or spacing between bars. Such interaction problems arise when dealing with multiple rods, thus requiring specific investigation to fully understand the behavior of the connection as a whole and to determine its capacity. In both cases, existing test procedures determine bond strength in specific geometrical configuration. The paper aims to determine the pull-out strength of single and multiple axially loaded steel rods bonded in glulam parallel to the grain differentiating the adhesive failure from the other failure modes. After an initial review of typical applications and existing design procedures, test results on single rod with confined or unconfined test setup on single rod at different embedment depths are presented and discussed, indicating that the confinement has a negligible influence on the pull-out capacity. Subsequently, interaction between bars is investigated by a specific unconfined configuration. The accounted parameters are the embedment depth of the bars, the dimensions of the timber section, and the spacing between bars. Results are discussed and compared with three-dimensional numerical simulations. Both experimental and numerical results suggest that the critical value at which the transition from pull-out to timber-related failure is observed depends on the mechanical properties of the timber and on the properties of the adhesive, such that a single value of spacing should not be provided in design standard if the full capacity of the adhesive is to be exploited.     

Fracture mechanics analysis of row shear failure in dowelled timber connections

A quasi-nonlinear fracture mechanics model is presented for the analysis of row shear failure in timber connections with multiple fasteners in a row. A prerequisite for use of the model is a known distribution on the fasteners of the total applied load. It is shown that the ideal plastic and linear elastic fracture mechanics solutions appear as special cases of the quasi-nonlinear model. The model offers strength predictions that include the effect of among others the number of fasteners in a row, fastener spacing, row spacing, end-distance, edge-distance, fastener diameter, and material properties, such as shear strength, fracture energy, and modulus of elasticity. Simple explicit expressions are obtained from the analysis for any known fastener load distribution. For bolted connections, where bolt-hole gaps cause high loads on certain bolts at random, the model may be used in simulation procedures or in probabilistic models.     

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     

Bearing properties of engineered wood products I: effects of dowel diameter and loading direction

The embedment tests of laminated veneer lumber (LVL) with two moduli of elasticity (MOE; 7.8 GPa and 9.8GPa), parallel strand lumber (PSL), and laminated strand lumber (LSL) were conducted in accordance with ASTM-D 5764. The load-embedment relation for each of these engineered wood products (EWPs) was established. The directional characteristics of bearing strength (_e), initial stiffness (k _e), and effective elastic foundation depth were obtained from the tested results. The effective elastic foundation depth (=E/k _e,E = MOE), based on the theory of a beam on elastic foundation, was obtained from thek _e and MOE. An of 90° (perpendicular to the grain) was calculated by dividingE ₉₀ [MOE of 90° from the compression test, but MOE of 0° (E ₀), parallel to the grain, obtained from the bending test] byk _e90, the initial stiffness of 90°. This study aimed to obtain the bearing characteristics of each EWP, taking into consideration their anisotropic structures, for estimating the fastening strength of a dowel-type fastener. The relations between the bearing coefficients ( _e,k _e,) on the loading direction and dowel diameter were established from the load-embedment curves. Based on the results of the embedment test, tested EWPs showed different tendencies in all directions from wood and glued laminated timber.Part of this study was presented at the 49th Annual Meeting of the Japan Wood Research Society, Tokyo, April 1999     

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     

Splitting strength of beams loaded perpendicular to grain by dowel joints

A linear elastic fracture mechanics model for calculation of the splitting strength of dowel-type fastener joints loaded perpendicular to grain (Van der Put/Leijten model) has previously been presented, and now forms the basis for design in Eurocode 5. The original Van der Put/Leijten model was derived using a number of simplifying assumptions, e.g., that the normal forces in the cracked parts of the beam can be ignored, leading to a solution that does not involve the effect of an initial crack. In the present article an extended version of the Van der Put/Leijten model is derived without any simplifying assumptions, and it is shown that the original Van der Put/Leijten model appears as a special case, namely by assuming that only contributions from shear deformations are significant. The model presented here involves the effect of an initial crack and may be characterized as a generalized linear elastic fracture mechanics model. Results of tests showing the influence of initial cracks of various lengths are presented and compared with the predictions.     

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     

The potential of young,green finger-jointed Eucalyptus grandis lumber for roof truss manufacturing

South Africa is a timber-scarce country that will most probably experience a shortage of structural softwood lumber in the near future. In this study the concept of using young, green finger-jointed Eucalyptus grandis lumber was evaluated for possible application in roof truss structures while the timber is still in the green, unseasoned state. Drying will occur naturally while the lumber is fixed within the roof truss structure. The objectives of this study were (1) to investigate the strength and stiffness variation of the finger-jointed E. grandis product in both the green and dry state for different age and dimension lumber, (2) to investigate the variation in density, warp and checking in the lumber when dried in a simulated roof-space environment and (3) to evaluate the potential of this finger-jointed product as a component in roof truss structures. Green finger-jointed E. grandis lumber of ages 5, 11 and 18 years and dimensions 48×73?mm and 36×111?mm from Limpopo province were evaluated. The study showed that the young finger-jointed E. grandis timber had very good flexural, tensile parallel to grain, and shear properties in both the green and dry state. The mean and characteristic modulus of elasticity and modulus of rupture values of the finger-jointed E. grandis product were higher and the variation lower in comparison to currently used South African pine sources. The tensile perpendicular to grain and compression perpendicular to grain strength did not conform to SANS requirements for the lowest structural grade (S5). Both tree age and product dimension were sources for variation in the physical and strength properties. Based on the results from this study the concept of producing roof trusses from green, finger-jointed young E. grandis timber has potential.     

Strength properties and effect of moisture content on the bending and compressive strength parallel to the grain of sugi (Cryptomeria japonica) round timber

Static bending tests and compressive test parallel to the grain of sugi (Japanese cedar, Cryptomeria japonica) green round timber were conducted to confirm whether its strength would satisfy the referenced strength determined by the Construction Ministry. The strength of green round timber and air-dried round timber were compared for bending and compression parallel to the grain. The strength change ratio in response to a 1 % change in the moisture content of round timber was compared with that of small clear specimens and timber. The results revealed that a 5 % parametric tolerance limit of bending and compressive strength parallel to the grain satisfied the referenced strength, even when using green round timber. The average strength of air-dried round timber was higher than that of green round timber, in both bending and compression parallel to the grain, with significant differences indicated at a 5 % significance level. The relation between the cross-section area that includes round timber, timber and the small clear specimens, and the strength change ratio in response to a 1 % change in moisture content change was fitted to a logarithm curve. Thus, the size of the specimen was considered to affect the strength change ratio.     

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.     

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.     

Mechanical response of wood perpendicular to grain when subjected to changes of humidity

This paper describes the deformations caused by stress and humidity interaction, mechano-sorption, in the cross grain directions of wood and the relaxation or accumulation of internal stresses caused by these deformations. Long-term tests on small clear specimens in cyclic climates with both tensile and compressive loads were carried out. The development of internal stresses in timber was measured indirectly at different times during the adsorption and desorption processes. Released deformations were measured from cross-sections after cutting them to small slices. These deformations were used to estimate the internal stresses caused by the humidity variations. Tests with constant loads and multiple humidity cycles show a mechano-sorptive strain that is ten times higher than the elastic strain. It is shown that existing models for describing mechano-sorption perpendicular to grain are inaccurate when applied to multiple humidity cycles. The present results demonstrate that if the mechano-sorptive behaviour and the moisture gradients in wood can be accurately described, it is possible to predict the stress distribution in a timber cross-section by knowing the climate history.