Seismic design of timber structures - A proposal for the revision of Chapter 8 of Eurocode 8

Sustainability and energy efficiency, together with reductions in construction times and costs are becoming fundamental issues in the housing policies of most of the European and other International Countries. From all these point of views, timber building systems represent a winning choice, due to: (i) the capacity of storing carbon dioxide by displacing it from the atmosphere and reducing energy consumption during the production, transportation and erection compared to other construction materials, (ii) the good thermal properties of wood which, together with insulation materials, easily lead to reach excellent rates in the energy performance of the overall building, and (iii) the great speed of construction due to a completely dry construction process. These advantages, together with the excellent seismic performance witnessed by recent research results based on extensive numerical simulations and full-scale tests on multi-storey buildings, explains the reason why timber systems are becoming more and more popular in the construction of medium to high rise buildings in different seismic areas of Europe, replacing day by day other materials like reinforced concrete, masonry or steel buildings. However this progress and the growing diffusion have not been accompanied by a corresponding update of the provisions to be used in the seismic design, which for the case of timber buildings are included within Section 8 of Eurocode 8. This chapter was published in 2004 and is very short and incomplete in many parts, especially when considering design provisions for modern construction systems nowadays widely used, thus causing real difficulties to structural engineers who have to apply these rules in the seismic design of timber buildings. Moreover, the differences from other building materials chapters of Eurocode 8 are significant, in some case substantial, and not always conservative. The aim of this Ph.D. Thesis is to propose a comprehensive review of the current version of Section 8 of Eurocode 8 with the modification of most of the existing provisions and the implementation of some new parts, always from the point of view of a practicing engineer, with the intent to provide clear and simple rules for the seismic design of timber buildings in seismic regions. In this respect, the research work was aimed first at analyzing and investigating the evolution of the subsequent versions of this chapter and the technical-scientific background that led to the drafting of the current version. Subsequently, a detailed analysis of the different problems which arises in the application of the existing provisions is provided in this Ph.D. Thesis. The research was then directed to the analysis of specific problems related to the seismic design of multistorey timber buildings through linear and non-linear modelling and design applications performed with a widespread software package among structural engineers like SAP 2000. More specifically, buildings made with the two most used structural systems for the construction of medium to high-rise timber buildings, i.e. the CLT and the Light-Frame system, were investigated by comparing the analysis results in terms of dynamic characterization and maximum displacements with those of experimental results conducted on full-scale buildings. Additionally a topic of growing interest in the seismic design of multi-storey timber buildings, i.e. the seismic analysis of hybrid buildings with different lateral load resisting systems, was investigated for a specific application, i.e. the seismic design of multi-storey buildings with mixed CLT/Light Frame shear walls. This study was carried out through non-linear dynamic analysis conducted on a case-study four-storey building with different combinations of CLT and Light-Frame shear walls at each level conducted in order to establish the value of the seismic behaviour factor to be used in the seismic design. The analysis were conducted with DRAIN-3DX, a program for inelastic analysis of structures developed at the University of Berkeley, California, implemented with a numerical model developed at the University of Florence in order to study the non-linear behaviour of mechanical joints and shear walls in timber structures. The existing model has been further implemented with new features like strength and stiffness degradation in order to have a more precise reproduction of the actual non-linear behaviour under seismic excitations. Finally the last part of the Thesis focuses on a proposal of Background Document for the drafting of a new version of Chapter 8 of Eurocode 8 with, according to the format of other material chapters of Eurocode 8, new provisions for the seismic design of existing and new structural systems not covered by the current version. Furthermore, capacity based design rules are implemented and new values of the over-strength factors and behavior factors to be used in the seismic design according to the different ductility classes currently proposed by Eurocode 8 are provided. Some changes related to the ductility rules for dissipative zones and partial safety factors for material properties to be adopted in the design according to the dissipative and non-dissipative behaviour are also given. New provisions regarding the design of buildings with different lateral load resisting systems, the different analysis methods to be adopted in the seismic design and inter-storey drift-limits are proposed. A code proposal for the evaluation of the behaviour factor q of mixed CLT/Light-Frame buildings is finally proposed. The research work conducted and the changes of the code provisions proposed within this Ph.D. Thesis could be a sound basis for the next revision of the chapter for timber buildings of Eurocode 8. Further research is nevertheless needed in order to provide more detailed provisions about (i) non-linear static and dynamic analysis methods in order to foster their use in seismic design, (ii) the use of displacement-based design as an alternative to the force-based procedure, (iii) the seismic design of innovative low-damage structural systems and passive base isolation systems for timber buildings, and (iv) design provisions for the retrofitting of existing timber buildings for the next generation of Eurocodes.