6. PERFORMANCE OF LVL IN FIRE 6.5 FIRE RESISTANCE DESIGN OF FLOOR & WALL STRUCTURES WITH CAVITIES The fire resistance of LVL wall and floor assemblies can be calculated according to Annexes C and D of EN1995-1-2 and its National Annexes. When the cavities between the joists or studs are filled with rock fibre insulation, Annex C shall be used. For void cavities Annex D shall be used. In the case of filled cavities, the design process according to EN1995-1-2 Annex C is the following: 1. Verification that the structure fulfils the boundary conditions of the design method, note that it is valid for max 60 min fire resistance. 2. Protection time tch for the start of charring and tf for failure of the panels are evaluated for the protection panel combination based on the equations in Annex C, and technical specifications of the panel suppliers. It is essential to use detailing that keeps the protective rock fibre insulation in place also after failure of the protection panels. One example for floor structure is fixing wooden battens to the joists to create a shelf to support the insulation, see Figure 6.11. 3. Residual cross section of the studs or joist is calculated with the assumption that they char only from the edge of the fire exposed side. Notional charring rate is calculated with adjustment factors which depend on the phase of the charring and the thickness of the joist or stud. 4. The resistance of the residual cross section is calculated with strength values fd,fi, see equation (6.2). The reduction factor kmod,fi values are defined in section C.3 of the Annex C. The reduction depends on the member depth and the notional charring depth and the kmod,fi has a significant influence on the resistance. In the case of wall structures, the same value of kmod,fi may be used both in the width and in the thickness direction of a stud. In the case of void cavities, the design process according to EN1995-1-2 Annex D is the following: 1. Protection time tch for the start of charring and tf for failure of the panels are evaluated for the protection panel combination based on the equations in Annex D, and technical specifications of the panel suppliers. 2. Residual cross section of the studs or joist is calculated with the assumption that they are charring from the sides and the edge of the fire exposed side. Charring depth is calculated with adjustment factors which depend on the phase of the charring, similarly to beams and columns that are initially protected from fire exposure in subsection 3.4.3.2. of EN1995-1-2. 3. The resistance is calculated for an effective cross section according to subsection 4.2.2 of EN1995-1-2 with strength values fd,fi, see equation (6.2). The resistance to fire of an LVL joist floor is mainly dependent on the properties of the protective panels. Since the thickness of LVL joists is usually 45–57 mm, their resistance reduces rapidly when their sides are exposed to fire. Fire resistance of rib slabs and box slabs may be calculated with the same principles, but manufacturer-specific design instructions according to their ETAs shall be followed for, e.g., the conditions when the structural gluing may be utilized. Figure 6.11. LVL joist floor structure with rock fibre filled cavities. Wooden battens fixed to the joists create a shelf that keeps the rock fibre insulation in place protecting the sides of the joists from charring. 166 LVL Handbook Europe
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