Connections In Timber Structures Research Articles

Effect of Wood Densification and GFRP Reinforcement on the Embedment Strength of Poplar CLT

Embedment strength is an important factor in the design and performance of connections in timber structures. This study assesses the embedment strength of lag screws in three-ply cross-laminated timber (CLT) composed of densified poplar wood with densification ratios of 25% and 50%, under both longitudinal (L) and transverse (T) loading conditions. The embedment strength was thereafter compared with that of CLT reinforced with glass-fiber-reinforced polymer (GFRP). The experimental data was compared with results obtained using different models for calculating embedment strength. The findings indicated that the embedment strength of CLT specimens made of densified wood and GFRP was significantly greater than that of control specimens. CLT samples loaded in the L direction showed higher embedment strength compared to those in the T direction. In addition, 50% densification had the best performance, followed by 25% densification and GFRP reinforcement. Modelling using the NDS formula yielded the highest accuracy (mean absolute percentage error = 10.31%), followed by the Ubel and Blub (MAPE = 21%), Kennedy (MAPE = 28.86%), CSA (MAPE = 32.68%), and Dong (MAPE = 40.07%) equations. Overall, densification can be considered as an alternative to GFRP reinforcement in order to increase the embedment strength in CLT.

Numerical Investigation of the Axial Stress Distribution of Self-Tapping Screws in Mass Timber Products during Wetting or Drying

Self-tapping screws (STS) are used in wood-to-wood and wood-to-steel connections in timber structures. Premature failure of STS during the construction phase has been reported by structural engineers and contractors in relation to a few North American mass-timber projects. This STS failure type is suspected of having been precipitated by additional axial stress exerted on STS from swelling of the wood resulting from prolonged wetting. This study investigates the axial stress distribution of STS installed in two mass timber products, cross-laminated timber (CLT) and glulam, under axial loading and changing moisture conditions in the linear elastic regime. The focus is on modelling the stress distribution of STS during wood-wetting. Properties of self-tapping screws under axial loads, such as tensile and withdrawal properties, along with swelling properties of CLT and glulam, have been investigated. A numerical method to predict the axial stress distribution of the screw from these material property tests has been developed. The numerical model has been calibrated with test results. The method developed in this research helps in understanding the premature failure of self-tapping screw connections under the moisture content variation of wood. The next step will be developing an analytical model to predict the axial stress distribution of STS.

Pull-out resistance of glued in rod connection in timber: Reliability analyses using an experimental database

Glue in rod (GIR) connections in timber structures are used, as they enable the prefabrication of timber sections and frames. However, uncertainties of achieving perfect GIR connection owing to possible non-uniformity in the glue application and rod alignment hamper widespread usage of GIR connection in timber structures. Although plenty of experimental studies have been conducted to explore the effectiveness of GIR in different timbers through pull-out tests and some rational design formulations are developed, the structural reliabilities of the predicted pull-out resistances are not well explored. Thus, in this study, the reliabilities of the existing design formulations to determine the pull-out resistances of GIR timber connections were assessed. A comprehensive experimental database of GIR pull-out resistances was established from the past studies to analyse the reliability of the existing predictive models. The database is divided into two categories (1) single and (2) multi GIR timber pull-out test results. The database comprised of 349 (single GIR) and 75 (multi GIR) pull-out test results from various experimental studies conducted in the past. Then, the appropriateness of the existing design formulations were assessed against the experimental database. The structural reliabilities of the design formulations considered were assessed through the First Order Second Moment method (FOSM) with an applicable target reliability index (i.e. 4.0). It was found that the design formulation given in DIN 1052:2008-12 has shown to be the most conservative compared to other formulations considered. The formulations outlined to compute the pull-out resistance of multi GIR timber connections have shown to be conservative, as their reliabilities were higher than the target reliability index (>4) defined. Finally, suitable resistance reduction factors are suggested to calculate the pull-out resistances of GIR timber connection with appropriate reliability for all the design formulations considered.

