European Yield Model Research Articles

Lateral Load Resistance of Laminated Veneer Lumber Sengonwooden Joints with Wooden Dowels

Analytical models for wooden-doweled joints need to be developed. The objective of this research is to determine whether the strength of wooden-doweled joints can be well predicted by equations developed for steel-doweled joints. In this experiment, various connection parameters, such as dowel diameter, species of the wooden dowel, and angle of joints, are studied. The joint specimens are loaded in a quasi-static state with four replications each. A5% offset diameter method is used to determine the lateral load resistance of the tested joints, while the predictions are evaluated by equations given in Indonesian National Standard (SNI) 7973-2013 and the European Yield Model (EYM). The results show that experimental lateral resistance is greater than that of the SNI prediction and is between the EYM prediction of elastic and the plastic conditions of the wooden dowel. Lateral load resistances given by the SNI and the elastic condition EYM are around 64.53% and 78.45% of the experimental result, respectively. In contrast to steeldoweled joints, the ductility coefficient of wooden-doweled joints increases as the dowel diameter increases. This is potentially due to the axial stiffness of the wooden dowels being relatively smaller than that of the wood member. Joint failure occurs due to plastic bearing deformation in the wood member surrounding the dowel, followed by the formation of two plastic hinges in the wooden dowel.

Studies of Nail Connectors Used in Wood Frame Shear Walls with Ply-Bamboo Sheathing Panels

Based on Johansen yield theory (European yield model) and experimental observations, mechanical models and capacity equations for nail connectors used in wood-frame shear walls with cross-prefabricated ply-bamboo sheathing panels were studied. Embedment strength of ply-bamboo panel was obtained in accordance with a British Standards Institution (BSI) standard. Monotonic and cyclic tests of nails applied in timber-bamboo shear walls were performed to validate the capacity equations. The adopted capacity equation based on European yield model meets the test results well, in the average and with a reduced scatter. The theoretical and experimental results obtained from the research suggested a capacity model and equations to predict the bearing capacity of timber-bamboo nail connections, indicating that ply-bamboo panels can be used as sheathings for light-weight timber structures, as well as included in existing timber design code.

Prediction model for the load-carrying capacity of nailed timber joints subjected to plug shear

Timber joints can experience ductile failures as modeled by Johansen in the European Yield Model adapted in Eurocode 5, or they can fail in a brittle manner. In nailed or screwed joints where the fastener does not protrude through the timber, plug shear failure can occur where an entire plug defined by the perimeter of the joint is torn away from the timber. The brittle plug shear failure, which can occur in joints loaded in tension parallel to the grain, results in a lower joint resistance than the ductile failure modes. The aim of this study is to evaluate existing prediction formulas for plug shear failure in timber connections, compare them to test results and observations, and propose a new prediction formula. Test results from four different experimental studies are presented. Using hypothesis testing, a prediction formula for plug shear failure is proposed based on 92 experiments. The resistance of the tensile failure mode of plug shear failure is best modelled by the tensile resistance of the end face of the plug. The resistance of the shear failure mode of plug shear failure is best modelled by the shear resistance of the bottom area of the plug taking into account the volume effect on shear strength. The model currently in Annex A of Eurocode 5 is overestimating the plug shear resistance compared to the test results analysed in this research. To avoid plug shear failure, short and wide joints are preferred, minimising the number of fasteners in line with the load and grain direction.

Failure mechanisms of dowel-type fastener connections perpendicular to grain

Dowel-type fastener connections perpendicular to grain may fail in either a ductile or a brittle fashion. Whether the failure mechanism should be classified as ductile, depends on the definition of ductility utilized. Generally, the load–slip response is considered as a means of ductility quantification. In this respect, the elastic potential energy is calculated (being the area underneath the load–slip response) or the ratio of the ultimate slip and the slip at the onset of yielding is used, amongst others. Another approach relates to the European Yield Model which is commonly adopted in design standards to calculate the load-bearing capacity of dowel-type fastener connections. In this respect, ductility may be considered to be associated with plastic deformation of the steel fasteners solely since the deformation ability of connections with dowel-type fasteners at yield is much larger compared to situations with non-yielding fasteners. In the case of connections with rigid dowel-type fasteners, it therefore may be suggested that less ductility is exhibited although it is recognized that timber in compression perpendicular to grain exhibits plastic capabilities as well. Whether these connections behave either brittle or ductile is related to the spacing requirements as well. In this paper, first an overview of potential fracture and failure mechanisms of dowel-type fastener connections is provided. Experimental results of a variety of connections and corresponding failure mechanisms are presented. Based on mechanical models to predict the load-bearing capacity, classification of these connections in terms of ductility is provided, and the governing parameters involved are distinguished. This paper does not intend to provide a quantification procedure of ductility. Yet, an extensive overview of failure mechanisms and an analytical analysis of parameters involved are given providing expressions to calculate the failure mechanism being governing.

