Dowel-type Fasteners Research Articles

Long-term load–deformation behaviour of timber–concrete joints

This paper discusses the long-term mechanical behaviour of timber-to-concrete joints made with dowel-type fasteners. Despite the influence that the long-term behaviour of joints has on the mechanical behaviour of a timber–concrete structure and consequently on its design, there is still a lack of research in this area. This paper presents experimental research, carried out at the University of Coimbra and Delft University of Technology, on seven joint configurations using different types of fasteners and different materials. For each joint configuration, either four or ten tests were performed resulting in a total of forty tests. A comprehensive description of the test specimens and test setup is given. The experimental creep–time curves were fitted to a creep–time model and used to predict joint creep values over longer timeframes (10 and 50 years). The values obtained were compared with values available in the literature for timber-to-concrete joints with other types of fasteners and timber-to-timber joints with dowel-type fasteners. The approach for timber-to-timber joints suggested by Eurocode 5 was used to determine creep values for timber-to-concrete joints. The results obtained were compared with test results to assess the accuracy of predicting creep values of timber-to-concrete joints with dowel-type fasteners. It was concluded that creep values measured in long-term experimental tests are usually higher than those obtained from the model indicated in Eurocode 5, particularly for environmental conditions corresponding the service class 2.

Development of a Specific Finite Element for Timber Joint Modeling

Widely used for light frame structures or for heavy laminated wood structures, dowel-type fasteners are the most commonly used kind of connectors in timber construction. The purpose of this work is to develop a tool for the semi-rigid analysis and design of such joints. Firstly, interests and approaches described in literature for the semi-rigid modeling of timber plane frames are summarized. Secondly, for a better understanding of the problem, the main characteristics of wood used as a structural material are presented. Finally, a method for an efficient study of joints built with dowel-type fasteners is proposed and developed. This method consists of the introduction of a specific finite element called “Finite Semi-Rigid Element (FSRE)” between the ends of the jointed members. This joint element consists of two nodes, each with three degrees of freedom. These nodes will be tied with common beamelements during the FE analysis. The stiffness of the FSRE is computed from the geometry of the joints and embedding stiffness of all fasteners, along and perpendicular to the grain. The embedding characteristics of fasteners are defined with help of their experimental load-slip curves (fitted with Foschi's models) leading finally to the resolution of a FE non-linear problem.

Stiffness of dowel-type fasteners in timber–concrete joints

The mechanical behaviour of timber–concrete structures is significantly influenced by the stiffness of the joints, both with regard to deformations and to strength. This paper analyses the experimental and numerical assessment of the stiffness of timber–concrete joints, made with dowel-type fasteners. The objective of this analysis is to provide information to be used in the design of timber–concrete composite structures. In order to achieve this, two approaches were used: experimental tests with timber–concrete joints and numerical predictions using models available in the literature. As part of the experimental assessment, shear tests were performed with various joint configurations aiming to be representative of a wide range of practical conditions. The results obtained are presented and discussed. In terms of numerical assessment, two types of models were considered: analytical models, based on the beam on elastic foundation and empirical models. The results obtained in the experimental tests are compared with those obtained from the design models. Based on that analysis the accuracy of the numerical predictions is discussed.

MECHANICAL BEHAVIOR OF THREE-DIMENSIONAL NAILING PLATES JOINTS: EXPERIMENT AND NUMERICAL DESIGN

The aim of this study is to characterize the mechanical behavior of connections timber-to-timber joist hangers with specific nail fasteners. The methodology proposed by technical design standards is based on the hypothesis of an elastic and linear joint behavior. However, the local behavior of joint is semi-rigid characterized by the ductility of wood material in embedment and the non linear behavior of the nail fasteners in bending. Hence, in this study, the combination of a comprehensive experimental setup and a numerical approach is developed to analyze the evolution of the forces in the joint. Firstly, various tests are realized on specific nail fasteners, using different glulam species, in accordance with European test standards. They concern the withdrawal push-out test, the single plane shear and the yield bending moment. Besides, the adequacy of the methodology proposed by Eurocode 5 for dowel-type fasteners is discussed. Secondly, a similar investigation is carried out on a whole timber-to-timber joint, with nailed tridimensional metal plates. Considering the hypothesis of a semi-rigid behavior of the joint, the stiffness of each fastener is identified in tension and shear slips. Finally, a finite element model is applied and discussed considering the fasteners represented by bi-directional mechanical springs. The stiffness of each spring is considered as the slip modulus of each fastener defined by tests. A semi-rigid formulation was adopted and the overall joint behavior is apprehended numerically.

