Light-frame wood structures are the most common type of construction for residential and low-rise buildings in North America. The 2015 edition of the National Building Code of Canada has increased the height limit for light-frame wood construction from 4 to 6 stories. With the increase in building height, it was noticed that light-frame wood structures may be governed by inter-story drift under wind and seismic loads. To reduce the inter-story drift, a hybrid system, consisting of CLT cores and light-frame structures, is proposed. The efficiency of this hybrid system is dependent on the performance of the connections between the two sub-systems. In this project, self-tapping screws (STSs) were used to connect the CLT core and light-frame wood structures on the floor level. Monotonic and reversed-cyclic tests were carried out on CLT-wood frame connections connected with STSs inserted at 45°, 90°, and mixed angles (45° and 90°). The connection performance was evaluated in terms of strength, stiffness, ultimate displacement, ductility, and energy dissipation capacity. Results show that a joint with STSs inserted at 45° had high stiffness and ductility but low energy dissipation, while connections with STSs installed at 90° had high ductility and energy dissipation but low stiffness. Connections with STSs inserted at mixed angles (45° and 90°) achieved the advantages of both configurations when the STSs were inserted at 45° or 90° individually, i.e., high stiffness, ductility, and energy dissipation. The ductility and energy dissipation were significantly improved compared with connections with STSs only inserted at 45° or 90°. This mixed angle connection can be an ideal design for connecting light-frame wood structures to a CLT core to resist wind and seismic load.
Long-span cross-laminated timber (CLT) floors are typically an assembly of prefabricated CLT panels connected together on the site. The actual connections are commonly neglected in design calculations. Hence, a CLT floor is modelled either as a monolith slab or more frequently as a set of CLT panels with no connections at all. This paper presents a numerical study designed to examine the influence of two most common inter-panel connections, i.e. single surface spline and half-lapped joint, on vibration modes and vibration responses of a range of different CLT floors due to pedestrian-induced loading. Although the inter-panel connections are relatively complex in reality, they are modelled here as an equivalent 2D elastic strip between the CLT panels. This relatively simple yet robust model can be used with ease in design practice, regardless finite element (FE) software used to extract vibration modes of a CLT floor. The corresponding monolith floors and floors without inter-panel connections are studied for the comparison of the results. Vertical vibration responses are simulated for low-frequency and high-frequency floors using the corresponding walking force models given in a popular design guideline for footfall induced vibrations of civil engineering structures. Vibration responses were calculated for single pedestrian occupants and their walking paths parallel and perpendicular to the line of connection. The results showed that including the inter-panel connections in a FE model resulted in up to 2.5 higher RMS acceleration levels. Hence, the common practice of modelling CLT floors as monolith slabs or as a set of panels without connections should be left behind.
Birch is a short-lived hardwood species widespread in the Northern Hemisphere. Plywood made from birch has superior mechanical properties compared with that made from most softwoods, which makes it suitable for structural application. In this study, the feasibility of using birch plywood as gusset plates in timber-timber connections is presented. Test frames consisting of birch plywood gussets and glulam beams connected by nails were built and tested. A 2D analytical model based on truss theory and a 3D finite element model were proposed and constructed. Both models showed satisfactory agreements with the test results in terms of stiffness and strength. Tensile failure on the birch plywood gussets along the outermost row of nail holes was observed in the experiment. The observed failure modes and the stress distributions in the 3D numerical model suggest that the spreading angle (Whitmore effective width) theory should be considered in the design phase of birch plywood gusset plates. Besides, a modified spreading angle theory is proposed to both approximate the stress distribution and predict the load-bearing capacity.
The hidden nature of a glued-in rod joint presents considerable challenges with regards to quality control, and there has been minimal research on the subject to fully understand the influence of defects on the joint performance. In this study, voids in adhesive line or glue-free zones were simulated in various depths of the embedded rod, and the results were compared to a reference sample population without defects. Withdrawal capacity of glued-in steel threaded rods were lower compared to reference group samples without gluing defects, when glue simulated voids or glue-free zones were positioned in the middle part and upper (closer to sample crosscut surface) part of the glued-in rods. And no difference was observed of simulated glue-free zones in the lower (the deepest) part, closer to the end of the glued-in rod, compared to reference group samples without gluing defects.
