Dowel-type Connections Research Articles

Analytical Model for Non-linear M-θ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\user2{M - }\\,\\,{\\varvec{\ heta}}$$\\end{document} Relationships of Dowel-Type Timber Connections Exposed to Fire

Recent experimental evidence has shown that wood–steel–wood dowel-type connections exhibit a semi-rigid behaviour even after 90 min of fire exposure. Because a semi-rigid behaviour influences the bending moment distribution among structural members, considering the moment–rotation relationships within frame analyses in which structural members are modelled as beam elements can enable a realistic fire response analysis that is significantly simpler than the three-dimensional finite element method. This study proposes an analytical methodology that accounts for the thermo-mechanical behaviour of timber and dowels, enabling the simulation of the non-linear moment–rotation relationships under fire conditions. The proposed analytical model divides dowels into a series of elements on an elastoplastic foundation and performs a direct stiffness method in a time-incremental procedure using an element stiffness matrix derived from beam-on-elastic-foundation theory. This study also presents the results of load-carrying tests on timber frames with dowel-type connections performed under ambient and fire conditions. The analytical results were consistent with the fire test results. Additionally, the analyses were also performed under three conditions in which the dowels were rigid, linear elastic, and elastoplastic bodies. These three results converged to the same value after 65 min of heating, which suggests that the ultimate states of beams with dowel-type connections exposed to fire can be modelled by assuming that dowels are rigid bodies.

Beam-on-Foundation modelling of dowel-type single fastener connections in cross laminated timber

In this study, a numerical model that aimed to predict the strength and stiffness, of a single laterally loaded mechanical fastener in steel-to-cross laminated timber (CLT) connections using a Beam-on-Foundation (BoF) model, is presented. The BoF model was used to predict the ductile failure modes of steel-to-CLT connections and considered the layered structure of CLT. When compared to the European Yield Model (EYM), the BoF model was found to be advantageous as it not only predicted the strength but also the stiffness of the connections. A comparison of the slip modulus and strength from BoF model simulations of experimental tests carried out on steel-to-CLT connections showed good agreement with the experimental results. Using the BoF model, the influence of; (i) density, (ii) deck layer orientation, (iii) embedment behaviour, (iv) dowel diameter, (v) steel plate thickness, and (vi) embedment length, on steel-to-CLT connections was investigated in a parameter study. The results presented herein highlight the benefit of the BoF model that considers CLT layer specific embedment behaviour in determining the shear capacity and stiffness of steel-to-CLT connections.

Enhance resilience in timber connections with shape memory alloy and densified veneer wood

Dowel-type timber connections are commonly found in historic buildings and require reinforcement and repair to ensure that their resilience against natural disasters is maintained. This study aims to enhance the resilience of dowel-type connections by equipping them with shape memory alloy (SMA) dowels and densified veneer wood (DVW) reinforcement. The research findings reveal that the performance of SMA bars under cyclic bending is superior to that of steel, in terms of self-centring effect and ductility. Dynamic testing of the SMA-dowelled timber connections shows that connections using SMA exhibit lower residual deformation and better self-centring than those using steel. Additionally, the DVW reinforcement improves not only the self-centring but also the resistance of the SMA-dowelled connection. The comparison between solid bars and tubes shows that the solid bars perform better in terms of damping, resistance and self-centring. These results demonstrate the potential of developing resilient timber connections and improving the performance of SMA-dowelled timber connections.

Behaviour of bonded-in dowel shear connections in timber lightweight concrete composite beams including effect of an interlayer

With the need of renovation in the current timber structures, use of a cement based layer is a popular solution, which improves the structural integrity and increases the structural mass. In this paper, the use of a lightweight concrete with dowel type connections is investigated since such a combination can serve for both new and existing structures. Two types of dowel connections are experimentally tested, screws and bonded-in rods, to observe their limits and the contribution of an adhesive. To extend the application to an existing structure where a wood-based flooring may be present (such as OSB), which acts as an interlayer in the composite solution, the connections are tested with an interlayer as well. To overcome this challenge, in this paper, the predrilled hole at the wood based material (timber and interlayer) is filled with an adhesive to form a bond to the connection. The experimental results indicate that the addition of an adhesive lead to a positive contribution in mechanical properties. The experimental results are compared with the analytical approaches given in the current codes. The difference in comparisons are justified and further supported with the literature.

