4. STRUCTURAL DESIGN OF LVL STRUCTURES 4.4.3 Multiple-glued GLVL panels In GLVL-C panels the mechanical properties are declared as homogenized values for the cross section assuming the cross veneers as zero layers. The cross layers are thin and a gamma method of effective cross section is not needed for multiple-glued GLVL-C panels when the LVL-C layers are all in the same direction. They can be designed based on the mechanical values of the LVL components with the addition that a size effect shall be taken into account for the flatwise bending strength and flatwise shear strengths. The reference sizes and size effect parameters s, sflat,m and sflat,v are declared in manufacturers’ technical documentation. 4.4.4 Rib slabs and box slabs LVL rib slab and box slab elements have LVL-P ribs and LVL-C panels that distribute the loads to the ribs and work as thin flanges of the composite cross sections. Structural design is made separately for each rib section. Their specific design instructions are given in Eurocode 5, clause 9.1.2. The effective width bef of the flange panel that can be taken into account in the calculations is limited due to shear lag and due to plate buckling in ULS design on the compression side according to Table 4.11. In ULS design the following verifications are required: • The mean axial compression stress σf,c,d and tension stress σf,c,d shall be smaller than ff,c,d and ff,c,d strengths respectively. Since the flange panels are thin, it is not required to verify the resistance against tension and compression stresses at the extreme fibres of the flanges. • Resistance against shear stress for the ribs at the neutral axis and with the equation (9.14) of Eurocode 5 for the glued joints between the ribs and flange panels. The critical material property is typically the flatwise shear strength fv,flat,0,d of the flanges due to the rolling shear of the cross veneers in LVL-C. • Resistance against bending and shear stress of the flange panels in the perpendicular direction of the slab Figure 4.32. Multiple-glued GLVL panels. Table 4.11. Maximum effective width bef of flange LVL-C panel parallel to the ribs (Modified from EC5 Table 9.1). Figure 4.33. Definitions of the parts of rib slab part (EC5, Figure 9.2). I-section is used for the intermediate rib and C-section for the edge rib of the element. In SLS design, bending and shear deformations are taken into account. For simplicity, the shear rigidity GA may be evaluated based on the ribs only. In that case, the kdef factor of LVL-P can also be used for the LVL-C flange panels in the flatwise direction in the calculation of final deformation, as the thin flanges are mainly axially loaded. Rib slab suppliers also have more advanced design instructions tailored and approved for their elements as part of their technical documentation. LVL 04, Table 4.11 Shear lag Plate buckling in ULS design I-section bef = bw + 0,1 ∙ l bef = bw + 20 ∙ hf,c C-section bef = bw + 0,05∙ l bef = bw + 10 ∙ hf,c Where bw is rib thickness, l is span length and hf,c is the thickness of the flange panel on the compression side. LVL Handbook Europe 141
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