Structural performance of rounded dovetail connections: experimental and numerical investigations

Rounded Dovetail Connections (RDC) are a relatively new wood-to-wood connection concept that, despite the lack of design guidance in standards, has become popular in timber construction due to the widespread of modern milling machinery. Because of the anisotropic nature of wood and the complex stress-strain state in RDC, the question of their dimensioning is very complex. Experimental and numerical investigations were carried out on full scale RDC used to connect two timber members as joist to beam connections subjected to quasi-static shear loading. The influence of two geometric parameters was investigated: the dovetail height (varied between 109 and 189mm) and the flange angle (varied between 5 and 20°). Both, serviceability and ultimate limit states were studied using analysis of variance. It was found that the joint capacity (i) depends on the dovetail height, with an optimum of approximately 2/3 of the beam height and (ii) can be considered almost independent of the flange angle. The development and implementation of a numerical model for the design process of RDC was examined and good agreement between experimental and numerical load deformation curves validated the model, thus making it suitable for developing a method to predict RDC capacity. The paper proposes a probabilistic method to predict the capacity of RDC taking into account the scale sensitivity of the material strength, which is modelled using Weibull statistics, and considers not only the magnitude of the stress fields, but also the volume over which these stress peaks act. The proposed method has immediate actionable application for the improvement of RDC desig

Adhesively bonded lap joints of pultruded GFRP shapes

The importance of Fibre Reinforced Polymers (FRPs) as a material used for civil engineering purposes has grown in the last decade. Especially the introduction of pultrusion at an industrial level as a way to produce big batches of FRP made it possible to offer the advantages – like the high strength-to-weight ratio or the good corrosion resistance – at a reasonable cost. One issue when designing with pultruded FRPs are the connections. Up to now, connections between pultruded FRPs have been designed in the same way as structural steel connections, mainly through bolts. Because of the fibrous and layered character and the anisotropy of pultruded FRPs, bolting is not a material-adapted way to connect. Adhesive bonding is by far better suited, but has not yet been investigated for the special case of pultruded FRPs. This research is intended to fill the gap by offering designing engineers a method allowing them to dimension safe and economic adhesively bonded joints of pultruded FRPs under static loads. The present Thesis is aimed to show the steps leading to this method. After a short introduction, where the objectives and methods used are listed, the actual state of the art is presented. The review of actual literature shows that not much has been done on the special field of adhesively bonded connections of pultruded FRPs, neither experimentally nor theoretically. Some publications treat the global aspect of bonded connections for special cases like the single and double lap joints, but all on idealized mechanical systems with isotropic adherents. Also, there are no detailed reports of a mechanical failure criterion for both describing and quantifying the failure of pultruded FRPs. To overcome this, experimental investigations were carried out at different levels: the basic FRP material has been investigated in both senses of revealing the fibre architecture – with the help of burn-off tests – and the material strength – using a device the Author especially developed for this purpose: the CCLab Shear-Tensile device. This device allows the determination of the material strength subjected to combinations of out-of-plane and shear stresses. The device was also used to determine an important basic material property necessary to numerically formulate the anisotropy: the out-of-plane E-Modulus. Besides the investigations on the basic material, experiments on bonded single and double lap joints were carried out where the influence of parameters like the length of the bonded overlap, the thickness of the adherents and stress reduction methods (like chamfers of fillets) on the ultimate load was investigated. All of these experimental investigations were carried out on relatively big specimens to avoid the influence of any size effects. The experimental results showed that the adhesive layer thickness and stress reduction measures like chamfers are by far less influential that former publications expected them. For selected geometric configurations, the axial strain development along the bonded splice was experimentally gathered using strain gauges. Comparisons with FEA showed that by using the right mechanical input parameters in regard to the anisotropy, it is possible to model, with sufficient accuracy the stresses inside adhesively bonded joints of pultruded FRPs. Some single and double lap joints were filmed using a high-speed camera (up to 2000 fps) to investigate the failure process. This failure process is closely linked to the fibre architecture in the sense that it has been shown that the failure is triggered inside the laminate. The entire group of experimentally investigated specimens were then modeled with the Finite Element Method using orthotropic elements. In combination with the experimentally gathered material failure criterion, it was possible to formulate a method based on the comparison of stresses in the joint and the material resistance to predict the ultimate load of single and double lap joints, which was validated for a wide range of geometrical configurations. A simplified version of this method, based on existing analytical formulae, was then developed to make the strength prediction of adhesively bonded joints of pultruded FRP shapes available for civil engineering purposes

Experimental and numerical investigations on full-scale adhesively bonded timber trusses