Distributed fiber optic strain sensing in axially loaded glued-in steel rods in glued-laminated timber

The rise in popularity of mass timber structures globally has increased the demand for high-performance timber connections. Glued-in rod connections which are advantageous compared to other dowel-type connections because of their high strength and stiffness, have been the subject of extensive research in the literature; However, the lack of an in-depth understanding of their behavior and a uniform set of guidelines for their design continues to limit the use of glued-in rods in practice. To develop an improved understanding of the behavior of glued-in rod connections this paper examines, for the first time, the use of distributed fiber optic sensors to measure the complete strain distribution along the length of a glued-in rod connection. Axial pullout tests were conducted on twenty-five glued-in rod connections in glued-laminated timber with varying embedment lengths and glue-line thicknesses. Results of the study demonstrate the ability of fiber optic sensors to capture the full strain distribution along the length of the steel rod and along the epoxy-adhesive interface, enabling the study of localized behavior that would be difficult to measure using conventional strain gauges. In addition, results demonstrate the ability of distributed fiber optic strain sensors to infer axial stress and force distributions along the length of a glued-in rod connection, which are compared with theoretical distributions from the Volkersen model, as well as measure bond-slip behavior.

Embedment strength of dowel-type fasteners for the case of cross-laminated timber plane side insertion

The strength property of dowel-type connections in cross-laminated timber (CLT) is dependent on the embedment strength of the adopted dowels. In this study, eight groups with 164 CLT cubic specimens were tested based on the half-hole embedment test configuration to address the effects of the potential factors on the embedment strength. In the embedment tests, the axial direction of the smooth dowels was perpendicular to the plane face of the CLT cubic specimens (i.e., in the case of CLT plane side insertion). The considered influencing factors included the loading angle θ, the ratio between the thickness of the transverse layers to the thickness of CLT (RTrLay), and the gap between the longitudinal laminas within the embedment area. The experimental results were further utilized to verify the current empirical equations that could predict the embedment strength. Results show that the embedment strength of the dowels in the case of CLT plane side insertion is enhanced by 19.96–68.40 % when changing the θ from 90 degree to 0 degree. When the gap between the longitudinal laminas exists in the embedment area, the distinction between the embedment strength measured through the θ of 0 degree and that measured through the θ of 90 degree can be weakened. When the θ is 90 degree, the gap existing in the embedment area has almost no effect on the CLT embedment strength. In contrast, when the adopted θ is switched to 0 degree, the gap existing in the embedment area can weaken the CLT embedment strength by up to 37.11 %. When no gap exists in the embedment area, the embedment strength declines with an increase of the RTrLay; whereas when a gap between the longitudinal laminas exists in the embedment area, a growing trend of the embedment strength is illustrated with the increasing RTrLay.

Reinforcement of Timber Dowel-Type Connections Using Self-Tapping Screws and the Influence of Thread Configurations

The application of self-tapping screws as reinforcement on glulam connections has been proven effective. However, the implication of different thread configurations on the effectiveness of reinforcement remains unknown. This paper conducted experiments using screws with various thread configurations in embedment-strength tests and tensile connection tests. Results show that self-tapping screws with one third of thread achieved similar improvement in the embedment strength and mechanical properties of connections as fully threaded screws. This implies that properly reducing the thread length on self-tapping screws ensures easier screw installation than using fully threaded screws. The influence of screw-to-dowel distance was also investigated and two distances (0.5 d and 1 d) were adopted, with ‘d’ being the diameter of the dowel. The difference in embedment strength due to different screw-to-dowel distances was insignificant. The group with screws placed in contact (0.5 d) with the dowel achieved 5% higher embedment strength than the group with screws placed at a 1 d distance. The connection tests showed good agreement with the embedment-strength tests. This confirms that self-tapping screws with reduced thread can enhance the load-carrying capacity and ductility of connections to a level similar to connections reinforced by fully threaded screws.

