# Sustainable Cross-Laminated Timber Structures in a Seismic Area: Overview and Future Trends
## 3 Traditional CLT Buildings
### Seismic Behavior  
Despite the high number of connections, CLT buildings exhibit a favorable box-like behavior under combined seismic actions and gravity loads.  
With regards the gravity loads patterns only, particular precautions on connection systems are not necessary. Due to the platform constructive technique, the compressive forces at the interface between the vertical panels and the interposed floor panels (in the sequel referred as \"panel to panel\" contact) are transferred by contact. Such compressive forces load in orthogonal to grain direction the floor panels in which a low-strength of material is expected, because the orthogonal to grain compressive strength (\(\left\langle{{\rm{c}}_{,90}}\right\rangle\)) is about 1/10 of the parallel to grain compression strength (\(\left\langle{{\rm{f}}_{,5,0}}\right\rangle\)). This issue sets limits also with respect to the maximum height of the buildings, which should be at most 10 storey in height in order to avoid high stress levels on the floor panels. On the contrary, the panels cross-section dimensions typically used for low and mid-rise buildings are overestimated with respect to the amount of the gravity loads. It is worth underlining that, in case of  
Figure 4: Typical connections for CLT buildings constructed with platform technique.  
Figure 5: Constructional techniques for CLT buildings: (**a**) platform, (**b**) balloon.  
balloon constructive technique the issue of orthogonal to grain loading of panel floors does not arise because they do not interrupt the vertical continuity of the wall.  
Seismic actions, instead, entail a more complex load path which involves the mechanical connections in the panel to panel and the panel to foundation contact zones, as schematically represented in Figure 6. Hold-downs are devoted to face uplift and then withstand principally the tension forces due to the overturning moment, while angle-brackets contrast principally the sliding and then absorb the shear forces. While the timber-to-timber contact ensures the transferring of compression forces due to horizontal actions at the panel-to-panel or panel-to-foundation interfaces.  
A full box-like behavior of the construction is achieved if CLT floors are sufficiently rigid in own-plane. This assumption could be compromised (and then requires verifications) especially in the case of a floor realized by adjacent panels connected by screws. The relative elastic sliding which occurs among the panels in the joined zones could make the floors not adequately rigid in the horizontal plane; this produces a transferring of seismic actions among the seismic-resistant walls as a function of their tributary area, encouraging potential out-of-plane behavior of the vertical panels themselves. In any case, this phenomenon can be prevented by stiffening the floor by means of reinforced concrete slabs connected with steel fasteners to the timber panels. On the contrary, in the case of high-intensity earthquakes, the dissipation in the panel-to-panel connections of the floors could lead to many issues such as the amplification of higher modes of vibration and increased structural damage. For these reasons it is suggested to design the floors as non-ductile elements, with overstrength connections [18].