4. STRUCTURAL DESIGN OF LVL STRUCTURES The shear strength of LVL is highly dependent on the direction of the shear stress. In the edgewise orientation the shear strength fv,0,edge,k is the highest. LVL-P and LVL-C strength values are quite similar at 3,2-4,5 N/mm2, but in practice LVL-C behaves more ductile (i.e. less brittle) under loading due to the cross veneer. In the flatwise orientation the shear strength fv,flat,k is smaller. For LVL-P the strength fv,0,flat,k is 2-3,2 N/mm2, but for LVL-C it is 1,1-1,3 N/mm2 due to the cross veneers which are in the rolling shear direction compared to the main direction of the panel. In the across direction of the panel the strength fv,90,flat,k is 0,6 N/mm2. Note: Multiple-glued GLVL products have manufacturer-specific rules for size effect in flatwise shear (reference size and size effect parameter sflat,v). 4.3.3 Tension parallel to the grain The following expression shall be satisfied: σ_(t,0,d)=F_(t,0,d)/A≤f_(t,0,d) (4.9) where σt,0,d is the design tensile stress along the grain; Ft,0,d is the design compressive force; A is the cross-sectional area of the member; ft,0,d is the design tensile strength along the grain. In addition to kmod and γM, for LVL the design value is dependent on the length l of the member in tension. This is taken into consideration by a factor kl which is defined as k_l=(3000/l)^(s/2)≤1,1 (4.10) (EC5 3.4) where l is the member length in tension; and s is the size effect parameter. For LVL-P and LVL-C strength classes it is 0,15, but individual manufacturer-defined values are possible. 4.3.4 Tension perpendicular to the grain The following expression shall be satisfied: σt,90,d ≤ ft,90,d (4.11) where σt,90,d is the design tensile stress perpendicular to the grain; ft,90,d is the design tensile strength perpendicular to the grain. Although in LVL-P the veneers are parallel to the main direction of the product, there is a small difference between the grain directions of the veneers. This makes the product less sensitive to cracking and the tension strength perpendicular to the grain edgewise of LVL-P ft,90,k = 0,5-0,8 N/mm2 is slightly higher than solid wood or glulam ft,90,k = 0,4-0,5 N/mm2. Figure 4.7. Tension parallel to grain of surface veneers. Figure 4.8. Edgewise and flatwise tension perpendicular to the grain of surface veneers. The cross veneers of LVL-C improve the tension strength perpendicular to the grain in edgewise direction ft,90,k = 4-5 N/mm2 is many times better than the strength of LVL-P. This property is an advantage especially in suspended connections and between main beams and secondary beams or diagonal struts. Tension strength flatwise perpendicular to the grain of LVL-P and LVL-C is low and it is not recommended to design a structure so that the stresses in this direction would become critical. The strength value in this direction is normally not defined in the DoPs for LVL products, but to have an idea of the strength level, fc,90,k,flat = ~ 0,2-0,3 N/mm2 could be used for estimations in predesign. 4.3.5 Compression parallel to the grain The following expression shall be satisfied: σ_(c,0,d)=F_(c,0,d)/A≤f_(c,0,d) (4.12) where σc,0,d is the design compression stress along the grain; Fc,0,d is the design compressive force; A is cross-sectional area of the member; and Fc,0,d is the design compression strength along the grain. For LVL strength classes a separate, 20% higher fc,0,k value has been defined for service class 1 in Table 4.5 and 4.6. The kmod factor of Eurocode 5 is similar in SC1 and SC2, but material testing has shown compression strength values to be different in SC1 and SC2. The same phenomenon is also known for other load-bearing wood products, but the strength values t,0,d = t,0,d ≤ t,0,d (4.9) l =�3000 � 2 ≤1,1 (4.10) (EC5 3.4) t,0,d = t,0,d ≤ t,0,d (4.9) l =�3000 � 2 ≤1,1 (4.10) (EC5 3.4) t,90,d ≤ t,90,d (4.11) ,0, = c,0,d ≤ c,0,d 122 LVL Handbook Europe
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