Analysis and test of stiffness of bolted connections in timber structures

The stiffness of bolted connections in timber structures is of particular importance in the evaluation of the lateral performance, yet the method to calculate the stiffness of connections remains largely unsolved. To facilitate evaluation of the stiffness of bolted connections, testing of the dowel-bearing performance of wood and the lateral performance of bolted connections were revisited. In addition, experimental evaluation of the moment-resisting performance of beam-column joints with bolted connections was conducted in this study. Three deformation modes, namely Pre-Yield Modes I, Ⅱ and III, in relation to the European Yield Modes were identified. Empirical formulae for calculating dowel-bearing stiffness both parallel and perpendicular to grain were obtained via regression of the dowel-bearing test results. By making simplifications to the beam-on-elastic-foundation theory to suit a particular bolted connection, formulae with respect to the Pre-Yield Modes for calculating the stiffness of bolted connections were derived. All formulae were validated by physical testing of both bolted connections and beam-column joints. Thus a reliable method to calculate the stiffness of bolted connections was established, making it feasible to predict the lateral performance of timber structures with bolted connections.

Application and potential of shape memory alloys for dowel-type connections in timber structures

ABSTRACT The present study aims at identifying and analyzing challenges and prospects related to the use of pseudoelastic shape memory alloys for well-established double shear connections in timber engineering. The unique behavior of shape memory alloys will allow to bear large reversible deformations without loss of cohesion. Thereby, the main objective of the present work is to study the behavior of a connection that is able to stay completely reversible even upon very large deformations. In this context, the stiffness of the connection as well as functional and structural integrity and damping behavior have been improved by stepwise adaption of the detailing. Proof of concept is provided, and current limitations are discussed, based on the application of bolts made from a commercially available shape memory alloy.

Loosening of Bolted Connections under Transverse Loading in Timber Structures

Bolted joints are widely used in timber structures, and the loosening of bolt connections will reduce the structural performance. In this paper, a mechanical model of bolt connection for timber structures is established, and the process of bolt loosening under a transverse load is investigated. By using the finite element method to construct an accurate thread model with a helix angle, the thread contact state during the bolt loosening procedure was analyzed in detail, and the factors such as load amplitude, load frequency, load location, and different timber materials on bolt loosening are also studied. In the timber structure, the load amplitude is the main factor affecting the bolt loosening, the decay rate of the preload in the bolted joint is positively correlated with the amplitude of the cyclic transverse load. The frequency of the loading has a smaller effect on the looseness, the preload decreases as the frequency increases. When the load is applied to the smooth rod part of the bolt, the preload force will decrease rapidly, and the distance between the load position and the bolt has no effect on the change in looseness. The decreasing range of the preload is different with different timber material, but the decreasing law is the same. The model can be applied to analyze the loosening rule of bolted connections in timber structures.

A probabilistic study of brittle failure in dowel-type timber connections with steel plates loaded parallel to the grain

ABSTRACTDowel-type connections in timber structures should be typically designed in a way that, if the load bearing capacity is exceeded, desirable ductile failure should likely occur, i.e. yielding of the metal dowel or embedment of the timber. However, the probability of brittle failures cannot be completely avoided in many cases. If the connection is loaded parallel to the grain direction, splitting, row-shear, plug/block shear and tensile failure are examples of such brittle failure modes. To ensure a beneficial structural performance locally and sufficient structural robustness globally, the probability of such brittle failures should be kept at a sufficiently low level. However, in the current version of EN1995, the probability of such failure is not considered explicitly. The final aim of the presented research study is to develop a rational approach for the consideration of brittle failure in timber connections with dowel-type fasteners and steel plates loaded parallel to the grain, consistently with the partial factor method. As a first step, a probabilistic model has been used to study the susceptibility to brittle failure and provide a rational basis for the calibration of a “brittle overstrength factor”. The results of this study are reported in the current contribution.

Glued-in basalt FRP rods under combined axial force and bending moment: An experimental study

Glued-in rods have potential for use as moment-resisting connections in timber structures. There has been minimal research to date conducted on the performance of glued-in rods acting under both axial force and moment. The influence of increasing embedded length of the glued-in rod on the strength of the system was assessed using a pull-bending test which subjected the connection to this force combination. The longest length (600 mm) had a pull-out capacity 213% greater than the shortest (80 mm). To reduce instances of splitting in the timber end distance was varied. Optimum end distance of 42 mm (3.5dr) was identified with further increase having minimal impact upon overall strength.

Fire resistance of primary beam – secondary beam connections in timber structures

Purpose This paper deals with the fire resistance of primary and secondary beam connections in timber structures. Design/methodology/approach This paper describes a series of unloaded and loaded furnace fire tests in different configurations of these types of connectors. Findings The main objective is the fire safety design of joist hangers and full thread screws. Originality/value Design recommendations are given.

