Issue 31

C.L. dos Santos et alii, Frattura ed Integrità Strutturale, 31 (2015) 23-37; DOI: 10.3221/IGF-ESIS.31.03 24 connections applied in timber structures. The singularity of this type of timber connections is associated to the combination of very distinct materials – wood and steel – and to the high anisotropy of wood. The knowledge of the mechanical behaviour of these dowel-type connections (e.g. load–slip relation, stress distribution, ultimate strength and failure modes) is of primordial importance for their rational application. This complex behaviour is governed by several geometric, material and load parameters (e.g. wood species, dowel diameter, end and edge distances, space between connectors, number of connectors, hole/dowel clearance, friction and load configuration). According to design codes of current practice [2, 3], the design of dowel-type timber connections has been based on the European Yield Model (EYM) proposed by Johansen [4]. According to the EYM, the embedding strength of wood is a material parameter governing the failure of wood members. This model has an empirical basis and assumes an elastic- perfectly plastic behaviour for both wood and dowel. It also considers that embedding strength is a material property, when in fact it is a combination of several geometric and material parameters. The EYM only predicts the ultimate loads associated with ductile failure modes; brittle failure modes (e.g., shearing out, splitting perpendicular to grain) are not foreseen [5]. Because the EYM does not allow the simulation of brittle failure modes, design codes prescribe empiric minimum dimensions for connections (e.g., dowel holes to end member distances) to avoid the brittle failure modes. In addition to the minimum distances suggested by EC5 [3], the fastener slenderness may be controlled to allow ductile failure modes. Generally, in order to verify the influence of parameters governing the mechanical behaviour of connections, a number of tests are required for assessing the embedding strength. These embedding tests are standardized such as in the EN383 standard [6]. Alternatively to the EYM, 2D models have been proposed: non-linear beam on elastic foundation models [5, 7] and finite element (FE) models [5, 8-10]. However, these models do not predict the brittle failure modes. The Finite Element Method has been used to simulate the non-linear behaviour of dowel-type wood connections involving different strategies, namely using constitutive models and failure criteria for specific failure modes [8-14], or using Linear Elastic Fracture Mechanics [15-17]. This paper presents experimental and numerical results from monotonic quasi-static compression tests of a double-shear single dowel wood connection, made of Pinus pinaster wood, which is one of the species with large implantation in Portugal. Despite the abundance of this raw material, its use for structural applications has been disregarded due to several reasons, such as cultural and lack of data about the behaviour of this material. The experimental program included tests of single-doweled T-connections. The T-connection consists of three wood members: two simply supported side members loaded along the perpendicular-to-grain direction and a centre member loaded in compression along the parallel-to-grain direction, according to the recommendations of the EN26891 standard [18]. The load–slip behaviour of the joint is determined until failure, including the characterization of the ultimate loads and the ductility. The connection configuration originated the occurrence of both ductile and brittle failure modes. For this reason, the numerical modelling included a plasticity model, to simulate the ductile behaviour observed essentially in the centre member, and a cohesive damage model implemented along with contact finite elements, to simulate the splitting occurred on side members. A three dimensional FE model of the wood connection is built using the commercial FE analysis code, ANSYS ® [19]. The dowel is modelled as isotropic elastic material; wood is considered as orthotropic elastic-plastic material, following the Hill’s criterion, in a similar approach applied by Moses and Prion [11-13]. Cohesive elements were added to the side members at locations where brittle failures are likely to occur. The interaction between dowel and wood members is simulated using standard contact finite elements, namely surface-to-surface contact elements available in the ANSYS ® . This approach involving mixed plasticity and cohesive damage modelling, is similar to the one followed by Kaliske and Resch [20, 21]. It should be noted that there is already experience documented in the literature about the fracture modelling of pine wood (the same species adopted in this study), using exclusively cohesive damage models [22, 23], but assuming the material in the elastic domain. The non-linear behaviour of the connection is evaluated and compared with the experimental data, allowing the calibration of the proposed models. E XPERIMENTAL RESULTS AND ANALYSIS he experimental program consisted of a series (10 tests) of a single-doweled T-connection. Each specimen comprises three wood members: a centre member loaded in compression along the parallel-to-grain direction and two simply supported side members loaded along the perpendicular-to-grain direction (see Fig. 1 and 2). The supports consisted of steel pipes with an inside diameter of 28 mm and an outside diameter of 34 mm. The distance T