Bonded-in Rods Research Articles

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.

CZM-based FE numerical study on pull-out performance of adhesive bonded-in-rod (BiR) joints for timber structures

Bonded-in-Rod (BiR) connections for timber structures are becoming rather popular solution, in place of other traditional fastening techniques. Besides, design rules and recommendations in support of safe and robust verification are still poor in existing standards. As such, the actual mechanical performance and load-bearing capacity – which is severely affected by a multitude of operational parameters – should be possibly addressed with the support of experimental investigations. In this paper, a Cohesive Zone Modelling (CZM) based, Finite Element (FE) numerical investigation is proposed for a total of 144 BiR joints characterized by various geometrical and mechanical properties, including grain orientation (α = 0° or 90°), adhesive type (polyurethane or epoxy glue), diameter (d = 10, 14, 20 mm) and anchorage length of rods (L = 60, 100, 200, 400 mm) is presented. Most importantly, the attention is also focused on the analysis of service class effects / moisture content (9, 18 or 27 %) on the actual residual capacity of the examined joints, which is of paramount importance for safe design applications in timber structures. Based on validation of input model properties to past experimental results, the sensitivity of basic mechanical performance indicators for the examined BiR samples is discussed in the paper.

Experimental Bond Behaviour of Glued-In Rod Connection for Mengkulang Glulam under Pull-Out Loading

Mechanical joint and adhesive joint are the two most common types of timber connections. In the case of timber construction, bonded-in rods offer a long-term, aesthetically acceptable alternative to some of the more conventional steel moment connections. Bonded-in rod connections in timber need many desirable attributes inefficiency, manufacture, performance, aesthetics and cost. This paper presents pull-out experimental testing for glued-in rod made of Mengkulang (tarrietia javanica) glulam joints. Hundred and seven (107) specimens were prepared, each with a single glued-in rod parallel and perpendicular to the grain. The steel treaded rod with a diameter of 12 mm, 16 mm and 20 mm with three numbers of adhesive (Sikadur-30) thicknesses of 2 mm, 3 mm and 4 mm were used in this experiment. The pull-out tests observations were on the effects of adhesive thickness, parallel and perpendicular grain directions and modes of failure of the specimens. The result obtained that 4 mm adhesive thickness was the strongest and parallel specimens showed better results than the perpendicular specimens. The pull-out failure modes are affected by the selected factors, i.e. the adhesive thickness, rod diameter and grain directions. Most of the specimens show failures in the timber besides the adhesive to timber interface.

Wood-fibre composite connectors for roundwood trusses: structure and material

Timber structures and buildings made from sustainable forestry might be the solution for today's ecological challenges when their renewable materials are locally processed with low energy consumption in production. The current market shows a dominance of rectangular cross-sections in engineered timber solutions. Roundwood – a traditional material in the past – is no longer common due to a lack of applications in modern structures, lost knowledge in handcraft skills and the high costs of computerised numerical control-based manufacturing. However, solid timber with a round cross-section outperforms other engineered wood products, such as glued laminated products, in terms of energy consumption and environmental aspects. To overcome restrictions in joint technologies and to increase the usability of roundwood, especially in truss structures, a new type of connector was developed. The investigation was split into two research topics: the connection design in roundwood with modified bonded-in rods and the design of wood-fibre composite moulded connectors, which is described in this paper.

Densified wood dowel reinforcement of timber perpendicular to the grain: a pilot study

ABSTRACT An investigation was carried out to examine the potential to utilise densified wood dowels as a reinforcement for timber subjected to compression loading perpendicular to the grain. While timber has a high strength-to-weight ratio parallel to the grain, it demonstrates poor strength perpendicular to the grain and in recent years there has been a significant number of studies examining the use of steel screws and bonded-in rods as reinforcement in this area. This is becoming more and more important with the increased use of timber in medium-to-high rise structures. In this study, thermo-mechanical densified wood in the form of dowels are utilised as compression reinforcement perpendicular to the grain and tested to failure. Thermo-mechanically densified dowel reinforcement arrangements of 2, 4, and 6 dowels are examined experimentally under a compressive load and compared to timber samples similarly reinforced but with steel self-tapping screws. The results have demonstrated the potential to utilised densified wood to create an all-wood solution to reinforce against compressive stresses perpendicular to the grain. Additionally, modifications to recently proposed Eurocode 5 recommendations for the design of compression reinforcement using self-tapping steel screws are presented, which are suitable for the design of compression reinforcement using densified wood dowels.