Lateral resistance of joints made with various screws in commercial wood plastic composites

In this study, the effects of screw diameter, pilot hole diameter, end distance, thickness of side and main members, loading speed and screw type on the lateral resistance of screw joints in commercial wood plastic composites were investigated and also compared with those calculated using the European Yield Model (EYM). The results have shown that for a given diameter of screw, increase in end distance, thickness of joint members, pilot hole diameter and loading speed, increases lateral resistance of the joint. Comparing screw effect it was found that lateral resistances of joints made with various screws did not have significant differences. The results have also shown that the accuracy of EYM prediction of lateral resistance for the joints fractured in mode I s is better than those fractured in modes III s or V.

Bolted and Dowelled Connections in Radiata Pine and Laminated Veneer Lumber Using the European Yield Model

Connections with mechanical fasteners are important for all cases of timber structures. The failure of these connections may occur in either ductile or brittle manner. For the calculation of the ductile failure strength or the load carrying capacity, the European yield model (EYM) is used in many standards and accepted as a very accurate model. In the current New Zealand timber standard NZS 3603:1993 (Standards New Zealand, 1993) and the Australian one AS1720.1–1997 (Standards Australia, 1997), the design concept for bolted or dowelled connections is not based on the EYM, and depends only on the diameter, the timber thickness and the species group. The most important parameters for the EYM are the fastener yield moment and the timber embedment strength, but embedment strength values are not available for New Zealand Radiata pine or laminated veneer lumber (LVL). To obtain the missing information and to implement the EYM into the New Zealand and Australian standards, embedment tests parallel, perpendicular and under various load-to-grain angles with different dowel diameters in Radiata pine lumber and LVL were conducted and compiled to build a database of embedment strength values. This paper includes the latest results of the investigations with dowel diameters extended up to 30 mm. Furthermore, different international testing standards are compared and their evaluation methods are used. The test results are also compared with the corresponding results using the Eurocode 5 formulas, and show that adjusted formulas of the Eurocode 5 can be used to predict the load carrying capacity of bolted and dowelled connections in Radiata pine lumber and LVL. Design examples comparing the current methods from the New Zealand/Australian design standards and the proposed method adopted from the EYM of the European design standard are given as well.

Tests and models for engineered wood product connections using small steel tube fasteners

Laminated Strand Lumber (LSL) is a type of engineered wood product widely used as a substitute for sawn structural lumber. This is because LSL can be manufactured to have superior mechanical and physical performance to competing products, including high resistance to splitting, high strength and dimensional stability. The authors studied connections in LSL with behaviour of similar connections in sawn Spruce lumber as a benchmark. Proneness to splitting is the Achilles heel of most wood-based construction materials and especially products like sawn lumber and glued-laminated-timber. This characteristic makes it especially difficult to make connections that are mechanically efficient. However, when constructing with LSL it is possible to create connections that not only have desirable response characteristics, but also achieve labour efficiencies. The particular type of connection studied by the authors uses small diameter steel tube fasteners inserted in tight-fitting holes in conjunction with steel plate elements located in slots in ends of joined members and links them together. Tube fasteners are driven perpendicular to the axes of wood members and pass through both those members and steel link elements. Tests were conducted under static tensile loads with multiple-fastener arrangements obeying spaced and end distance rules typical for solid bolts in sawn softwood lumber. The response was highly ductile and failure occurred mainly in the fasteners. Consequently the so-called European Yield Model (EYM) for doweled timber connections yields good predictions of connection strengths. A detailed finite element analysis was also performed to assist interpretation of test results and validate predictions from the EYM beyond the range of test data. The finite element model was developed using the ABAQUS software and results from that model agreed closely with experimental findings in both qualitative and quantitative aspects.