Timber architecture, taking advantage of modern production techniques, is increasingly moving towards free forms; however, traditional joining techniques are not yet adapted to echo the new expression at the level of the details. This paper reports on adhesively bonded joints as a way to help architects fully unleash their creative potential. For this purpose, experimental and numerical investigations on full-scale adhesively bonded timber trusses were performed, in which adhesive bonds were compared to traditional doweled connections. The adhesively bonded trusses achieved significantly higher failure load and stiffness. Tests on small clear specimens were conducted to determine input parameters for finite element analyses. The sole timber connection was characterised, giving valuable insights into the mechanical behaviour of this truss component. At this end, the influence of the embedded length of the applied sleet plates was experimentally determined, delivering data to benchmark the subsequent dimensioning method. The trusses were then modeled and excellent agreement was found between numerical and experimental results. Finally, a dimensioning method, based on a realistic multi-axial failure criterion coupled with size effects was implemented and yielded very good agreement when with experimental results. The reported investigation demonstrates the high potential of adhesive bonding in timber structure

Capacity prediction of welded timber joints

Linear vibration welding of timber structural elements provides new opportunities to potentially achieve structural joints. This paper investigates to which extent welded joints can be considered for load-bearing structural joints. On the basis of a series of experimental and numerical investigations on a series of welded single-lap joints, failure modes were identified, and the associated failure criterion was quantified. A probabilistic method subsequently allowed accurately predicting the capacity of the tested wood welded joints exclusively based on objective input data, including an estimate of the scattering due to the material's inherent variabilit

Moment resisting connections composed of friction-welded spruce boards: experimental investigations and numerical strength prediction

Research on friction welded wood-to-wood connections has shown high potential for further development regarding the industrial application of timber construction. This paper addresses the question to which extent welding of wood can be applied to fabrication of cross-laminated timber panels (CLT). Those panels can be used as wall elements, in which load transfer occurs by transmission of moments and shear stresses. For this purpose, experimental investigations on friction welded L-shaped specimens were carried out. The welded bond between the boards, arranged perpendicularly to their fibre directions, represents points of intersection of those CLT panels. The results allowed for (a) the determination of a fracture mechanism of welded timber connections under torsion, (b) the definition of a set of design parameters influencing the resulting joint strength and (c) the comparison between the performance of welded timber joints and those of commonly glued alternatives. The experimental investigations were compared to results obtained by numerical modelling, thus allowing a probabilistic strength prediction algorithm for predicting the joint capacity as a function of the surface size

Schnellaushärtung im Holzbau. Die 5-Minuten-Verbindung

Eingeklebte Stangen haben sich im Holzbau als ein sehr leistungsfähiges Verbindungsmittel erwiesen. Allerdings bringt die aktuelle Praxis, mit der Verwendung von kalthärtenden 2K-Klebstoffen, eine Reihe von Nachteilen mit sich. Die Zeit zur vollständigen Aushärtung wird in Stunden, wenn nicht in Tagen gezählt. Außerdem erfordert sie Mindesttemperaturen auf der Baustelle, die nicht immer vorliegen.Einen Ausweg aus dieser Situation bietet die hier vorgestellte induktive Erwärmung, bei der in mehreren Schritten gezeigt werden konnte dass sie die Aushärtung in fünf Minuten gewährleisten kann. Durch die Verwendung von speziellen Partikeln, hier auf der Basis von Curie-Material, konnte gezeigt werden dass die induktive Erwärmung frei von aktiver Regelung zu tragfähigen Anschlüssen in einem reproduzierbaren Prozess durchgeführt werden kann.Bevor die induktive Erwärmung zum Stand der Technik wird müssen jedoch noch Fragen zum Einfluss der Partikel auf die Festigkeit und Dauerhaftigkeit der entsprechenden Klebverbindungen beantwortet werden, aber auch eine auf die Baupraxis zugeschnittene technischen Umsetzung erfolgen. An beiden Fragen arbeitet das Fraunhofer IFAM zurzeit intensiv

Shear loaded friction-welded crosswise arranged timber boards

Friction welding of wood is a bonding technology that can be used for joining timber elements, and in which the adhesive is formed from thermally modified cell wall material within the substrate during the welding process. In this paper, this principle is used for manufacturing prototypes of friction welded crosswise arranged timber boards, structural element bearing similitude to cross-laminated timber (CLT) tested under in-plane shear loads. In addition to the experimental investigations, three different approaches of strength prediction were performed. Beside a simply analytical method, two different probabilistic principles based on the non-local Weibull theory of brittle failure were conducted

Capacity prediction of welded timber joints

Linear vibration welding of timber structural elements provides new opportunities to potentially achieve structural joints. This paper investigates to which extent welded joints can be considered for load-bearing structural joints. On the basis of a series of experimental and numerical investigations on a series of welded single-lap joints, failure modes were identified, and the associated failure criterion was quantified. A probabilistic method subsequently allowed accurately predicting the capacity of the tested wood welded joints exclusively based on objective input data, including an estimate of the scattering due to the material’s inherent variability. Introductio