Design of connections with metal dowel-type fasteners in double shear

The EC-5 is the European Structural Standard which develops the design of timber structures. One of its principal aims is to give the engineers the rules to design the connections that are used in these kind of structures. Dowel-type fasteners are one of the most extended kind of joints that can appear, and they work in single or in double shear. The focus of this research is to study the design stresses in this last case. The results obtained using the rules included in the official standard will be compared with those arising from the finite element method. The interaction between stresses and other variables such as geometry, section and material will be presented, as well as an analysis of the results.

Experimental analysis of laterally loaded nailed timber-to-concrete connections

This work intends to assess the accuracy of the methodology proposed in the Eurocode 5 for laterally loaded nailed timber-to-concrete connections. First, the adequacy of the methodology proposed by Eurocode 5 for the dowel-type fasteners is analyzed and discussed, using timber elements from Pinus pinaster Ait. Square and round smooth nails have been used, initially on push-out tests of single and double shear dowel-type connections. Later, a similar investigation is carried out on timber–concrete connections, using the same kind of fasteners (nails), in single shear, wood species and experimental procedure. The use and the influence of permanent formwork are also investigated. As a result, comparisons between the values obtained for the load-carrying capacity and the slip modulus using the analysis provided by the Eurocode 5, when existing, and the experimental results are presented.

Load-Carrying Capacity of Timber–Concrete Joints with Dowel-Type Fasteners

This paper analyzes the load-carrying capacity of timber-concrete joints made with dowel-type fasteners. The scope of the work was to obtain information to be used in the design of timber-concrete composite structures. In order to achieve that, two different approaches were used: experiments with timber-concrete joints loaded in shear, and the use of nonlinear analytical models. Shear tests were performed using various timber species, concrete mixtures, and fasteners. The results from these tests are presented and compared with the results determined with three different analytical models. These models differ in the approach used to simulate concrete: linear-elastic, linear-elastic with crushing, and elastic-perfectly plastic. The analysis of the test results shows that the load-carrying capacity of this type of joint is significantly influenced not only by the strength of the materials but also by the shape of the fastener. From the three models proposed, the one assuming elastic-perfectly plastic behavior for concrete leads to the results closer from the test results, however, the best statistical correlations between model results and test results were obtained with the model assuming linear-elastic behavior with crushing.

Fully fashioned biaxial weft knitted and stitch bonded textile reinforcements for wood connections

Multiaxial stitch bonded and biaxial weft knitted textiles show high potential for reinforcement in wood construction. These technologies enable the production of two and three dimensional pre-forms. The necessary steps for the enhancement of load bearing capacity of timber joints are explained for the reinforcement of dowel type fasteners.

Quasi-non-linear fracture mechanics analysis of splitting failure in moment-resisting dowel joints

This article addresses the splitting failure of moment-resisting dowel-type fastener joints, in which the failure may be attributed to the perpendicular-to-grain loading of one single dowel located close to the end of a beam. A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied. A simple approximation suitable for practical design is also proposed. Model predictions of the influence of edge distances and end distances are compared with test results.

Capacities of Dowel-Type Fastener Joints in Australian Pine

An experimental study is reported aimed at determining stiffness and strength of laterally loaded dowel-type fastener joints in Australian grown pine. Emphasis is on joints in Radiata pine lumber or glued-laminated (glulam) timber, and Slash pine lumber. Information was collected in a manner that makes it useful for load and resistance factor design. It was found that characteristic “one-fastener” design capacities of joints are most reliably based on the yield load rather than the ultimate load, and that for practical purposes behaviors of joints in Radiata pine and Slash pine can be regarded as identical.

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.

Improving the evaluation of multiple-dowel-type connection strength

Our study focuses on predicting the ultimate short-term load carrying capacity of timber-to-timber connections with dowel-type fasteners. The wide range of possible configurations in practice makes the resolution of these values by tests unrealistic. Moreover, different current regulations do not consider some specific failure mechanisms. In many countries, the reduction of resistance involved by this phenomenon is taken into account by considering an effective number of dowels (nef) smaller than the actual number of dowels (n) in the connection. However, these different regulations disagree on the values of nef and on other points (spacing, partial coefficient of security, formulas). These discrepancies in design rules invite the fundamental research on this topic and, therefore, new methods are sought in order to estimate the load carrying capacities of the connections. In the light of these, our approach consists of predicting the load carrying capacity with the neural network numerical tool. The results obtained by this analysis tool are satisfactory, although the model remains complex. Subsequently, we focus on the simplification of this numerical model with classical regression techniques in order to implement it in a design code.