Guidelines for a Finite Element Based Design of Timber Structures and Their Exemplary Application on Modelling of Beech LVL

Design verifications of buildings are usually carried out supported by a finite element analysis (FEA), for which, however, there are only a few and almost exclusively non-binding application rules. Within the Cluster of Excellence Integrative Computational Design and Construction for Architecture (IntCDC) at the University of Stuttgart, Guidelines for a Finite Element-Based Design of Timber Structures have been developed. The scope of the guidelines is daily engineering practice, expert engineering applications and product development and certification. Essential parts of the guidelines are design procedures, modelling (including geometrical, material and imperfection modelling), analysis, model verification and validation and design. The content and application of the guidelines are described and illustrated in this paper using two benchmarks. These two benchmarks, which are based on experimental investigations, deal with the elastic material modelling of glulam made of beech laminated veneer lumber (beech LVL) and dowel-type connections for beech LVL members. The experimental basis of the benchmarks is described. With the experiments for the benchmarks, all Poisson’s ratios and the complete elastic material stiffness matrix of beech LVL are determined by means of an optical measuring system. The experimentally determined stiffnesses of the investigated dowel-type connections in beech LVL are compared with normative values. Based on the experiments, a numerical model is developed in RFEM (Dlubal).

Numerical and Experimental Analysis of the Load-Carrying Capacity of a Timber Semi-Rigid Dowel-Type Connection.

The paper deals with the analysis of the load-carrying capacity of a timber semi-rigid connection created from a system of two stands and a rung. The connection was made from glued laminated timber with metal mechanical dowel-type fasteners. Not only a common combination of bolts and dowels, but also fully threaded screws were used for the connection. The aim of the research and its motivation was to replace these commonly used fasteners with more modern ones, to shorten and simplify the assembly time, and to improve the load-carrying capacity of this type of connection. Each of these two types of connections was loaded statically, with a slow increase in force until failure. The paper presents results of the experimental testing. Three specimens were made and tested for each type of the connection. Experimental results were subsequently compared with numerical models. The achieved results were also compared with the assumption according to the currently valid standard. The results indicate that a connection using fully threaded screws provides a better load-carrying capacity.

Statistical correlation investigation of a single-doweled timber-to-timber joint

Dowel-type joints are widely used in timber structures given their ease of construction, strength, and capacity to deform before failure. The embedment strength of timber and the bending moment capacity of dowels are considered key properties in the design. On the other hand, these properties have an inherent variability that increases the uncertainties related to the connection’s strength and associated failure modes. This study proposes to quantify the uncertainty related to the statistical correlation behavior between the timber embedment strength and dowel bending moment capacity while comparing analytical solutions to the results of double shear single doweled timber joints. Traditional distribution fitting procedures, as well as copula functions, are implemented to capture their marginal and dependence behavior. Since their source of mutual correlation is known, the effectiveness of the different approaches in describing the statistical dependence structure can be assessed. This is done by investigating how equivalent are the descriptions of dependence by copula functions and directly from the correlation origin. Results obtained here indicate that, for single dowel-type connections in double shear, the impact of the copulas on the results is small, which means that improving their joint characterization represents a minor improvement in the reliability results. Besides the minor differences, the results show that copula functions are a viable tool capable of capturing the nuances of the joint behavior between random variables.

Numerical and Experimental Analysis of the Rotational Stiffness of a Timber Semi-Rigid Dowel-Type Connection

The paper deals with the analysis of the rotational stiffness of a semirigid connection created from a system of two stands and a rung. The connection was made from glued laminated timber with metal mechanical dowel-type fasteners. Not only a common combination of bolts and dowels but also fully threaded screws were used for the connection. The aim of the research and its motivation was to replace commonly used fasteners with more modern ones, to shorten and simplify the assembly time, and to improve the load-carrying capacity of this type of connection. Each of these two types of connection was loaded to the level of 60%, 80%, and 100% of the ultimate limit state value. Subsequently, the rotational stiffness was determined for each load level after five loading and unloading cycles. This paper presents the results and comparison of the experimental testing and the numerical modeling. The obtained results were also compared with the assumption according to the currently valid standard.

Probabilistic model for determining the failure time of steel-to-timber connections with multiple dowel-type fasteners exposed to fire

There is a lack of knowledge on the structural reliability of dowel-type connections exposed to fire. The objective of this paper is to propose a model for the structural reliability analysis of steel-to-timber connections with multiple dowel-type fasteners two shear planes exposed to standard fire to determine the failure time probabilistically. A thermomechanical model was developed in two uncoupled phases. The phase to calculate the temperature field was based on the finite element method, while the phase to calculate the load carrying capacity was based on Johansen's theory, modified to consider the influence of temperature on the materials thermomechanical properties. The model error was calculated for the failure time of the connections. The structural reliability analysis of the failure time was carried out using Monte Carlo simulations. The analysis indicated that modes I and II, which are influenced by the solicitation rate and the relation between thickness of timber member and dowel diameter, have the largest probability to occur. The structural reliability analysis provided the mean and the 5th and 20th percentiles failure times of probability distributions for 25 connections. A sensitivity analysis through Sobol’ indices showed that the model error is the random variable with the higher relative importance.