Short-Term Analysis of Adhesive Types and Bonding Mistakes on Bonded-in-Rod (BiR) Connections for Timber Structures

Bonded-in rods (BiR) represent a structural connection type that is largely used for new timber structures and rehabilitation (repair or reinforcement) of existing structural members. The technology is based on steel / Fiber Reinforced Polymer (FRP) / Glass Fiber Reinforced Polymer (GFRP) rods bonded into predrilled holes in timber elements. The mechanical advantages of BiRs include high local force capacity, improved strength, a relatively high stiffness and the possibility of ductile behaviour. They also offer aesthetic benefits, given that rods are hidden in the cross sections of wooden members. As such, BiR connections are regarded as a solution with great potential, but still uncertain design formulations. Several research projects have dealt with BiRs, but a final definition of their mechanics and a universal design procedure is still missing. This research study explores the typical fracture mechanics modes for BiR connections. A special focus is given to the evaluation of the impact of adhesive bonds under various operational conditions (i.e., moisture content of timber). A total of 84 specimens are tested in pull-out setup, and investigated with the support of digital image correlation (DIC). The reliability of empirical equations and a newly developed analytical model in support of design, based on linear elastic fracture mechanics (LEFM), is also assessed.

Bond Behavior between Softwood Glulam and Epoxy Bonded-In Threaded Steel Rod

AbstractThis study aims to develop an improved understanding of the interfacial bond behavior of softwood glulam joints with bonded-in threaded steel rod. A total of 39 glulam joints with bonded-in single-threaded steel rods were tested to failure in the pull-pull configuration. The test results were presented in term of failure modes, load-relative movement response, pullout strength, and the corresponding slip. The distributions of bonded-in rod axial strain, interfacial bond stress, and relative movement were also analyzed to evaluate the local bond stress–relative movement response in the bond line. The results confirmed that the bond-relative movement response is dependent on the locations along the anchorage length, and the bond-relative movement responses located near both the loaded end and the anchorage end were observed to be stiffer than those at other locations. Finally, the predictions for the load capacity of the glulam joints with bonded-in threaded steel rod were carried out based on sever...

Effect of temperature on the mechanical performance of glued-in rods in timber structures

Structural bonding technology has proven to be an economically and attractive connection process in timber engineering. Within old or historical wooden buildings, local reinforcement of weak zones is often performed with glued-in rods. This kind of connection typically allows the transfer of loads within wooden elements by means of threaded steel rods glued with a structural adhesive. This paper relates to experimental and numerical investigations on small sized specimens, with the aim of providing a better knowledge about the elastic behavior according to temperature. Experimental results reveal that stiffness of bonded-in rods significantly decreases once the glass transition temperature of the adhesive is reached. However, the ultimate shear strength is constant and sudden failures occur in the wood close to the adhesive whatever the temperature is. Then, an elastic finite element model allows the evolution of the Young modulus of the adhesive with temperature changes and also reveals the stress distribution along the glued-in depth during the elastic regime.

Effect of Timber Grain Orientation on Bonded-In Rod Connection Systems

Presented in this paper are the results of practical tests to determine some key material properties of engineered timber, in particular glulam. The results are discussed and compared with three known failure criterion, Hankinsons formula, The Tsai-Wu criterion and Maximum stress theory to determine if they are appropriate models for predicting the properties of glulam. The properties considered have been chosen with respect to a connection system for use in folded plate structures utilizing embedded rods. This paper considers the effect of the timber grain angle on the compression, tensile and steel dowel rod pull-out strengths. The test data shows that Hankinson and Tsai-Wu are both good models to predict both the compression and tensile behavior of engineered timber products at non-tangential angles, whereas maximum stress theory had less correlation and over predicts the results and would not be recommended for use in engineered timber materials.

Rehabilitation of timber structures: novel test method to assess the durability of bonded-in rod connections

Rehabilitation/restoration systems involving bonded-in rods represent an efficient alternative method for the repair and/or reinforcement of structural timber members. However, in spite of their advantages, their use is still restrained by the lack of knowledge about their long-term performance. In order to correct this situation it is necessary to gain a greater understanding of their durability, creep and fatigue behaviour. However, reliable and realistic accelerated ageing tests do not yet exist, and the application of the existing European or American standards to epoxy bonded products are much too penalising, since they merely impose severe conditions that are not verified in service although these may be suitable for other adhesives. To address these concerns a test procedure to assess long-term behaviour of bonded-in rod connections was developed with the aim of providing a simple method for the selection of candidate repair system materials for specific applications. The data collected so far using the realistic performance assessment test (RPAT) appears to predict correctly the end-use performance for different adhesives as well as for different timbers.

State-of-the-art review on timber connections with glued-in steel rods

Adhesive joints have been known and applied for timber structures for decades. Hybrid joints with glued-in rods are nowadays successfully used for both constructing new and strengthening existing timber structures. Since the 1980s the research and development of timber joints with bonded-in rods have been going on, however agreement regarding design criteria for these connections has not been reached. Today, connections with glued-in rods are not included in the European design code. Thus, it is desired to gather the current state of knowledge to enable application in practice of the existing and documented knowledge and experience. This paper summarizes practical and theoretical approaches from research done regarding joints with glued-in steel rods mostly in Europe and published in English, German or Swedish. The review considers manufacturing methods, mechanisms and parameters governing the performance and strength of the joints, theoretical approaches and existing design recommendations.

