# Sustainable Cross-Laminated Timber Structures in a Seismic Area: Overview and Future Trends
## 2 Cross-Laminated Timber (CLT) Panels  
CLT panel is a two-dimensional mass timber element composed of an appropriate number of layer of boards. The layers are made with adjacent boards or lamellae (usually not glued on the edges), rotated around at a right angle one each other and glued among them (Figure 2). Panel dimensions are variable as regards thickness (t), width (b) and length (l). The length can in theory be unlimited because the individual boards are finger-joined to create long boards. The number of layers is odd and vary from a minimum of three to a maximum of nine. In Table 1 are indicated the common dimensions used in building practice. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Commonplace** \\ \hline Thickness (t) & 60–300 mm \\ Width (b) & 1.20–4.80 \\ Length (l) & up to 30 m \\ No. of layers & 3–5–7–9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Range of the geometrical characteristics of cross-laminated timber (CLT) panels. Figure 2: CLT panel lay-out. The raw material consists of timber boards that have been strength graded according to standards (e.g., EN-14081-1 in Europe [27]). The cross-section of CLT usually comprises boards of the same strength in the two main direction, here indicated as \"longitudinal\" (axis of major strength and stiffness) and \"transversal\" (axis of minor strength and stiffness) in Figure 2. To make the most of the panel strength, wood with higher strength can be used in the main direction of the load. As for the majority of engineered wood products, also for CLT panels soft-wood species are mainly used, such as spruce or less frequently larch or douglasia (that ensure greater durability to the panel). Depending on geographic area, the boards arrangement can be different [11], as represented in Figure 3.  
The cross-layered morphology provides high structural properties to CLT members, that can compete with other more traditional structural materials adopted for medium and high-rise buildings. The main advantages of CLT panels regard the structural features, thermal and sustainable aspects, as summarized in the following:  
* In relation to their own-weight, CLT panels have a higher load-bearing capacity than most other construction materials: this means that high-rise buildings can be constructed with reduced masses. * The cross-layered nature provides in-plane and out-of-plane load-carrying capacity and can be used either for vertical shear-walls (membranes) or floors (plates), as well as for other structural applications (bridges, sport arenas, curved elements, etc.). * High in-plane strength and stiffness, even if differentiated between the longitudinal and transversal directions of the panel. * Good dimensional stability against moisture variations: crossed layers reduce the shrinkage and swelling phenomena with respect to other timber-based products. * Excellent energy efficiency and capacity of storing moisture and thermal energy. * Ecofriendly, recyclable and renewable construction material with long service life (when correctly protected against moisture). * Workability, slenderness and different sizes do not limit architectural design. Despite the significant use in construction practice, CLT panels are not yet codified in national or international standards. In Europe, the European Technical Assessments (ETAs) represents the main reference document that codifies the CLT products indicating their specific properties for structural applications [28]. In compliance with ETAs, CLT panels are made with boards having a strength class corresponding to C16, C24 and C30. Instead, the National Annexes to the European standards [29; 30] specify the design rules and checks, although they have not been yet included in the Eurocode 5 [12]. In Eu  
Figure 3: Number of layers and boards arrangement in CLT panels (image from [11]). rope, the revised Italian Technical Document CNR DT 2016/2018-R1 [31] contemplates the use of the CLT and indicates its main characteristics for structural purposes.