Influence of the earthquake and progressive collapse strain rate on the structural response of timber dowel type connections through finite element modelling

This paper aims at understanding the influence of the strain rate, as those experienced under earthquake and progressive collapse events (referred to as “medium” strain rates), on the structural response of metal dowel type timber connections through the development of a finite element model. Past studies on timber connections have been mainly performed under quasi-static loadings. However, as metal and timber show different degrees of sensitivity to increased strain rate, such connections may exhibit different dynamic failure modes and load-carrying capacities than quasi-statically. To understand this phenomenon, a finite element model for dowel type timber beam-to-column connections, having a T-section aluminium bracket, was developed in this study. The model was validated incrementally, against (1) quasi-static and dynamic embedment tests, (2) single bolted connection quasi-static shear tests and (3) complete dowel type connection quasi-static shear and bending tests. Material properties under medium strain rates were then applied to the static model to study the structural response of the studied connection during earthquake and progressive collapse events. It was found that the change in material properties only contributes to a slight increase in capacity (up to 6.5%) but insignificant variations in initial stiffness (less than 4.1%) for the studied connection. This indicates that the connection can be safely designed statically for earthquakes and progressive collapse events. The methodology proposed in this paper and the finite element model developed can be extended to other types of timber connections.

Brittle failure of laterally loaded self-tapping screw connections for cross-laminated timber structures

The performance of structural timber connections is of utmost importance since they control the global response of the building. A ductile failure mechanism on the global scale is desirable, especially in the design of structures in seismic areas, where dissipative components in which ductile failure modes need to be ensured are considered. Therefore, the knowledge of possible brittle failure modes of connections is crucial. The paper investigates the brittle failures of laterally loaded dowel-type connections in cross-laminated timber subjected to tensile load in a lap joint configuration through experimental investigations and analytical estimations. A set of 13 different test series has been performed with fully threaded self-tapping screws of 8 mm diameter and different lengths (40 to 100 mm) in cross-laminated timber composed of 3 or 5 layers (layer thickness range from 20 to 40 mm), giving rise to the activation of different brittle failure modes at different depths. Plug shear was among the most typically observed failure modes. A previously proposed model for the brittle capacity was applied to the tested connections at the characteristic level. As shown by the performed statistical analysis, the existing model is not reliable and mainly unconservative. A very low performance is observed (CCC = 0.299), but with a good correlation (c = 0.750) for the tests in the parallel direction. Further research work is required to improve the current model predictions and to gain a better understanding of the underlying resisting mechanisms.

Splitting capacity of Eucalyptus globulus beams loaded perpendicular to the grain by connections

In timber structures, knowledge of the splitting capacity of beams loaded perpendicular to the grain by dowel-type connections is of primordial importance since brittle failure can occur. In the present work, single- and double-dowel-type connections following different loaded edged distance arrangements are experimentally investigated to derive the splitting behaviour of Eucalyptus globulus L., which is a hardwood species of increasing interest for structural use due to its high mechanical performance, fast growth, and good natural durability. The correlation of experimental failure loads with those theoretically predicted by the expression included in Eurocode 5 and by eight analytical models based on an energetic approach is discussed. Most of the analytical models studied overpredict the splitting capacity. However, the code splitting expression, derived from softwoods, proves to be very conservative in predicting the eucalyptus splitting failure load.

Reduced and test-data correlated FE-models of a large timber truss with dowel-type connections aimed for dynamic analyses at serviceability level

The rise of wood buildings in the skylines of cities forces structural dynamic and timber experts to team up to solve one of the new civil-engineering challenges, namely comfort at the higher levels, in light weight buildings, with respect to wind-induced vibrations. Large laminated timber structures with mechanical joints are exposed to turbulent horizontal excitation with most of the wind energy blowing around the lowest resonance frequencies of 50 to 150 m tall buildings. Good knowledge of the spatial distribution of mass, stiffness and damping is needed to predict and mitigate the sway in lighter, flexible buildings. This paper presents vibration tests and reductions of a detailed FE-model of a truss with dowel-type connections leading to models that will be useful for structural engineers. The models also enable further investigations about the parameters of the slotted-in steel plates and dowels connections governing the dynamical response of timber trusses.