Stiffness of dowel-type timber connections under pre-yield oscillating loads

The dynamic behaviour of timber structures in service is becoming a more important consideration in design as modern engineered wood products allow longer spans and taller timber buildings, which can be sensitive to dynamic loading such as that from wind or footfall. Connections in timber structures have a pronounced effect on their structural behaviour. In dowel-type connections, the fasteners bend under load and embed into the surrounding timber and, since embedment is a nonlinear process, the stiffness of those connections varies depending on the nature of the applied load. Here, single-dowel connections are tested under cyclic loads representative of in-service vibration. One-sided and reversed cyclic loads are applied. The specimen stiffness is observed to reduce with the amplitude of one-sided cyclic load. For small-amplitude one-sided load, the specimen stiffness is seen to tend towards that predicted by an elastic model, and an analytical elastic model is presented to represent the embedment resistance of the timber and the behaviour of the connector.

Viscoelastic embedment behaviour of dowels and screws in timber under in-service vibration

Dowel and screw connections in timber structures behave nonlinearly, even at loads which would be experienced in a structure in normal service. They exhibit hysteresis and creep as a result of both the viscoelastic behaviour of the timber itself and the frictional interaction between the timber and connecting elements, and stress concentrations are created which behave plastically, even at loads well below the nominal yield force of the connection. A fundamental process in the load transfer through such a connection is the embedment of the connector into the timber that surrounds it, and the frictional, nonlinear and time-dependent properties in that process are investigated here. A simple rheological model, a combination of Kelvin–Voigt viscoelastic elements, was fitted to the measured response of a block of timber in embedment by a plain dowel or screw. Experiments were performed in which an oscillating force was applied to the screw or dowel, representative of in-service vibration in a timber structure. The effects of plasticity and viscoelasticity were quantified by comparing equivalent linear stiffnesses for an oscillating load, a short-term change in static load, and an initial static loading. The results showed a stiffness, on average, 3.8 times higher under oscillating load than under initial static loading with the same peak force. By quantifying and modelling viscoelastic behaviour in timber around a connector, this work contributes to the development of damping and stiffness models for joints under oscillating load. Such models could be used to determine the contribution of connections to the dynamic response of long spans and tall buildings in timber.

Epoxy Adhesives Modified With Nano- and Microparticles for In Situ Timber Bonding: Fracture Toughness Characteristics

Adhesives used for bonded-in steel or composite pultruded rods and plate to make connections in timber structures are commonly room temperature cure adhesives. The room temperature cure, applied without pressure, thixotropic, and shear thinning characteristics of the adhesives, is for ease of application when repairs and reinforcement are being made in situ in the field. The room temperature cure adhesive may not fully cross-link and this may cause brittleness. Therefore to improve the toughness properties of such adhesives, nanoparticles can be added. This paper reports the experimental investigation carried out on the fracture toughness of three thixotropic and room temperature cured epoxy-based adhesives formulated specifically for in situ timber bonding, namely, CB10TSS (standard adhesive), Albipox is CB10TSS with the addition of nanodispersed carboxyl-terminated butadiene acrylonitrile (CTBN), and Timberset is an adhesive formulation containing ceramic microparticles. The fracture toughness behavior of the adhesives was investigated using the Charpy impact test on unnotched and notched specimens conditioned at 20∘C/65%RH to evaluate notch sensitivity, and a single-edge notched beam (SENB) test was performed to evaluate the stress intensity factor KIC. The fracture surfaces were investigated using scanning electron microscopy. Under high impact rate, toughness was in the order of CB10TSS, Albipox, and Timberset. CB10TSS and Albipox were found to be ductile in the unnotched state and brittle when notched. Timberset was brittle in both unnotched and notched states. Under low strain rate (SENB) conditions the addition of CTBN significantly improved the fracture toughness of Albipox compared with CB10TSS and Timberset. Examination of the topography of the fractured surface revealed marked changes in crack propagation due to the addition of nano- or microfillers accounting for the variation in toughness properties.

