8. BUILDING PHYSICS Table 8.1. Swelling and shrinkage factor αH for LVL products in % per 1% change in moisture content below fibre saturation point. Note: due to its cross band veneers, LVL-C undergoes much less dimensional change in the width direction than LVL-P (FprEN 14374:2018). Figure 8.3. Directions of LVL dimensions Table 8.2. Example of dimensional changes due to moisture : If the relative humidity changes from 50% to 85%, the moisture content of a LVL beam increases by approximately 7%. The resulting effect on beam dimensions is as follows: 8.1.3 Dimensional changes due to moisture LVL products are delivered from the factory at a moisture content that is close to the moisture content of a service class 1 end use. This is an advantage, as it significantly reduces dimensional changes due to moisture, provided that the members are protected against weather exposure during transport, storage and construction work. LVL 08, Table 8.1 Dimension LVL-P LVL-C Thickness t 0,32 0,32 Height h (or width of a panel) 0,32 0,03 Length l 0,01 0,01 LVL 08, Table 8.2 Product Direction Original dimension Dimension after +7% increase of MC Difference LVL-P or LVL-C Length l 4200 mm 4200 + (7 × 0.01/100 × 4200) = 4203 mm +3,0 mm LVL-P or LVL-C Thickness t 57 mm 57 + (7 × 0.032/100 × 57) = 58,3 mm +1,3 mm LVL-P Height h 260 mm 260 + (7 × 0.32/100 × 260) = 266 mm +6,0 mm LVL-C Height h 260 mm 260 + (7 × 0.03/100 × 260) = 260,6 mm +0,6 mm LVL products swell when their moisture content increases and shrink when their moisture content decreases. The extent of these dimensional changes depends on the grain direction. Due to the hot press production process, a proportion of the swelling in the thickness direction occurring upon first wetting of the product is permanent. However, despite the dimensional changes due to moisture, the nominal thickness of the product should be used in the structural capacity design of LVL members. Dimensional change ΔL due to variation in moisture can be calculated as follows: ∆L = ∆ω ∙ αH/100 ∙ L (8.2) where Δω is the change in product moisture content [%]; αH is the product’s dimensional variation coefficient, see Table 8.1 for values and Figure 8.3 for directions; and L is the product dimension in the corresponding direction. Notably, due to its cross band veneers LVL-C has a very low αH factor in the member width direction: only 10% of the value specified for LVL-P products. This advantage can be utilized in structures that are sensitive to dimensional changes due to moisture. LVL products can warp if the moisture content of opposite surfaces is not equal, for example if one surface is exposed to a higher relative humidity than the other. LVL-P products are more sensitive to such warping than LVL-C, especially if the height of the product is more than 8 times the thickness (h > 8t). Therefore, it is normally recommended to limit the slenderness of the LVL-P beams to this ratio. If careful moisture management of the components and structures can be assured throughout the logistic chain and construction process, e.g. in off-site element production, a h/t ratio of max ~12 may be considered. LVL Handbook Europe 175
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