Simulation of large deformations on timber joints using 3D FEM models

Abstract

The seismic behaviour of timber structures is highly dependent on the performance of their joints. Furthermore, the performance of the joints in such an extreme event, is significantly influenced by its plastic deformation capacity. Many times this plastic deformation capacity is low due to the brittle behaviour of timber. There are, however, certain types of joints that show considerable plastic deformation capacities when slender fasteners are used as, for example, the dowel type fasteners.

Usually the prediction of the load and deformation characteristics for this type of joints is done based on the embedding tests. Most of the times, the plastic deformation capacity of the joints can be anticipated by the tests. The basis of these tests consists on a rigid fastener that is pushed against timber up to the failure or to a limit deformation of 5mm. This method is, however, expensive and takes much time, becoming worthless in many practical situations. An alternative to the tests are the numerical simulations using various types of models, as for example, 3D FEM models.

This paper presents a 3D FEM model developed to simulate the stresses and deformations that occur on such tests. The model mesh is constituted by solid eight node elements. Both materials, timber and steel, are simulated considering non-linear material behaviour. Timber was simulated assuming an orthotropic behaviour and an anisotropic yield criteria, while steel was simulated assuming isotropic behaviour and an isotropic yield criteria. The model was validated using test results available for test specimens made of Spruce. Most of the data necessary to perform the simulations was available from the tests: when it was not available, a range of possible realistic values were assumed and verified by performing extra numerical simulations. Once all these parameters were decided the numerical results, load, deformation and stress distribution were compared with test results. The developed FEM model was also used to perform a parametric analysis considering two of the most relevant properties in terms of timber behaviour: yielding strength and modulus of elasticity. The results are presented and discussed in this paper. From that discussion it is concluded that the model is generally able to simulate the behaviour of timber in embedment tests.