Fracture toughness of thixotropic and room temperature cured epoxy-based adhesives for in situ timber bonding

Abstracts An experimental investigation has been carried out on the fracture toughness of three thixotropic and room temperature cured epoxy-based adhesives formulated specifically for in situ timber bonding namely CB10TSS (standard adhesive), Albipox is a modification of CB10TSS with the addition of nano-dispersed carboxyl-terminated butadiene acrylonitrile (CTBN) and Timberset is a commercially used adhesive formulation containing quartz, mica and bentonite and cured with an aliphatic diamine (TMD). The fracture toughness behaviour of the adhesives was investigated using the Charpy impact test on unnotched specimens conditioned at 20 °C/65%RH and 30 °C/95%RH to observe the effect of temperature and moisture. The Charpy impact tests were also performed on notched specimens to evaluate notch sensitivity, and a single-edge notched beam (SENB) test was performed to evaluate the stress intensity factor, KIC. The fracture surfaces were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Under high impact rate at 20 °C/65%RH, toughness was in the order of CB10TSS, Albino and Timber set. CB10TSS and Albino were found to be ductile in the unmatched state and brittle when notched. After exposure to 30 °C/95%RH, Albino was tougher than CB10TSS due to an increase in ductility, which allowed the rubber particles to cogitate. Timber set was brittle in both unmatched and notched states. Under low strain rate (SENB) conditions the addition of CTBN significantly improved the fracture toughness of Albino compared with CB10TSS and Timber set. Examination of the topography of the fractured surface revealed marked changes in crack propagation due to the addition of nana- or micro-fillers accounting for the variation in toughness properties. The toughness of thixotropic adhesives, specified for bonding in connections in timber structures, could therefore be enhanced by the inclusion in the epoxy matrix of phase separated liquid rubber.

AN EXPERIMENTAL STUDY ON EMBEDMENT STRENGTH FOR BOLTED CONNECTIONS LOADED PERPENDICULAR TO THE GRAIN

For bolted connections in timber structures, calculation method based on European yield theory is well known and widely used in the world. On this equation, we need some material properties, embedment strength of wood, yield strength of bolt and each size of specimens. Based on some test results, the strength on the latest standard in Japan announced in public. On the other hand, failure modes in wood aren't considered enough. On this study, we conducted bearing tests based on BS EN 383 and 3-point-bearing test, using end-distance, bolts diameter and wood species as tests parameter. We arranged the specimens with or without clearance between specimen and loaded-steelplate to observe crack behavior. Additionally tension-loaded bolted specimens were analyzed by failure modes and the bearing strengths were calculated according to EYT equation. As a result, 2%off-set strength was corresponding with the crack-starting-strength on bearing tests. The cracked-strength of bolted connection was almost the same with the calculation result with 2%off-set strength.

Behaviour of timber connections using glued-in GFRP rods under fatigue loading. Part I: In-line beam to beam connections

Glued-in rods are an attractive option for making connections in timber structures. This paper describes the static and fatigue behaviour of in-line beam to beam connections made using glass fibre-reinforced plastic (GFRP), pultruded rods bonded into the end grain of LVL beams with epoxy resin. The beam to beam samples were connected with three rod configurations and the mechanical performance of these in-line jointed beams was compared with solid unjointed LVL. The mode of fatigue loading selected was flexure in compression–compression at R = +10. The results show that solid LVL beams have significantly higher static and fatigue strengths than beams jointed using GFRP glued-in rods, but the capacity of solid LVL to dissipate energy under cyclic loading is much less than for the jointed beams. The pattern in the development of the shape of hysteresis loops for beams in fatigue is, in general, an increase in loop area and decline in dynamic modulus with increasing number of cycles.

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.

Ancient Chinese Timber Architecture. II: Dynamic Characteristics

It is imperative to preserve ancient Chinese timber architecture because the impacts of time and the environment have seriously reduced the resistance capability of these historical relics. However, little information can be found to provide basic knowledge for maintenance and reinforcement. Based on an experimental study of an ancient timber structure—the front tower over the North Gate of the Xi'an City Wall in China—a semirigid finite-element model (FEM) was developed to simulate the behavior of dou gong (corbel brackets) and tenon joints and a 3D FEM model was used to study the dynamic characteristics of the structure. The FEM program SAFATS was developed to conduct dynamic and earthquake response analysis for this kind of timber structure. By comparison with experimental results, the equivalent stiffness of the semirigid element was defined and the dynamic characteristics of the structure were obtained. Based on the analysis, it was found that the use of semirigid connections in timber structures, su...

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.

Glulam rivets in radiata pine

Glulam rivets are special nails for making high strength connections in timber structures. This paper describes an investigation of glulam rivet behaviour in radiata pine timber grown in New Zealand. Test results are compared with those on Canadian species, and with several international codes. The European yield theory is shown to give excellent prediction of glulam rivet strength in those connections where wood tension failures do not occur. Simple techniques for obtaining rivet bending strength and wood embedment strength are described, to provide input to the yield theory equations. Key words: fastenings, glulam, laminated, nails, pine, radiata, rivets, timber, wood.