1. LVL AS A CONSTRUCTION MATERIAL Table 1.10. Example dimensional changes due to a 3% increase in moisture content (MC) %. Figure 1.64. Dimensional changes due to increased moisture content. LVL 01, Table 1.11 Product type Direction Original dimension Dimension after +3% increase in MC Difference LVL-P Length l [mm] 5000 5001,5 +1,5 mm Thickness t [mm] 57 57,5 +0,5 mm Height h [mm] 260 262,6 +2,6 mm LVL-C Length l [mm] 5000 5001,5 +1,5 mm Thickness t [mm] 57 57,5 +0,5 mm Height h [mm] 260 260,3 +0,3 mm 1.12 BASIC PROPERTIES OF LVL 1.12.1 Strength and stiffness properties LVL has homogeneous material properties, firstly, due to the breakdown and uniform distribution of natural defects, such as knots, in the product and, secondly, due to the effect of lamination, which further eliminates their impact. Strength grading of the veneers also reduces variation within each strength class of the product. This results in strength levels that are close to defect-free wood for the highest LVL grades and, due to low variation, the characteristic 5% fractal values used in structural design are also high. LVL-P has the highest strength and stiffness properties parallel to grain. LVL-C has about 20% lower values parallel to grain due to its cross-bonded veneers, but is stronger and stiffer perpendicular to the grain direction of the surface veneer, properties which can be utilized in panel structures. Table 1.11 presents the basic mechanical properties of the typical strength classes of LVL. The variation in bending strength and stiffness properties for LVL is typically less than 10% compared to 12-20% for glulam and plywood and 15-30% for structural timber. Therefore, the characteristic 5% fractile values of non-LVL materials for structural design are significantly lower 16. Table 1.12 compares the basic mechanical properties of some common structural wood products. For more information on the mechanical properties of LVL, see Section 4.2. 1.12.2 Building physics properties Moisture LVL products are delivered from the factory at a moisture content (MC) of 8-10%, which is close to the MC of service class 1 end uses. This significantly reduces initial dimensional changes due to moisture in structures if the members are protected against weather exposure. LVL swells when its moisture content increases and shrinks when its moisture content decreases 18. The extent of these dimensional changes depends on the grain direction and the product type. Table 1.10 shows an example dimensional change for a 3% increase in moisture content. LVL-C undergoes a much smaller change in beam height because the cross veneers efficiently prevent movement in the height direction. Untreated wood surfaces are hygroscopic, meaning that they absorb moisture from humid air and release moisture to the surrounding air when the RH is low. This moisture buffering phenomenon may be useful for improving the indoor air quality of buildings. Thermal properties LVL has a thermal conductivity λ of about 0,13 W/mK depending on its density and moisture content, and a specific heat capacity cp of 1600 J/(kg K) according to EN ISO 10456. Thermal expansion of LVL is negligible and its dimensions remain stable during temperature changes. Therefore, temperature variation does not need to be considered in structural design, unlike swelling and shrinkage due to moisture changes. For further information on building physics, see Section 8. 44 LVL Handbook Europe
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