4. STRUCTURAL DESIGN OF LVL STRUCTURES Figure 4.2. Examples of service class 1 (SC1), 2 (SC2) and 3 (SC3) conditions in wooden structures. Note: Assignment of structures to service classes are given in the National Annex to Eurocode 5 for each country. 4.1.3 Load duration In the Eurocodes, load-duration classes are characterized by the effect of a constant load acting for a certain period of time in the life of the structure. For a variable action, the appropriate class is determined on the basis of an estimate of the typical variation of the load with time, see Table 4.1. 4.1.4 Service classes Moisture content and changing humidity conditions have a significant influence on the strength and stiffness properties of LVL as well as on other wood-based materials. In Eurocode 5 this is taken into account by defining 3 service classes: • Service class 1 (SC1) is characterized by a moisture content of the materials corresponding to a temperature of 20 °C and the relative humidity of the surrounding air only exceeding 65% for a few weeks per year. This corresponds typically to heated indoor air conditions. In service class 1 the average moisture content (MC) of softwood LVL is usually between 6 and 10%. The MC of most solid woods is in those conditions typically a couple of % higher, but will not exceed 12%. The manufacturing process of LVL keeps the product dryer because the veneers are dried at high temperature to less than 5% MC, which alters the wood cell structure making the material less moisture absorbent. • Service class 2 (SC2) is characterized by a moisture content of the materials corresponding to a temperature of 20 °C and the relative humidity of the surrounding air only exceeding 85% for a few weeks per year. This corresponds typically to ventilated outdoor conditions under a roof protecting from direct weather exposure. In service class 2 the average moisture content of softwood LVL is usually between 10 and 16%. The MC of most softwoods will not exceed 20% in SC2. • Service class 3 (SC3) is characterized by climatic conditions leading to higher moisture contents than in service class 2. This corresponds typically to conditions where the structures are under direct weather exposure, high humidity or direct contact with water. LVL cannot be used in service class 3 without preservative treatment against decay. Standard EN 335 defines use classes with respect to durability. These correspond to the service classes of Eurocode 5 with the exception of service class 3, which is divided into sub-classes UC3.1, 3.2, 4 and 5, which describe the conditions more precisely. When choosing the service class for the design, in addition to the moisture content of the wood material, special attention must be paid to cyclic changes in the moisture conditions, which may have a more significant effect on timber constructions than higher constant humidity conditions. In service class 1 special attention must be paid to the risk of cracking of timber structures. 4.1.5 Partial safety factor of material γM and modification factors kmod and kdef The partial safety factor γM of different materials are defined in the National annex of Eurocode 5. For LVL it is typically γM =1,2 - 1.3. kmod is a modification factor taking into account the effect of the duration of load and moisture content. In Eurocode 5 the same values of kmod are used for all mechanical properties of LVL in service class 1 and 2. However, experimental studies have shown that the compression strength fc,0,k is lower in SC2 than in SC1. This is taken into consideration in the strength classes by defining a 20% lower strength value of fc,0,k for SC2 15. If a load combination consists of actions belonging to different load-duration classes a kmod value that corresponds to the action with the shortest duration should be chosen. For example, for a combination of dead load and a short-term load, a value of kmod corresponding to the short-term load should be used. In serviceability limit state design the influence of creep is dependent on the service class and the duration class of the loads. ψ0 and ψ2 factors for the quasi-permanent value of the action take this into account in load combinations. The deformation factors kdef take into account the service classes. For characteristic combination of actions, the final deflection including creep is calculated using equation (4.2). LVL Handbook Europe 115
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