Timber structures are strongly depending on the design of connections, which are mostly constructed from steel components. However, these joints have a number of limitations such as the tendency to be heavy, proneness to corrosion and often poor aesthetic appearances. Therefore, this study aims to replace metallic joints by non-metallic materials. An experimental testing program was performed to investigate the use of glass fiber reinforced plastics (GFRP), densified veneer wood (DVW) and laminated veneer lumber (LVL) in the form of plates and dowels in different test configurations. Analytical and numerical models were developed to better understand the load-bearing behaviour and to perform static verifications. The models were validated based on the experimental results. The results demonstrate that the use of GFRP dowels in combination with GFRP plates can provide a robust connection system for contemporary applications.
The aim of the experimental study presented herein is the assessment and quantification of the behavior of individual dowels in multi-dowel connections loaded by a bending moment. For this purpose, doubleshear, steel-to-timber connections with nine steel dowels arranged in different patterns and with different dowel diameters were tested in 4-point bending. In order to achieve a ductile behavior with up to 7° relative rotation, the connections were partly reinforced with self-tapping screws. The reinforcement did not influence the global load-deformation behavior, neither for dowel diameters of 12 mm nor for 20 mm, as long as cracking was not decisive. The deformation of the individual dowels was studied by means of a non-contact deformation measurement system. Thus, the crushing deformation, i.e. the deformation at the steel plate, and the bending deformation of the dowels could be quantified. In case of 12 mm dowels, the bending deformation was larger than the crushing deformation, while it was smaller in case of 20 mm dowels. Moreover, dowels loaded parallel to the grain showed larger bending deformations than dowels loaded perpendicular to the grain. This indicates that the loading of the individual dowels in the connection differs, depending on their location.
This paper presents an experimental study on ductility and overstrength of dowelled connections. Connection ductility and overstrength derived from monotonic testing are often used in timber connection design in the context of seismic loading, based on the assumption that these properties are similar under monotonic and cyclic loading. This assumption could possibly lead to non-conservative connection design. Therefore, it is necessary to quantify ductility and overstrength for cyclic loading and contrast them with their monotonic performance. For this purpose, monotonic and quasi-static cyclic experimental tests were performed on dowelled LVL and CLT connections. The experimental results were also compared with strength predictions from state-of-the-art analytical models in literature that were verified for ductile and brittle failure under monotonic loading. This work also allowed investigation into a generally applicable overstrength factor for push-pull loaded dowelled connections.
Information on ductile and brittle failure modes is critical for proper design of timber connections in Crosslaminated Timber (CLT). While considerable research has been conducted in Europe and Canada on the ductile performance of connections in CLT, little is known about the brittle behaviour. This paper presents new information from testing programs and analysis performed in Canada and in New Zealand on the brittle performance of dowel-type fasteners in CLT. The testing programs have been designed to trigger brittle failure modes based on minimum end distances and fasteners spacings specified in the Canadian timber design standard. Timber rivets and bolts/dowels are covered under this study. At the time of writing of this abstract, the testing program is advancing and results will be available at the time of paper submission.
Recent studies showed the need for timber connections with high fire performance. Connections of members in timber structures commonly comprise steel connectors, such as dowels, screws, nails and toothed plates. However, multiple studies have shown that the presence of exposed metal in timber connections leads to a poor performance under fire conditions. Replacing metallic fasteners with non-metallic fasteners potentially enhances the fire performance of timber connections. Previous studies showed that Glass Fibre Reinforced Polymer (GFRP) dowels can be a viable replacement for steel dowels and that Densified Veneer Wood functions well as a flitch plate material. However, as the resin matrix of GFRP dowels is viscoelastic, connection creep, which is not studied before, can be of concern. Also no research has been carried out on the fire performance of these connections. Therefore, a study of the creep behaviour and the fire performance of non-metallic timber connections comprising GFRP dowels and a Densified Veneer Wood flitch plate was performed, as is discussed in this thesis. Predictive models were proposed to determine the connection slip and load bearing capacity at ambient and elevated temperatures and in a fire. The material properties and heat transfer properties required for these models were determined experimentally and predictions of these models were experimentally validated. Furthermore, an adjustment of the predictive model of connection slip at ambient temperature allowed approximating the creep of the connection. The material properties, required for the creep model, were determined experimentally and predictions of the model were compared to results of longterm connection tests. The study confirmed that timber members jointed with non-metallic connectors have a significantly improved fire performance to timber joints using metallic connections. Models developed and proposed to predict fire performance gave accurate predictions of time to failure. It was concluded that non-metallic connections showed more creep per load per connector, than metallic connections. However, the ratio between initial deflection and creep (relative creep) and the ratio between load level and creep were shown to be similar for metallic and non-metallic connections.
Cross-laminated timber (CLT) constitutes a promising solution for numerous structural applications, including for large and tall residential and commercial buildings. The prospect of building larger timber structures creates some structural challenges, amongst them being that lateral forces created by high winds and strong earthquakes are higher and create higher demands of “holddowns”. The Canadian Standard for Engineering Design in Wood CSA-O86 does not (yet) provide any specific procedures to estimate the resistance of mass-timber Lateral Load Resisting Systems (LLRS) nor how to facilitate the targeted kinematic mode, especially for multi-panel walls where the LLRS behaviour is a function of connection behaviour.
The project investigated the viability of internal-perforated-steel-plates (ISP) with self-drilling dowels as high-performance connections for CLT LLRS. The project objective was to contribute towards the development of reliable design guidance for ISP connections. To achieve this objective, first at the material level, the properties of the used steel-plates and dowels were verified. Then, at the component level, the performance of shear connections and hold-downs were investigated by performing quasi-static monotonic and reversed cyclic tests.
The most significant finding of the component level tests was the proof that it is possible to control the strength, stiffness, and ductility only through the IPSP and avoid bending of the SDD or crushing of the wood. Furthermore, the length of the steel perforations had a large impact on the performance with the steel-plates with the long slots (Type-D and Type-E) exhibiting lower strength and stiffness. For the hold-down tests, the same perforation geometry as for the shear-connection tests was chosen. As already determined in the shear-connection tests, the hold-down specimens with the short perforation slots resulted in the strongest and stiffest connection.
The results from this project will be used to design and test CLT shear walls with ISP connections.
The load distribution in multi-dowel timber connections under bending moments was investigated by means of an integrative evaluation of a hierarchically organized test program, which encompassed component tests as well as single-dowel and multi-dowel connection tests. It was demonstrated that the anisotropic material behaviour of Laminated Veneer Lumber, and consequently of wood in general, leads to a non-uniform load distribution among the dowels, even for multi-dowel connections with a circular arrangement of dowels. Model predictions from this study highlight inefficiencies of the simplified calculation approach, based on the polar moment of inertia, i.e., based on isotropic theory. Loads of dowels loaded parallel to the grain were found to be underestimated by up to 50%. Through the hierarchically organized experimental campaign with full-field deformation measurement techniques, load distribution effects could be related to the orthotropic material behaviour of wood expressed in terms of load-to-grain angle dependent slip curves of single-dowel connections.