Various design approaches for establishing the resistance of connections in cross-laminated timber (CLT) structures have been developed and adopted in timber design standards worldwide. Although the fundamental principles are similar, the new design provisions for CLT connections have been aligned in some standards with the existing design philosophy and format adopted for sawn timber and glulam using traditional fasteners such as dowels, nails, and wood screws for consistency and simplicity, in the other standards, alternate approaches have been developed. This article presents a snap shot of the various design approaches for connections in CLT adopted in Europe, Canada, the United States, and New Zealand. The intent is for the reader to have a better knowledge of the underpinning assumptions, principles, and the adopted design rules in each of these standards.
According to Eurocode 8 moderate to high dissipative behaviour of timber structures requires sufficient ductility of the critical regions. Earlier experiments on timber connections with slotted-in steel plates and laterally loaded common steel dowels rarely achieved high ductility values. Connections consisting of LVL-C members, dowel-type fasteners with different post-elastic steel properties, full confinement of the timber member and measures to restrain the notch effect were investigated by means of monotonic and cyclic tests with regards to the displacement ductility. The measures taken proved to be effective in enhancing the plastic deformation capacity of the steel dowels to a large extent. However, a new aspect was observed: the constriction of the dowels in the contact area with the plate. The research results provided a better understanding of the factors influencing the behaviour of slotted-in steel plate connection.
Sustainability issues are driving the civil construction industry to adopt and study more environmentally friendly technologies as an alternative to traditional masonry/concrete construction. In this context, plantation wood especially stands out as a constituent of the cross-laminated timber (CLT) system, laminated wood glued in perpendicular layers forming a solid-wood structural panel. CLT panels are commonly connected by screws or nails, and several authors have investigated the behavior of these connections. Glass-fiber-reinforced polymer (GFRP) dowels have been used to connect wooden structures, and have presented excellent performance results; however, they have not yet been tested in CLT. Therefore, the objective of this study is to analyze the glass-fiber-reinforced polymer (GFRP)-doweled connections between CLT panels. The specimens were submitted to monotonic shear loading, following the test protocol described in EN 26891-1991. Two configurations of adjacent five-layer panels were tested: flat-butt connections with 45° dowels (x, y, and z axes), and half-lap connections with 90° dowels. The results were evaluated according to the mechanical connection properties of strength, stiffness, and ductility ratio. The results showed higher stiffness for butt-end connections. In terms of strength, the half-lap connections were stronger than the butt-end connections.
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.
A new timber frame structural system consisting of continuous columns, prefabricated hollow box timber decks and beam-to-column moment-resisting connections is investigated. The hollow box timber decks allow long spans with competitive floor height and efficient material consumption. To achieve long spans, semi-rigid connections at the corners of deck elements are used to join the columns to the deck elements. In the present paper, experimental investigations of a semi-rigid moment-resisting connection and a mock-up frame assembly are presented. The semi-rigid connection consists of inclined screwed-in threaded rods and steel coupling parts, connected with friction bolts. Full-scale moment-resisting timber connections were tested under monotonic and cyclic loading to quantify rotational stiffness, energy dissipation and moment resistance. The mock-up frame assembly was tested under cyclic lateral loading and with experimental modal analysis. The lateral stiffness, energy dissipation, rotational stiffness of the connections and the eigen frequencies of the mock-up frame assembly were quantified based on the experimental tests in combination with a Finite Element model, i.e., the model was validated with experimental results from the rotational stiffness tests of the beam-to-column connections. Finally, the structural damping measured with experimental modal analysis was evaluated and compared with FE model using the material damping of timber parts and equivalent viscous damping of the moment-resisting connections.
In platform-type multi-story cross-laminated timber (CLT) buildings, gravity loads from upper floors, and vertical reaction forces from horizontal actions, like wind loads, cause substantial compressive forces in the CLT-floor elements. The combination of these high forces with a comparable low compression stiffness and strength perpendicular to the grain of timber, makes the compression perpendicular to the grain (CPG) verification of CLT an important design criterion. In this experimental study, CPG of CLT was investigated by means of typical wall-to-floor connections in CLT platform-type structures. CLT-wall elements were used for load application to transmit forces through the CLT-floor element by CPG. Compared to load application by steel elements, as it commonly is done in experiments, lower stiffness but similar strength were found for CLT walls. The study of different connection types showed the highest stiffness and strength for connections assembled with screws, followed by pure wood-to-wood contact, while connections with acoustic layers between the floor and wall elements showed the lowest stiffness and strength. In addition, these connections were tested for center and edge load position on the CLT-floor element. The strength for center and edge position compared to full surface loaded specimens increased linearly with the activated material volume, as determined by earlier proposed stress dispersion models. The stress dispersion effect was visualized by surface strain measurements using digital image correlation technique. Also, the stiffness increased with the activated material volume. Stress dispersion in the CLT-floor allowed to explain the increase in stiffness and strength with decreasing CLT-wall thickness. Strength values at different strain levels, and stiffness and strength increase factors suitable for the engineering design of CLT structures are provided.
