# Seismic Design of Timber Buildings: Highlighted Challenges and Future Trends
## 3 Timber Buildings--Future Trends and Challenges in Seismic Design
### Hybrid Systems with Timber  
Combining timber with other materials, to achieve composite action, enables the strength of timber to be enhanced and its shortcomings to be strengthened or eliminated. Timber is most commonly combined with concrete and steel, but recently also with glass and polymers. Pre-stressed timber buildings [82], timber-steel hybrid systems [83; 84] and timber-concrete hybrid systems [33] are already in use and are proven as earthquake-resistant structural systems. Nevertheless, due to tendency towards taller and larger timber construction, and consequently higher structural performance demand, these topics continue to be furtherly explored in terms of experimental, numerical and analytical studies [36; 85; 86]. In this paper, hybrid systems with structural glass will be briefly addressed. While timber-concrete and timber-steel hybrid structural systems are already widely applied to mid-rise construction in practice, the timber-glass hybrid structural system is still emerging. In this section, current state-of-the-art of this hybrid structural system is presented and key open questions and challenges for its future possible launch to the construction market are discussed. Timber and glass composite systems are lately intensely investigated due to extremely high-aesthetic and -ecological value in addition to their cost-effectiveness and the possibility of significant load transfer [40]. Nevertheless, design models and current European standards include the usage of glass panels as the secondary elements [42], which means the positive impact of these elements when transferring transverse loads caused by the earthquake have to be ignored [39]. According to the EC8, it is necessary to calculate the primary structural elements within the allowed displacements regarding the protection of the secondary elements. If the problem is approached as defined in Eurocode 5 (EC5) [87], timber wall diaphragms shall be designed to resist both horizontal and vertical actions imposed upon them, and shall be adequately restrained to avoid overturning and sliding. Racking resistance is provided by in-plane plane stiffness of board materials, diagonal bracing or moment connections. Method A from EC5 defines that shear diaphragms with windows and doors do not contribute to stability of the structure. Method B is less restrictive and it proposes to regard panel parts from each side of the opening as separate panels. Since the openings decrease the racking resistance and significantly reduce the horizontal stiffness of precast elements, glass helps to enhance these properties. In recent years several articles on quasi-static and dynamic tests of timber-structural glass composite systems were published [39; 40; 88; 89; 90; 91; 92; 93; 94] (Figure 4). The researchers have concluded that timber-structural glass load-bearing systems can be used in various construction applications, depending on the required bearing or ductility levels. [PERSON] et al. [95] investigated deformation capacity, lateral strength, stiffness and strength deterioration and energy dissipation capacity in order to provide data for the future development of computational models and design guidance for the new codes. Nevertheless, it is possible to devise a combined system with timber and structural glass in which each material could transfer load, and in mutual interaction of constitutive elements could be resistant to earthquake. Several extensive tests of composite systems timber-structural glass have been conducted with various types of bonding timber and glass. Bonding glass on timber proved as a good example for accomplishing high-load-bearing of composites, but deficiencies are noticed in the level of ductility along with possible problems with the durability of the structure. Further research on timber-glass composites is needed to get the structural response in seismic actions. In addition, factor q (force reduction factor) of structures constructed with hybrid laminated glass-CLT structural panels need be derived.