# Sustainable Cross-Laminated Timber Structures in a Seismic Area: Overview and Future Trends
## 6 Future Trends in CLT Building Practices
### New Frontiers for More Sustainable CLT Structures  
It is recognized that CLT buildings are provided with high environmental sustainability during their life cycle, with respect to other structural types (e.g., reinforced concrete). This depends, firstly, on the excellent eco-sustainability attitude of raw material and, secondly, on the attention paid by the manufacturers to produce certified environmentally sustainable materials. The sustainability of CLT buildings can be furtherly enhanced within the life cycle of the construction if the amount of material disposal and/or its recycling is reduced. Seismic damage to timber parts is one of the main causes of material disposal and recycle, and it appears fundamental to mitigate seismic damage in the connection zones. In order to reduce damage in timber panels, some alternative connection systems have been recently introduced in literature. The common goals of such systems are those of:  
* concentrating the hysteretic behaviour in metal parts (i.e., steel plates or similar devices) only;
* eliminating possibilities of damaging in timber components (both timber embedding of CLT vertical panels and orthogonal to grain crushing of horizontal ones). To satisfy these requirements, a new way of conceiving the connection zones is needed. The current hierarchy of strength criterion based on the concept of the strength of metal plates-weak timber to steel connections (i.e., timber to fasteners interaction) should be inverted as strong CLT panels-weak metal devices. To obtain this, cyclic dissipation must be concentrated in steel devices properly conceived as regards this aim. Such solutions are recognized as low-damage connections. They represent an innovative aspect in timber buildings practice, even if they arose in the United States in the 1990s for post-tensioned precast concrete structures [80] and subsequently were adapted in New Zealand to post-tensioned rocking timber structures [81; 82]. Dissipative connections proposed in New Zealand consist of fuse-type elements made with mild steel (or viscous damping) dissipaters placed at the base of the panels, replaceable after seismic damaging (reason for which are also named \"plug and play\" systems) (Figure 14). A low-damage philosophy can be also applied to the traditional CLT buildings, adopting connection systems devoted to hysteretic energy dissipation similar to those used for post-tensioned structures. The main objective is that under seismic actions the vertical timber panels remain elastic and undamaged, thus they can be reused to attach new connections after seismic damage. With respect to traditional HD and AB, the low-damage connections rely on the dissipation of steel part only, with further advantages: cyclic behaviour presents wide loops without pinching and strength and stiffness degradation is that of steel. Figure 14: Plug and play fuse-type external dissipaters (from [82]). Replaceable connections can be made with mild-steel bars, or thin steel plates nailed or screwed to timber panels having a properly designed shape. For instance, [PERSON] and [PERSON] [25] proposed a new type of hold-down called the XL-stud, where the main idea is to shift the dissipative zone of the hold-down from the stem zone to the flange plate (Figure 15a). The stem zone has to be over-strengthened by adopting a proper number of nails and by checking the resistance of stem and wood with respect to the force needed to yield the flange plate. Moreover, to maximize the plate energy dissipation, the proposal is to give an hourglass shape to the flange plate of the angle (Figure 15b). [PERSON] et al. [26] proposed another innovative dissipative connection (named X-bracket) composed of X-shaped metallic plates nailed or screwed to timber panels, used to withstand either tensile and shear forces in panel to panel, panel to foundation and wall to wall connections (Figure 16a,b respectively). Energy dissipation involves the steel plates only, while timber parts are overstrength.