The use of moment-resisting frames with semi-rigid connections as a lateral load-carrying system in timber buildings can reduce the need for bracing with diagonal members or walls and allow for more open and flexible architecture. The overall performance of moment-resisting frames depends largely on the properties of their connections. Screwed-in threaded rods with wood screw thread feature high axial stiffness and capacity and they may be used as fasteners in beam-to-column, moment-resisting timber connections. In the present paper, a structural concept for a beam–to-column, moment-resisting timber connection based on threaded rods is presented and explained. Analytical expressions for the estimation of the rotational stiffness and the forces in the rods were derived based on a component-method approach. The analytical predictions for stiffness were compared to experimental results from full scale tests and the agreement was good.
The aim of the study was to create and verify a theoretical model for single-shear steel to bamboo nailed connections in natural bamboo. To this end, existing timber strength and fracture theories were used to model the brittle and ductile failure strength of the studied bamboo connection. Experimental tests of steel to bamboo nailed connection subjected to short-term loading in single shear parallel to fibre direction in Moso bamboo confirmed the suitability of the timber models to predict the connection behaviour. The findings constitute a basis for development of theory for screws, which is the advocated connector type by the authors. It is anticipated that the predictability of bamboo dowelled connection behaviour, along with the latest developments in bamboo design standardization, will enhance the designers confidence to implement dowelled bamboo connections in practice.
Cross laminated timber volumetric construction (CLTVC) is an innovative construction technology that combines the sustainability of timber and the efficiency of modular construction, as opposed to conventional construction. However, the connection installing methods of CLTVC, such as fastening, are laborious with limited accessibility for connection installations, thus hindering the application of CLTVC in mid- and high-rise structures. Therefore, a new way of connecting CLT modules by sliding and stacking is explored herein with a proposed damage-control interlocking connection system, aiming to provide a more efficient assembly solution to CLTVC that does not require onsite screwing. Quasi-static monotonic and cyclic test, and numerical analyses were conducted to assess the mechanical performance of the proposed connections, which possessed adequate translational stiffness and strength of the proposed connections. The connections' ability to control deformation – damage is moved away from timber and the embedded fasteners, was also well demonstrated in the test, as both screws and timber remained mostly intact after testing. The proposed connection design showcases a new concept of modules’ assembly in volumetric construction with higher efficiency and flexibility; meanwhile demonstrates the potential in reducing the permanent damage to structural materials during service life and enabling reuse.
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.
Inaccuracies within timber step joints are a perennial problem of the wood construction industry. Even perfectly constructed carpentry step joints can become imperfect due to a change in moisture content. The predominant question when looking at step joints with gaps is to what extent the load-bearing behaviour is influenced by these inaccuracies. The authors look beyond this question and investigate if intentionally manufactured gaps could have a positive influence on the load-bearing behaviour and the failure mode of regular and newly designed carpentry joints.
Timber elements, such as timber bridges, are exposed to heavy loads. Therefore, reinforcement might be useful. Due to a lack of wood supplies, poplar, a fast-growing tree, could be used to construct CLT (cross-laminated timber). The low density of fast-growing wood species directly impacts the mechanical properties of CLT. Therefore, in this study, a CLT panel was reinforced with GFRP (glass-fiber-reinforced polymer), and the lateral resistance of double shear lap joints in reinforced CLTs with 0-90-0° arrangements in two strength directions was investigated. Lag screws (Ø = 8 mm) at the end distances of 1 and 3 cm were employed for making the lateral test specimens. First, the effect of the number of GFRP layers on lateral resistance of the joints was investigated. The results revealed that, as the number of GFRP layers changed from one to three, the lateral resistance increased by 45.47%, and then, by four layers, it decreased by 1.3%. Since the joints with three layers of FRP had the highest strength, the effects of the end distance and the CLT panel strength directions on the lateral performance of the reinforced and non-reinforced specimens were investigated. The results indicated that the lateral resistance of reinforced CLTs with GFRP was about 26.5% more than the unreinforced ones. Moreover, CLTs in the major strength direction showed 4.2% more lateral resistance than those in the minor strength direction. Moreover, lag screws at the end distance of 3 cm had 60% more lateral resistance than those at the end distance of 1 cm. In terms of failure modes, bearing, shear, and net-tension modes were observed in the CLTs, while Is, IIIs, and IV modes were observed in the lag screws.