Pull-out strength of axially loaded steel rods bonded in glulam parallel to the grain

Bonded-in steel rods are very efficient in withstanding high forces applied to timber members. Investigations of bonded-in rods started in the late eighties of the last century and several design models were published since. By comparing these approaches on base of an extensive literature review a certain degree of discrepancy and partly even contradiction can be found. The paper describes a test program which aimed to study the influence of timber density and of geometric parameters on the pull-out strength of single axially loaded steel rods. Following the GSA®-procedure, which is a well established glued-in rods joint in Switzerland, rods with metric thread were bonded in glulam made of Norway spruce lamellas using an epoxy-type adhesive. The tests showed that the influence of the timber density can be quantified by a power function of ρ with an exponent of 0.6. The parameters length of the glued zone l and diameter of the hole d H can be summarized in the slenderness ratio λ = l/d H, which itself is related to the mean shear strength in the anchoring zone by an exponent of approximately −1/3. In order to prevent the specimens from premature splitting, distances between the axis of the rod and the edge of the specimen of 2.3 times the diameter of the rod have been used.

Influence of glue rheology and joint thickness on the strength of bonded-in rods

The maximum pull-out strength of bonded-in rods was used for investigating the relationship between the joint geometry and the rheology of the structural adhesive. Tests with a brittle and a ductile glue type, both formulated with epoxy resins, showed that the glue rheology has a strong influence on the joint behaviour. The joint thickness is also an important parameter, having a good correlation with pull-out strength and with the fracture typology. Wood density and moisture content had a very low influence on strength, in these test conditions. A modification of the Eurocode 5 equation was developed, allowing an improved use of the adhesive properties for a safer design. Results suggest that a standardised test method for bonded-in rods would be very helpful for the determination of basic joint properties such as the maximum joint thickness and the glue strength parameter that were used in the modified design equation.

Bonded-in rod connections for timber structures—development of design methods and test observations

Bonded-in rods provide the potential for engineering design of connections in timber structures to be expanded beyond the common types of connection devices currently employed. The achievement of improved connections is the major challenge in timber engineering and has great potential in a wide scope of new build and renovation projects. The achievement of transfer of technologies and techniques from the realm of research to actual practice is reliant upon usability of codified design methods and compatibility with the wider scope of design codes. This paper provides an overview of the key aspects for address in relation to British Standard and Eurocode design of timber structures and draws upon examples of significant knowledge development to support code development achieved through full-scale structural testing of joints. This includes duration of load tests involving adhesive bonded rods with three types of adhesive, subject to high fractions of ultimate load in an outdoor climate, and fatigue studies.

Pull-out behaviour of steel rods bonded into timber

Connections for timber structures using concealed bonded-in rods offer one possible solution to the development of more efficient joining methods. However the importance of certain basic joint characteristics, and the influence of adhesive type and properties, are not yet fully understood and as such design criteria are lacking. This paper describes a comprehensive experimental and numerical investigation into the fundamental material and joint geometry characteristics of rods bonded in to timber. The adhesive type and performance are both considered, along with test configuration and joint parameters that included rod embedment length, rod diameter, annular bondline thickness, multiple rods and rod spacing. A novel approach that involved end-profiling of the bond area was also investigated. It was found that epoxy adhesives out-performed all other types investigated, and pull-out strengths can be significantly improved through careful selection and optimisation of the joint geometry.

Adhesive systems for structural connections in timber

Adhesives can be used to form load-bearing joints in timber structures, both in repair and in new-build applications. An efficient and cost-effective method of making joints is to use rods or dowels, bonded into pre-drilled holes in timber elements, for transferring structural loads between such elements. This paper reviews the important issues in bonded-in rod technology, the adhesives and reinforcing materials used, the results of theoretical and experimental studies, and the steps involved in design and specification.

Effect of timber moisture content on bonded-in rods

Concealed bonded-in rods represent one example of an efficient joining method for modern structural timber composites and structures, exhibiting properties such as good joint stiffness and strength, reduced end-grain splitting, greater fire resistance and they have the added advantage of being more aesthetically pleasing than other joining methods simply because there is no visible joint. However, the effect of the timber moisture content (MC), at the time of bonding, on the strength of these connections is at present unknown. This paper describes the effect of MC on concealed bonded-in rod connections using ash and oak specimens. It was found that both strength and, in particular, the failure modes of the various specimen combinations were significantly influenced by the MC of the timber at the time of bonding.

A REVIEW OF MOMENT-RESISTANT STRUCTURAL TIMBER CONNECTIONS.

This paper presents a review of methods which can be employed for the formation of concealed moment-resisting connections in timber structures. The review considers technologies and connection systems which are commercially available and also those under development. It is concluded that the wide range of structural timber connection systems can be categorized into five generic types: concealed bonded-in rods, concealed bonded-in plates, adhesive bonded surface contact joints, timber connectors within lapped joints and dowel-type connections.