Modelling the influence of boundary conditions on the concentrated load performance of oriented strand board floor decking

Concentrated load performance tests to failure have been carried out on 18-mm thick one-way simply supported, screwed and bolted oriented strand board (OSB) panels with spans of 300 and 450 mm. The screwed and bolted panels simulated the near minimum and maximum edge restraints, respectively, associated with dowel-type fasteners. The mechanical properties of the panels enabled the calculation of the service responses of the simply supported models of the panels that are used in routine design. The measured stiffnesses of the screwed and bolted panels included the effect of joint slip on stiffnesses. Comparison of the test and analytical results reveals that the stiffnesses of the dowel-fastened panels are between 10 and 20 percent greater than the analytically determined stiffnesses of the simply supported models. All the panels exhibited punching shear-type failures. The experimentally determined load capacities support the theoretical prediction that the concentrated load capacity of a one-way panel, loaded at the centre of span, is almost independent of the boundary conditions.

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 (σe), initial stiffness (ke), and effective elastic foundation depth were obtained from the tested results. The effective elastic foundation depth (α=E/ke,E = MOE), based on the theory of a beam on elastic foundation, was obtained from theke and MOE. An α of 90° (perpendicular to the grain) was calculated by dividingE90 [MOE of 90° from the compression test, but MOE of 0° (E0), parallel to the grain, obtained from the bending test] byke90, 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 (δe,ke,α) 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.

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.

Empirical Models Depicting Grain Angle Effects on Load-Embedment Response of Dowel-Type Fasteners in Wood

Abstract Predicting the moment-rotation response of a multifastener timber joint requires the use of empirical models that describe adequately the influence of load-to-grain angle on embedment response of wood under a dowel fastener. This paper presents two models. The first model describes the relationship between load, embedment, and load-to-grain angle, and can be used for predicting moment-rotation response of a multifastener joint. The second model relates the ultimate embedment at failure to load-to-grain angle. This second model is intended for predicting failure of timber joints. Comparison with test data confirms the suitability of these models to describe the influence of load-to-grain angle on embedment response of wood.

Hollow steel dowels — a new application in semi-rigid timber connections

Dowel type fasteners used in timber joints are traditionally of solid cross section. The rigidity of those with a larger diameter increases the chances of brittle failure in the timber components. Hollow fasteners of variable wall thickness, whilst maintaining the required timber performance, offer variable fastener rigidity and the likelihood of enhanced joint ductility. This paper describes a finite element based model which was validated against the results of a series of tests on single hollow steel dowel timber joints and then used to conduct a parametric study of the effect of fastener wall thickness on joint performance.

A three-dimensional finite element model for locally reinforced timber joints made with hollow dowel fasteners

Brittle premature failure caused by splitting parallel to the timber grain is a common failure mode in glulam joints made with solid dowel type fasteners. It is thought that this problem can be alleviated by using hollow steel dowels as the fasteners and reinforcing the timber locally in the area of the joint. In this way, by varying the wall thickness of the dowels and the thickness of the reinforcing members, a chosen combination of strength and stiffness should be attainable together with a ductile failure mode. In this paper, three-dimensional nonlinear finite element models are developed to simulate (i) the structural performance of a timber joint made with a single hollow steel dowel and (ii) a moment transmitting joint made with a number of the dowels, each type being locally reinforced by densified veneer wood. The models incorporate suitably defined elastoplasticity and orthotropic elasticity and also allow for large deformations of the joints as well as for frictional contact between the timber and the dowel. They are calibrated against physical test data from joints loaded to failure.Key words: glulam, densified veneer wood, resin injected, hollow dowel fastener, moment transmitting, ductile failure, finite element.

Influence of semi-rigidity of joints on the behaviour of timber structures

The review starts by identifying the conditions for taking account of the semi-rigid behaviour of joints with dowel-type fasteners, i.e. well-defined load–displacement behaviour, no initial slip and reliable ductility. It is described how reinforcing a wood member perpendicular to the grain with fibre fabrics, plywood or densified veneer wood ensures that joints have reliable ductile behaviour. New types of fastener with desirable properties are described; with the expanded tube fastener being especially interesting. Modelling the load–displacement curves for single fasteners and joints with multiple fasteners is discussed. The so-called Plane Group of Mechanical Fasteners Model is most appropriate. The Spring Model is less suited to timber than steel. Some examples show how the semi-rigidity in the joints influences the behaviour of structural systems. The conclusions are that taking semi-rigidity of joints into account can lead to more efficient and economical structures.