In this paper, the short-term behaviour of innovative aluminium–timber composite beams was investigated. Laminated veneer lumber panels were attached to aluminium beams with screws. Recently conducted theoretical, experimental, and numerical investigations have focused on aluminium–timber composite beams with almost full shear connections. However, no experiments on aluminium–timber composite beams with partial shear connections have yet been conducted. For this reason, composite action in composite beams with different screw spacing was studied in this paper. Four-point bending tests were performed on aluminium–timber composite beams with different screw spacing to study their structural behaviour (ultimate load, mode of failure, load versus deflection response, load versus slip response, and short-term stiffness). The method used for steel–concrete composite beams with partial shear connection was adopted to estimate the load bearing capacity of the investigated aluminium–timber composite beams. The resistance to sagging bending of the aluminium–timber composite beams with partial shear connections from the theoretical analyses differed by 6–16% from the resistance in the laboratory tests. In addition, four 2D numerical models of the composite beams were developed. One model reflected the behaviour of the composite beam with full shear connection. The remaining models represented the composite beams with partial shear connections and were verified against the laboratory test results. Laminated veneer lumber was modelled as an orthotropic material and its failure was captured using the Hashin damage model. The resistance to sagging bending of the aluminium–timber composite beams with partial shear connections from the numerical analyses were only 3–6% lower than the one from the experiments.
The use of timber in construction in medium–high rise construction has increased in recent years largely due to the significant innovation in engineered wood products and connection technology coupled with a desire to utilise more environmentally sustainable construction materials. While engineered wood products offer a low-carbon solution to the construction industry, the widespread use of adhesive and metallic fasteners often limits the recyclability of the structural components at the end of life of the structure and it may be beneficial to reduce this where possible.
To establish the possibility of an all-wood connection solution, this preliminary study examines a series of beam-column connections designs to evaluate the relative performance of the different designs, which are connected with modified or compressed wood (CW) connectors. The connection designs are formed between glued-laminated beam and column members in the first instance and later examined when connecting dowel-laminated timber (DLT) members.
The results show that significant moment capacity and rotational stiffness can be achieved for connections solely connected using CW fasteners. Furthermore, the all-wood solution utilising CW fasteners to connect DLT members has also demonstrated significant moment capacity and rotational stiffness capacity without the use of adhesive and metallic components.
The lateral resistance of dowel-type connections with CLT is related to its lay-up, species of the laminations and even the manufacture method. Treating the CLT as homogeneous material, current methods develop new equations through test results or make use of the existing equations for the embedment strength already used in design codes; thus, the lateral resistance of dowel-type connections of CLT can be calculated. This kind of approach does not take the embedment stress distribution into account, which may lead to inaccuracy in predicting the lateral resistance and yield mode of the dowel-type connections in CLT. In this study, tests of the bolted connections and the screwed connections of CLT were conducted by considering the effects of the orientation of the laminations, the thickness of the connected members, the fastener diameter and strength of the materials. The material properties including yield strength of the fasteners and embedment strength of the CLT laminations were also tested. Using analysis of the dowel-type connections of CLT by introducing the equivalent embedment stress distribution, equations for the lateral resistance of the connections based on the European Yield Model were developed. The predicted lateral resistance and yield modes were in good agreement with the test results; the correctness and the feasibility of the equations were thus validated.
This paper presents an investigation of the load-slip behaviour of aluminium-timber composite connections. Toothed plates with bolts are often used for connecting timber structural members with steel structural members. In this paper, toothed plates (C2-50/M10G, C2-50/M12G or C11-50/M12) have been used as reinforcement in aluminium-timber screwed connections for the first time. The push-out test specimens consisted of laminated veneer lumber slabs, aluminium alloy beams, and hexagon head wood screws (10 mm × 80 mm and 12 mm × 80 mm). Of the specimens, 12 additionally had toothed plates as reinforcement, while 8 had no reinforcement. The load carrying-capacity, the mode of failure and the load-slip response of the strengthened and non-strengthened screwed connections were investigated. The use of toothed plate connectors was found to be effective in increasing the strength of aluminium-timber composite connections and ineffective in improving their stiffness. The examined stiffness and strength of the connections can be used in the design and numerical modelling of aluminium-timber composite beams with reinforced screwed connections.
