# On the Role of Seismic Damage Tolerance on Costs and Life Cycle of CLT Buildings
## 2 Role of the Design Criteria  
Despite the fact that damage tolerance, costs and LCA together represent three fundamental parameters governing a rational design approach for CLT structures, the relationship between them is not investigated in the scientific literature and each topic is treated sepa  
Figure 2: Optimized design approach for timber buildings. rately [22; 27; 28]. In this perspective, this paper investigates the correlation between damage tolerance, costs and LCA of CLT buildings, highlighting that the relationship is mainly governed by the design criteria adopted for the (dissipative) zones in wall-to-wall and wall-to-foundation connecting systems. In the context of the capacity design methodology, the design of the connections is severely governed by the Strength Hierarchy (SH) criterion established among the components present in the connecting zones [29]. The SH criterion aims to ensure that seismic loads generate an inelastic, ductile response in selected regions of the structure given that the rest of the components behave in the elastic range withstanding the forces associated with the plastic strength of the critical regions. As a result, overall and local protection from brittle phenomena can be achieved. To date, national and international codes [9; 29; 30; 31] and guidelines [32] refer to a single type of dissipative mechanism--those required for the traditional CLT buildings--based on Johansen's failure modes. For such an approach, identified as Approach A1 in the following, the non-dissipative elements are the metallic ones (e.g., HDs and ABs), while the dissipative ones are the timber-to-steel interactions characterized by a combination of timber embedding and cyclic yielding of steel metal fasteners (Table 1). Thus, in this case, the axial strength of metallic elements HDs and ABs (N\({}_{\rm pl}\)) must be greater than the Johansen's strength (R\({}_{\rm d}\)) of the connection amplified from the overstrength factor (\(\gamma_{\rm Rd}\)). Due to the cyclic nature and intensity of earthquakes, high and permanent damage to timber panels in the connecting zones is accepted in the absence of detailed evaluations of the damage level of the timber panels. This may be the case for buildings located in high seismicity regions, where high levels of ground shaking may even appear at the Damage Limit States, and high demand at the connection level may occur to activate the overall ductility required to survive at the Life Safety and Collapse limit states [33]. As a consequence, a relevant impact on the costs related to demolition and/or reconstruction of structural and non-structural components, recycling of materials and sustainability of the construction seems to be intrinsically accepted. As discussed above, alternative damage-tolerant connections can represent an effective solution to reduce costs related to damage to structural components after an earthquake and to improve the sustainability of CLT buildings, although specific HS rules are still not included in the current standards. The seismic dissipation is concentrated into sacrificial (plug and play) metallic elements which work in tension or shear (bars, steel plates, etc. ), while vertical timber panels are non-dissipative and behave elastically. As an advantage, timber components are preserved from plastic damage after the seismic event and no costs related to demolition and/or reconstruction or recycling arise. For this approach, named Approach A2 in Table 1, the HS requires that the Johansen's strength (R\({}_{\rm d}\)) of the connection must be greater than the axial strength of metallic elements HDs and ABs (N\({}_{\rm pl}\)) amplified from the overstrength factor (\(\gamma_{\rm Rd}\)). Nevertheless, in [PERSON] et al. 2021 [29], different structural behaviors achieved by CLT walls designed according to Approaches A1 and A2 have been compared.