Wood Materials, Nails, and Sheathing Connections from Early 20th Century Residential Buildings

Assessment of light-frame wood structures for wind and seismic loadings requires engineering information about the wood materials and connections in the subject building. The knowledge base can be developed by inspection and further enriched by knowledge of age effects on materials and connection properties. Nineteen wood-frame structures built between 1905 and 1970 and scheduled for demolition were located through local municipal building departments and sampled for wood materials and nail connections prior to demolition. The project plan was to evaluate the materials from the buildings while watching for trends in materials over time, calculate the expected connection capacity using the materials’ test results, and then compare the calculated allowable connection capacity to tests of single-fastener connections extracted from the same buildings. Insect damage and fungal deterioration were present in many of the structures and were avoided when sampling for test specimens. Inferences are not made regarding the effect of biological deterioration and climatic conditions on the wood material properties. The Douglas-fir studs and plank-shiplap sheathing extracted from the source structures had an average dowel bearing strength (Fe=29 MPa) that was statistically similar to the National Design Specification for Wood Construction table value for a specific gravity (SG) of 0.47 for dowels with diameters of less than 6.35 mm. The nails had average bending-yield strength of 671 MPa as compared to the contemporary reference value of 690 MPa for nails in this diameter class. No time trend was observed in the SG of the wood or the bending-yield strength of the nails. In general, the connection tests yielded good agreement between the European yield model equations and calculated connection capacity. Validation of the material conditions in older structures requires cautious inspection. The inference from this investigation as related to rehabilitation is that nailed connections without deterioration due to decay, insects, temperature, and/or moisture perform in a manner consistent with the current design standard.

Monotonic and Reverse-Cyclic Loading of Lag Screw Connections for Log Shear Wall Construction

The log building industry favors simple to install mechanical connections between log layers, also referred to as log courses, to facilitate fast and efficient construction. In active seismic regions, lag screws are commonly used: however, research is lacking on how these connections between log courses perform in monotonic and reverse-cyclic loading scenarios. For the current study, 406-mm-long logs were used to fabricate connection test specimens in a single-shear configuration to mimic a typical lag screw layout used in log shear wall construction. The intent of this study was to develop baseline data on these connections to relate log connection performance to full-scale log shear wall performance. A preliminary study of four commonly used lag screws of different sizes was conducted to assess the monotonic performance within log connections. Predictions of design capacities were calculated using the National Design Specification for Wood Construction which is based on derived equations from the European yield model. Predicted NDS design capacities were compared to experimental data. Design recommendations for deriving appropriate fastener spacing were provided along with fabrication parameters that were based on observed failure mechanisms. Tested connections exhibited high ductility, which is considered a positive attribute for seismic resistance, although reverse-cyclic tests revealed failures due to low-cycle fatigue of the lag screws, leading to prevalent cyclic stiffness degradation as displacements were increased.

Evaluation and modelling of perpendicular to grain embedment strength

Different test setups for determining perpendicular to grain embedment strength of timber have been reported in literature. In addition, different definitions of strength have been used associated with the deformation level underneath the fastener. It is shown that all reported experimental results can be related, which enables comparison on a common basis. Furthermore, several models for embedment strength perpendicular to the grain which primarily depend on timber density (specific gravity) and fastener diameter are evaluated. It is shown that the model currently prescribed by the European structural timber design code [Comité Européen de Normalisation (CEN) EN 1995-1-1: 2004: Eurocode 5—design of timber structures. Part 1.1: general rules and rules for buildings. CEN, Brussels, 2004] is unable to accurately predict the strength and an alternative is proposed. This may result in more reliable timber connections in applying the European Yield Model to determine the connection strength.

Withdrawal capacity of washers in bolted timber connections

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.

