4. STRUCTURAL DESIGN OF LVL STRUCTURES Table 4.1. Load-duration classes and examples of loads 31. in load is defined by multiplying the characteristic value of a load Qk by factor ψ0, ψ1 or ψ2 depending on the case: • Characteristic combination (ψ0Qk) is used for verification of ultimate limit states and for irreversible (permanent) deformations of a structure in serviceability limit states • Frequent combination (ψ1Qk) is used for verification of ultimate limit states involving accidental actions and for verification of reversible deformations of a structure in serviceability limit states • Quasi-permanent combination (ψ2Qk) is used for verification of ultimate limit states involving accidental actions and for verification of reversible serviceability limit states. Quasi-permanent values are also used for the calculation of longterm effects. Factor ψ2 can be concluded as a factor that converts short-term loads to permanent loads which have a similar long-term influence in the calculation of creep deformations. Loads are defined in EN 1991 and the load combinations in EN 1990. The rules in these standards define how permanent actions and variable actions shall be taken into consideration in load combinations. The general equation of load combination in the ultimate limit state is: Ed=∑j≥1γG,j ∙ Gk,j + γQ,1Q k,1 +∑i≥1γQ,i ∙ ψ0,i ∙ Qk,i (4.1) where γG,j = partial safety factor for permanent actions j; Gk,j = characteristic value of permanent load j; γQ,1 = partial safety factor for decisive variable action 1; Qk,1 = characteristic value of decisive variable load 1; γQ,i = partial safety factor for variable action i; Qk,i = characteristic value of variable load i; and ψ0,i = reduction factor in load combination for variable action i. The values of γG and γQ are set in national annexes, but in common ULS design cases for unfavourable actions γG = 1,15 - 1,35 and γQ = 1,5 - 1,6. In SLS design γG and γQ are 1,0. 4.1.2 Consequence class, reliability class and factor KFI For the purpose of reliability differentiation, consequences classes (CC1-CC3) may be established by considering the consequences of a failure or malfunction of the structure. Class CC1 is used for low consequence for loss of human life or when economic, social or environmental consequences are small or negligible. For example, agricultural buildings and storage houses may belong to class CC1. CC2 is a normal class with a medium consequence level and is used as a default class for residential and office buildings. CC3 is used for buildings where the consequences of failure are high such as concert halls or similar monumental structures. The requirements for different consequence classes are set in the associated reliability classes RC1-RC3. They include requirements for the level of reliability index β, the supervision of design and execution of the structures and resistance properties of materials and products. In partial safety factor design the reliability classes are taken into account by the KFI factor for actions. The values of KFI are given in national annexes, but according to the default values of EN1990, in RC1 the actions in equation (4.1) of the ULS are multiplied by KFI = 0,9, in RC2 by KFI = 1,0 and in RC3 by KFI = 1,1. KFI is not used in SLS. Load-duration class Order of accumulated duration of characteristic load Examples of loading Notes Permanent More than 10 years • Self-weight • Permanently installed machinery • Compartment walls in some countries Long-term 6 months – 10 years • Long-term storage Medium-term 1 week – 6 months • Medium-term imposed floor load • Snow Snow in Finland Short-term Less than one week • Short-term snow • Wind • Staircase loads • Imposed point loads • Service maintenance loads on roofs Snow in several countries Wind in several countries Instantaneous • Instantaneous wind • Accidental load Wind in Finland d =∑ G,j ∙ k,j + j≥1 Q,1 k,1 +∑ Q,i ∙ 0,i ∙ k,i i≥1 (4.1) 114 LVL Handbook Europe
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