Load-Carrying Capacity of Double-Shear Bolted Connections with Slotted-In Steel Plates in Squared and Round Timber Based on the Experimental Testing, European Yield Model, and Linear Elastic Fracture Mechanics

Nowadays, the use of timber as a building material is gaining more prominence. When designing timber structures, it is necessary to pay increased attention to the design of their connections. The commonly used connections are dowel-type connections, which are often used in combination with steel plates slotted into cut-outs in timber members. The presented paper deals with the behavior of double-shear bolted connections of squared timber and round timber with slotted-in steel plates. Several variants of connections with different distances between the fastener and the loaded end were selected for the experimental testing. A total of six types of test specimens were made from spruce timber, for which their selected physical properties were determined and evaluated before the experimental testing. Test specimens of bolted connections were first tested in tension parallel to the grain until failure under quasi-static loading. The connections were broken by splitting. Ductile failure preceded brittle failure. The actual load-carrying capacities were lowest for the lowest end distance. The load-carrying capacities for the middle and the longest end distances were comparable. The results of the experiments were then used for comparison with calculation procedures according to the standard for the design of timber structures and with calculations according to the theory of linear elastic fracture mechanics. The experiments and the analytical models were supported by a simple numerical analysis based on the finite element method.

A serviceability investigation of dowel-type timber connections featuring single softwood dowels

Dowel-type connections are common in timber engineering, but current design codes are largely based on empiricism and are oversimplified. This inhibits the optimised use of connections, which is essential for the design of economically efficient timber structures. This study investigates the use of 3D computational modelling to predict the slip modulus (a key measure of stiffness) of single and double shear, dowel-type connections featuring a single softwood dowel. Initial modelling was conducted on parallel- and perpendicular-to-grain connections to establish a suitable mesh size and to validate the model against experimental work. The slip modulus in angled orientations was then investigated, a relationship given no consideration in design codes. The results show that the current design codes greatly overestimate the slip modulus in both single and double shear connections involving timber dowels. In comparison, the models in this study are more than twice as accurate at predicting slip modulus. Furthermore, slip modulus was shown to vary sinusoidally as the grain-to-grain angle changes between the parallel and perpendicular orientations. Differences between model and experimental values can be attributed to uncertainty in the mechanical properties of the timber in the experiments, the assumption of uniform properties for timber in each principal direction in the models, and the inherent variability of timber which affects experimental results.

Performance evaluation of a wood treatment for connections with dowel-type bolts

Recent studies have shown the advantage of wood impregnation on increasing the dowel-bearing strength of black spruce wood by almost 50%. The aim of the present study was to improve the mechanical performance of a dowel-type connection through an impregnation method for black spruce wood. The results showed that wood treatments improved the mechanical performance of dowel-type connections. The dowel-bearing strength increased up to 25%, while the stiffness increased up to 52%. The increase obtained was lower in comparison with the previous studies, however. A lower polymer quantity, resulted in a shorter vacuum time, and a lower temperature polymerization used in wood treatment brought the process closer to an industrial application.

Numerical Model Validation for Mengkulang Glulam Timber Bolt Withdrawal Capacity

The adequacy of timber joints determines how much load it can sustain, commonly called load-carrying capacity. European Yield Model (EYM), also known as Johansen yield theory, has been widely adopted in the design of timber joints for predicting load-carrying capacity. In EC5, the pulling out capacity is known as the 'rope effect' and becomes one parameter that governs the load-carrying capacity in a dowel-type timber connection. Due to the high cost of preparing the timber specimen, computer modelling always becomes the alternative in measuring the load-carrying capacity for timber connections. However, the computer modelling results need to be validated with the experimental laboratory test before being extended to different sizes and materials of fasteners. This study presents a finite element method (FEM) for numerical modelling and analysis to validate the experimental performance of timber's 12mm, 16mm, and 20mm bolt withdrawal capacity. This method adopted Abaqus 6.14.4 software package to create four (4) FEM models consisting of a bolt inserted into a glulam timber block at a different insertion depth, parallel and perpendicular to the timber grain direction. The axially inserted bolt was subjected to a pull-out force while the glulam timber block was held in position. The mild steel bolt and tropical Mengkulang glulam timber blocks were used. The validation showed an acceptable agreement between the FEM and the experimental results.