Wood and Engineered Wood Product Connections Using Small Steel Tube Fasteners: Applicability of European Yield Model

Capacities of wood connections with laterally loaded dowel fasteners by plastic analysis can be predicted using the European yield models (EYM) equation. The underlying concept is that failure occurs due to crushing of wood beneath fasteners and / or formation of plastic hinges in fasteners with a rigid-plastic representation of the materials involved. EYM prediction is now “the” way within design codes to calculate the maximum attainable capacity per fastener. Subsequent adjustments discount those predictions through factors which account for unequal sharing of force between fasteners and premature connection failure by fracturing. In North America the EYM is the basis for lateral load capacities of connections made with bolts, drift-pins, screws, nails, and timber rivets. This paper addresses the capability of the EYM for predicting load carrying capacities of solid wood (sawn softwood lumber) and laminated strand lumber (LSL) connections employing tight-fitting steel tube fasteners with external diamet...

Reliability of Steel Doweled Wood Connections Designed to ASCE 16-95

First-order reliability analyses were performed for wood connections, using bolts, nails, lag screws, and wood screws, including both wood and steel side plates, designed to ASCE 16-95. The reliability analyses were performed using the European yield model to determine the connection capacity. Using reasonable statistics for the random variables in the yield equations, a reliability index was determined for each of the possible failure modes for both single and double shear connections. Simple bounds were then determined for the reliability index of the series system consisting of multiple failure modes. These reliability analyses show that designs to ASCE 16-95 do not exhibit consistent reliability across the range of possible designs. One way to increase consistency is to use the full set of yield equations rather than the simplified set of design equations now present in ASCE 16-95 for most of the connectors. Other modifications may also be necessary since there are inconsistent reliability levels even for those connectors designed to the full set of equations.

Ductility Aspects in Nailed Glue Laminated Timber Connection Design

A nailed joint system, comprising of two slotted-in steel plates with shot-through smooth round steel nails in glulam, was analyzed from the viewpoint of an engineering description of the ductility behavior. Experiments on 15 specimens, with 10 nails in each specimen, were performed. Based on the results, a model describing the ductility behavior is proposed. Ductility is defined as the ratio of ultimate-to-elastic deformations with the ultimate deformation defined as the first load drop corresponding to wood splitting. The model is based on “engineering” parameters and estimates the engineering ductility from the slenderness ratio, timber density, and the fastener yield strength in bending. The model is compared with similar experiments in the literature and the validation shows correlation. The method of solution, based on the plastic theory of the European yield model for dowels, is general, and is applicable to other timber-to-timber or timber-to-steel dowel-type connections. The engineering ductility...

Design method for bolted connections loaded perpendicular-to-grain

A new design approach to evaluate the resistance of bolted timber connections loaded perpendicular-to-grain is presented in this paper. The design method consists of calculating the resistances of the ductile failure modes, based on the original European yield model, and that of the splitting mode, and of using the smallest of the two. The splitting calculations are adapted from the wood resistance calculation for timber rivet connections. The new design method is based on the assumption that the bolted connection is forming a cluster considered to be equivalent in dimension to a cluster of timber rivets. The rivet design equations were modified to reflect the fact that bolts extend to the full thickness of the wood members, whereas rivets do not. In this paper, the research program is described, results are presented and the alternative design approach is proposed to predict the failure modes and the ultimate strengths of steel–wood–steel, wood–wood–wood, and wood–steel–wood bolted connections.Key words: connections, strength, design, bolt, connection, prediction, failure, perpendicular-to-grain.

On the failure modes and strength of steel-wood-steel bolted timber connections loaded parallel-to-grain

The current Canadian code provisions for the design of timber bolted connections were essentially developed based on connections showing a ductile behavior and then further modified to account for situations where connections fail in a brittle way. An experimental study was undertaken to evaluate the strength of bolted connections specifically experiencing a brittle mode of failure. Specimens consisting of steel-wood-steel connections with either 19.1 mm or 12.7 mm bolts were tested in tension. Test variables included end distance, bolt spacing, row spacing, number of bolts per row, number of rows, thickness and species of wood member, glulam or sawn lumber members. Connections were tested to the ultimate to observe possible modes of failure as variables were changed. Results show that the current Canadian standard approach to evaluate the resistance of timber bolted connections is not optimal although conservative. Brittle modes of failure such as row shear-out, group tear-out, and splitting were observed. The resistances calculated using the O86.1 design provisions are as little as a third as compared to tested results. Also, the design equations do not allow the designer to take advantage of the increases in strength as a result of increases in row spacing, as observed in tests. Analysis of the results show that the longitudinal shear stress at failure is related to a parameter which is a function of the smaller distance (end distance or bolt spacing) and the specimen thickness. This relation was used to formulate design equations to predict the row shear-out and group tear-out strengths of glulam specimens using the specified strength values listed in O86.1. As well, it was found that Mode I of the European yield model is the only ductile ultimate failure mode and that other equations for bearing failure can be neglected. In this paper, the research program is described, results are presented, and an alternate design approach is proposed to predict the failure mode and the ultimate strength of steel-wood-steel bolted connection groups.Key words: bolt, connection, strength, failure, design, code, ductile, brittle.

