This thesis deals with the response of timber structures undergoing large deformation levels. It is motivated by the current rise in popularity of timber structures in earthquake-prone regions and contributes to a better understanding of wood’s behaviour both numerically and experimentally. Although a very good seismic performance during past earthquakes has been documented for low-rise timber buildings, the seismic reliability of mid to high-rise timber-only or hybrid timber-steel buildings has not been proved yet. For these reasons, laboratory tests and advanced numerical tools which allow an accurate prediction of the structural global nonlinear response of hybrid-timber steel connections under the action of cyclic loads are investigated throughout this thesis.
First, a detailed literature review on constitutive models for wood and other brittle materials, wood failure criteria, timber structures three-dimensional FE models, experimental studies on the cyclic behaviour of timber dowelled connections, and damage mechanics applied to the seismic performance assessment of structures, was performed . As a result the main objective of the thesis was defined to be the proposal of a material constitutive model capable of accurately simulating the nonlinear behaviour of wood under large deformation levels and load reversals until failure. The model’s conceptual development, its mathematical formulation, numerical coding and implementation in a commercial FE software is documented in this thesis. Moroever, the proposed wood material model was developed within the framework of Continuous Damage Mechanics (CDM) theory, which makes it ideal for damage evolution modelling and for performance-based seismic assessment of timber dowelled structures. Experimental studies available in the literature were employed to evaluate the accuracy and the overall performance of the proposed constitutive model at the material and at the structural (or component) levels. On the other hand, an experimental investigation of the cyclic behaviour of an hybrid timber beam-to-steel column moment connection was carried out. The hysteretic response of this connection was obtained with the objective of gaining further insight into its load-carrying capacity, energy dissipation capability, failure modes, cyclic strength and stiffness degradation, among other characteristics. Moreover, some useful modelling techniques were presented for the elaboration of a 3D continuous model of timber dowelled connections, including the formulation and implementation of a timber-steel fastener interface contact model. Additional local responses not easily identified from the experiments, like internal potential cracking and crushing zones, damage extent or plastic deformations magnitudes, were obtained from the 3D continuous model.
Finally, a simplified component-based model for the type of timber beam-to-steel column connection previously investigated was developed. The precision of this simplified model, which is inspired in the full 3D model developed herein, was assessed by comparing the numerical and experimental results. The applicability of this simplified model to the seismic performance assessment of innovative hybrid timber-steel self-centring moment connections was also demonstrated through an illustrative case study. The proposed novel plasticity-damage model captures for the first time the key features of wood cyclic nonlinear behaviour at the material and component levels, including: i) cyclic stiffness and strength degradation, ii) brittle local failure due to tensile and shear stresses, iii) ductile local failure due to compressive stress, iv) stiffness recovery after load reversal (crack opening and closing), and v) permanent plastic deformation due to compressive stress.
Overall, the ability of the proposed finite element model, including both continuous plasticity-damage and contact modelling, to simulate the behaviour of complex timber beam-to-steel column connections for different levels of deformation is demonstrated. The proposed plasticity-damage constitutive model for wood under cyclic loads relies on an efficient and reliable algorithm which is implemented in a widely-used FE software and opens the door for realistic simulations. Therefore, this thesis represents a fundamental step towards a faithful estimation of the ultimate capacity of timber structures subjected to extreme loads such as earthquakes and, as a consequence, a more reliable assessment of their performance.