4. STRUCTURAL DESIGN OF LVL STRUCTURES 4.3.13 Serviceability limit state design: Deflections Instantaneous deflection of a member is calculated using the actions in equation (4.1). In serviceability limit state (SLS) the partial safety factors γG and γQ of loads are 1,0. The following serviceability limit state principles are defined in Eurocode 5, clause 2.2.3: The deformation of a structure resulting from the effects of actions (such as axial and shear forces, bending moments and joint slip) and from moisture shall remain within appropriate limits, having regard to the possibility of damage to surfacing materials, ceilings, floors, partitions and finishes, and to the functional needs as well as any appearance requirements. Instantaneous deformation, uinst, see Figure 4.27, should be calculated for the characteristic combination of actions, see EN 1990:2002, clause 6.5.3(2) a), using mean values of the appropriate moduli of elasticity, shear moduli and slip moduli. Final deformation ufin, see e.g. wfin in Figure 4.27, should be calculated by superimposing the creep deformation ucreep calculated using the quasi-permanent combination of actions, see EN 1990:2002, 6.5.3(2)(c), onto the instantaneous deformation uinst calculated from 2.2.3(2). The creep deformation should be calculated using mean values of the appropriate moduli of elasticity, shear moduli and slip moduli and the relevant values of kdef given in Table 4.3. Note: When calculating the creep deflection of LVL-C in the edgewise direction, the kdef value is similar to LVL-P. In flatwise direction, the kdef is larger due to the rolling shear deformation of the cross veneers, similar to plywood, see Table 4.3 of kdef values. If the structure consists of members or components having different creep behaviour, the long-term deformation due to the quasi-permanent combination of actions should be calculated using the final mean values of the appropriate moduli of elasticity, shear moduli and slip moduli according to Eurocode 5, clause 2.3.2.2 (1). The final deformation ufin is then calculated by superimposing the instantaneous deformation, due to the difference between the characteristic and the quasi-permanent combination of actions on the long-term deformation. For structures consisting of members, components and connections with the same creep behaviour and under the assumption of a linear relationship between the actions and the corresponding deformations, as a simplification of EN 1990:2002, 6.5.3(2)(c), the final deformation, ufin, may be taken as: u_fin=u_(fin,G)+u_(fi (4.72) (EC5 2.2) where ufin,G=uinst,G (1+kdef ) for a permanent action G; ufin,Q1=uinst,Q1 (1+ψ2,1 kdef ) for the leading variable action, Q1; ufin,Qi=uinst,Qi (ψ0,i+ψ2,i kdef ) for accompanying variable actions, Qi (i>1); uinst,G , uinst,Q1 , uinst,Q,i are the instantaneous deformations for actions G,Q1,Qi respectively; ψ2,1 , ψ2,i are the factors for the quasiperma- nent value of variable actions; ψ0,i are the factors for the combination value of variable actions; and kdef is the creep deformation factor for timber and wood-based materials. The components of deflection resulting from a combination of actions are shown in Figure 4.27, where the symbols are defined as follows: − winst is the instantaneous deflection; − wcreep is the creep deflection; and − wfin is the final deflection. Note: LVL is not pre-cambered. Only in some very special cases LVL beams may be cut to a camber by special sawing from an LVL billet. The net deflection below a straight line between the supports, wnet,fin, should be taken as: wnet,fin = winst + wcreep (4.73) Note: The recommended range of limiting values of deflections for beams with span l is given in Table 4.10 depending upon the level of deformation deemed to be acceptable. Information on national limit values can be found in the National Annex for Eurocode 5. Figure 4.27 Components of deflection of LVL members. fin = fin,G + fin,Q1 +∑ fin,Q1 (4.72) net, fin = inst + creep =5 ∙ d,i,SLS ∙ 4 384 ∙ mean ∙ + ∙ d,i,SLS ∙ 2 8 ∙ mean ∙ = d,i,SLS ∙ 3 48 ∙ mean ∙ + ∙ d,i,SLS ∙ 2 4 ∙ mean ∙ 136 LVL Handbook Europe
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