2. LVL STRUCTURES IN FLOORS, WALLS, ROOFS AND IN SPECIAL APPLICATIONS LVL joist floors can be built on a wooden, steel or concrete frame or walls. The 1:8 width to depth ratio of rigid LVL-P joists is ideally suited to the structural depths of floor constructions even with long spans. The recommended minimum width of supports and LVL floor joists is 45 mm in order to provide proper support for decking panels. Floor elements can be prefabricated off-site. This saves time on the construction site, reduces waste, and improves quality as the elements can be made in dry factory conditions. An element floor usually has a slightly higher material consumption as the edge joists that seal the elements at the sides form double joists in the completed floor structure. Floor deflections and vibrations are not only dependent on the floor joists themselves, but on the decking on the joists and on the blockings installed for transverse bracing perpendicular to the span of the floor. Due to the rigidity of typical decking panels, a joist spacing of c/c 400 mm is recommended. The spacing should not exceed 600 mm to avoid excessive deflection of the panels in the perpendicular direction to the joists. It is also recommended to glue the decking panels to the joists with polyurethane adhesive for best results. Depending on the gluing conditions (on-site or off-site) and country-specific requirements, at least half of the advantage of the composite effect of gluing can be utilized in structural calculations. Blockings between the joists reduce deflection under point loads, but they need to be fixed well to the joists to give the desired transverse stiffness and to avoid creaking in the long term. The best improvement can be achieved by fixing the Figure 2.4. LVL-P joist floor members and details. 1. Floor joist 2. Rim board 3. Blockings at the centre of a span 4. Blockings at support 5. Trimmer beam 6. Staircase opening, trimmer connection 7. Joist hanger connection blockings to a transverse tension flange board installed under the floor joists, see Figure 2.6. Multi-span structures reduce deflection and enable longer maximum span lengths compared to single-span structures. However, if the floor joists are continuous between separate rooms more conservative span lengths are recommended as people are more sensitive to vibrations originating from spaces external to the space that they are occupying. Floor vibration is controlled in the design based on: 1) deflection under point 1kN load when the lowest natural frequency f1 is higher than 8-9 Hz depending on country-specific design rules and 2) acceleration when f1 is 4,5-8 Hz. It is recommended to design joist floors to f1 > 8 Hz, because achieving the requirements of the acceleration criterion requires the addition of considerable extra weight to the structure. For more information on floor vibration design, see subsection 4.3.7. Residential floors within apartments can be light, without acoustic or fire resistance requirements, in which case deflection under 1kN point load is the governing requirement. Intermediate floors between apartments require protective cladding underneath the floor and extra mass on the top of the floor, which usually make the fundamental natural frequency f1 > 8-9 Hz the most critical requirement. Figures 2.7 and 2.8 gives the maximum span lengths for different joist sizes. Span lengths up to ~6 m can be achieved with normal floor thicknesses, but with the biggest joists over 8 m spans are possible. 52 LVL Handbook Europe
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