4. STRUCTURAL DESIGN OF LVL STRUCTURES It is recommended to have the tapered edge on the compressive side, especially for LVL-P, since the tension perpendicular to grain strength ft,90,edge,k is low, which can lead to cracks and brittle failure. LVL-C may be used for special shapes, also when the tapered edge is on the tensile side, as its ft,90,edge,k is higher due to the cross veneers and it behaves more ductile. Figure 4.22 shows the km,α factors as a function of the angle α. For high pitched roof beams (α ≥ ~10°) the maximum shear stress τv,max,d and tension perpendicular to the grain σ90,max,d shall be calculated at the point of the maximum bending moment stress with the equations: τ_(v,max,d)=σ_(m,0,max,d)∙tanα (4.55)30 σ_(90,max,d)=σ_(m,0,max,d)∙tan^2 α (4.56)30 Figure 4.23. Stress distributions in single and double-tapered beams. When the angle between loading and the grain is large (α ≥ 10°), shear stress at the point of maximum bending moment stress may become more critical than the shear stress at the support 30. Figure 4.24. Stresses at the tapered edge of a beam: bending stress σm,α at the direction of the edge, bending stress at the grain direction σ0, shear stress τ = σ0∙tanα and stress perpendicular to the grain σ90 = σ0 ∙tan2α 30. For double-tapered, curved and pitched camber beams design instruction are given in Eurocode 5 clause 6.4.3. Additional information to the clause: • Factor kr is 1,0 for LVL in the edgewise direction, as the shape of the beam is cut directly from a panel and no reduction due to bending of the laminates during production is needed. • km,α is not used together with the equations for checking the stresses at the apex point. • It is not necessary to take kl into consideration in the resistance against lateral torsional buckling of the beam (4.38). v,max,d = m,0,max,d ∙ tan (4.55)30 90,max,d = m,0,max,d ∙ tan2 (4.56)30 v,max,d = m,0,max,d ∙ tan (4.55)30 90,max,d = m,0,max,d ∙ tan2 (4.56)30 LVL Handbook Europe 133
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