## 4 Design for Durability  
According to Eurocode 0 [23], \"The durability of a structure is its ability to remain suitable for use during its useful life, provided it is adequately maintained\". The UNI EN 1001-2 [24] standard defines the durability of wood and wood-based products as the \"resistance of the material to degradation induced by wood-borne organisms\", including molds and fungi that cause its degradation by attacking the material under certain humidity conditions. Eurocode 5 [12] defines three important service classes in order to assign the resistance and deformation values of the structural components, and therefore, of timber constructions in general. With the aim of evaluating the moisture content, which is particularly important for biological durability, the European Standard EN 335 [25] also defines five use classes, i.e., five environmental conditions in which the wood can be found. The concept of use class is related to the probability that a wooden element is attacked by biological agents, and mainly depends on its moisture content, that should not exceed 20%. The two classification systems differ in their consideration of the effects of humidity on the wood, i.e., of the mechanical type and the biological type, respectively. It should be noted that the classes of service and use are not performance classes; therefore, no indications are given on how long the wooden structures will remain in service. Similarly, the codes do not furnish any precise method by which to assess durability. In addition to the aforementioned codes, the European Standard EN 350-2 [26] only provides information about the natural durability of the wood species existing in Europe, while the European Standard EN 460: 1995 [27] indicates the requirements for wood corresponding to each use class. Within this framework, in this study, the following approach is suggested to properly account for durability, and so, improve the bridge service life: (1) definition of the design durability; (2) individuation of the use class according to [25; 26]; (3) choice of a design strategy, including protection measures and maintenance plan; (4) verification of durability on the basis of the rules proposed in [28]. The latter can be summarized in a set of design principles called the 4 Ds: deflection, drainage, drying and durable materials [28]. Deflection concerns rain penetration control and includes all devices that minimize rainwater loads on the construction envelope, while drainage, drying and durable materials are concepts dealing with the management of water after it has reached the envelope. The above strategy was applied to the pedestrian bridge described in this study. Durability was chosen based on the service life of the bridge, assumed equal to 50 years. Successively, focusing on the environmental context where the construction is planned, the third use class was set, corresponding to a moisture content \(>\) 20%, and so, to wood elements which are vulnerable to the attacks from fungi and insects. The consequential choice of the service class depended not so much on the design environmental conditions, i.e., the structural components in laminated wood which are protected by the roof, but from the predictable conditions that could involve structural elements if the \"water barrier\", created by the lining vertical wooden staves, were lost. Furthermore, the Gravina stream, which flows at about 10 m below the bridge, although often dry, could lead to a moisture content \(>\) 85%. For these reasons, it was decided to design the bridge according to service class 3. On the basis of the above observations, the larch was chosen as the wood species to be used for the bridge structure. Focusing on the design strategy for wood protection, it was fundamental to work on the cross section of the bridge, trying to apply the so-called 4D rule, known in the design of wooden buildings in the American context, but more recently also applied in Europe [28].