Modeling Bolted Connections in Wood: Review

This paper presents a discussion of salient issues surrounding the modeling and performance of bolted connections in wood construction. To understand the complex behavior of bolted joints under a load, a number of key issues related to connection geometry, material, and mechanics need to be understood. Issues addressed in this paper include the design philosophy of bolted joints (allowable stress and reliability-based design), the influence of connection geometry on behavior under load, and a discussion of previous models that have been used to predict connection performance. Discussions of the key models used to predict connection load behavior, including the European yield model, which currently serves as the basis for allowable connection load in U.S. codes, and various failure criteria are presented. Of particular interest is the discussion of the strengths and weaknesses of the two-dimensional (2D) models that have been used historically to quantify connection behavior. These are seen relative to developments in three-dimensional (3D) modeling of bolted connections. The contributions of modeling pin/wood contact surfaces to connection behavior prediction are also considered.

Bolted Connection Design Values Based on European Yield Model

This paper discusses the calibration of connection strengths predicted by the European yield model (EYM) to the design values for bolted connections tabulated in the 1986 National Design Specification (NDS). Predicted EYM strengths were compared with 1986 NDS design values to arrive at recommended calibration factors for two‐ and three‐member connections with wood and steel side members loaded parallel and perpendicular to grain over the design space. Design values were changed as a result of basing the bolt‐bearing strength of the wood on the specific gravity of a species. Use of Hankinson's formula with bolt‐bearing strength was found to be more expedient for loads at an angle to the grain than using the formula with design values for parallel‐ and perpendicular‐to‐grain loading. The recommended calibration factors result in some increases and decreases in design values. These factors have been incorporated into the 1991 NDS and a proposed specification for load and resistance factor design (LRFD).

Plywood‐Solid‐Wood Nailed Joints under Lateral Loading

Data collected from tests on 240 nailed joints connecting Douglas‐fir plywood side members to solid‐sawn Douglas‐fir main members were analyzed with the intention of applying the European yield model (EYM) to predict joint behavior under lateral loading. All joints were of approximately similar geometry, with the EYM predicting a failure mode consisting of a single plastic hinge being formed in the nail within the thicker main member. The 5% offset and ultimate loads of the connection were parameters of greatest concern. In addition, a model to predict the embedding strength of solid wood was used to estimate the same parameter for plywood. The results showed that the joint plastic load predicted from the EYM was of a magnitude similar (within 6–28%) to the 5% offset load. The ultimate load was on average approximately 3.3 times the EYM plastic load. However, the high degree of variability found experimentally made deterministic predictions of joint behavior difficult. The study suggested that a model for wood embedding stress used in conjunction with the EYM can be a rational mechanism to estimate the 5% offset load for nailed joints in plywood‐solid‐wood connections.

Application of the European Yield Model to Nailed Joints in Southern Hardwoods

Abstract A study was conducted which utilized data obtained from 1056 tests of nailed joints in 18 commonly used species of southern hardwoods. The data was developed in 1950 by Scholten. This data included information regarding joint geometry, material specific gravity, discrete load-slip curves, and the ultimate load for each joint. The test specimens were laterally loaded and contained one 8d common wire nail in each joint. Joints were tested in compression at a rate of 0.07 in./min (1.78 mm/min) to ultimate load. With a British model to predict wood embedding strength from specific gravity and nail diameter, and an estimate of nail yield strength,the European Yield Model was employed to predict the failure mode and yield load of the joints. Failure modes which create one or two theoretical plastic hinges in the nail were predicted for all joints. The data suggested that behavior modes of the joints were consistent with model predictions. Further, theoretical nail yield load showed a high correlation (r2 = 0.90) with the experimentally obtained load at 0.015 in. (0.381 mm) slip, 5% offset load, and ultimate load (joint capacity).