6. PERFORMANCE OF LVL IN FIRE For timber surfaces facing a void cavity in a floor or wall assembly (normally the wide sides of a stud or a joist),the following applies: • Where the fire protective cladding consists of one or two layers of gypsum plasterboard type A, wood panelling or wood-based panels, at the time of failure tf of the cladding, k0 should be taken as 0,3. Thereafter k0 should be assumed to increase linearly to 1,0 during the following 15 minutes; • Where the fire protective cladding consists of one or two layers of gypsum plasterboard type F, at the time of start of charring tch, k0 is 1. For times t < tch, linear interpolation should be applied, see Figure 6.6 (b). The effective cross section should be used for the calculation of the stiffness and fire resistance of an LVL member. Note: The effective cross section method is recommended. However, depending on the National Annex, the reduced properties method of Eurocode 5 may also be used. 3. Determination of design values of strength and stiffness For the calculation of the design values of mechanical resistance Rd,t,fi in a fire situation, the design values of strength properties shall be determined by the equation: f_(d,fi)=k_(mod,fi) f_20/γ_(M,fi) (6.2) (EC5 2.1) where fd,fi is the design strength in fire; f20 is the 20 % fractile of a strength property at normal temperature. It can be calculated as f20 = kfi ∙ fk. For LVL kfi is 1,1, so f20 is 1,1 times the characteristic strength fk; kmod,fi is the modification factor for fire. It replaces the modification factor for normal temperature design kmod given in EN 1995-1-1. kmod,fi is 1,0 in most cases, except when the method of annex C of EN 1995-1-2 is used; and γM,fi is the partial safety factor for timber in fire. The recommended factor for material properties in fire is γM,fi = 1,0. Information on national choice may be found in the national annex. For example, the design value of bending strength for LVL36 C: f_(m,d,fi)=k_(mod,fi)∙(k_fi∙f_(m,k))/γ_(M,fi) =1,0∙(1,1∙36 N/ mm^2 )/1,0=39,6 N/mm^2 For stability calculations, the characteristic values of stiffness properties at normal temperature are used. 4. Determination of the design values of actions The design effect of actions Ed,fi for the fire situation is determined in accordance with EN 1991-1-2:2002, including the effects of thermal expansions and deformations. In typical cases of timber structures where the own weight is relatively low, the design values of actions Ed,fi are 0,2-0,4 times the actions Ed in normal temperature design. 5. Verification that design resistance is larger than design action It shall be verified for the required duration of fire exposure t that: Ed,fi ≤ Rd,t,fi According to Eurocode 5, clause 4.3 Simplified rules for analysis of structural members and components, compression perpendicular to the grain and shear resistance may be disregarded. 6.4.2 Charring rates of LVL There are two different types of charring rates β0 and βn. For panels and wide cross sections one-dimensional charring rate β0 is used in the calculations. This is also used as the basis value in some more advanced calculation methods. When the characteristic density of LVL is ρk ≥ 480 kg/m3, the one-dimensional charring rate β0 is 0,65 mm/min. The design charring depth for one-dimensional charring dchar,0 [mm] should be calculated as follows when the surface is unprotected throughout the time of fire exposure: dchar,0=β0 t (6.3) (EC5 3.1) where t [min] is the time of fire exposure and β0 [mm/min] is the one-dimensional charring rate. For all other structures that are exposed from multiple sides, generally columns and beams, the notional charring rate β0 is used in the calculations of the notional depth dchar,n. When the characteristic density of LVL is ρk ≥480 kg/m3, the notional charring rate βn is 0,70mm/min. The design charring depth for notional charring dchar,n should be calculated as follows when the surface is unprotected throughout the time of fire exposure: dchar,n=βn t (6.4) (EC5 3.2) where t [min] is the time of fire exposure and βn [mm/min] is the notional charring rate In the test report VTT-S-04746-16 the one dimensional charring rate of different wood products was evaluated in 120min fire exposure according to a standardized time-temperature exposure curve (EN 1363-1:2012) 39. According to the report, wood products behaved predictably and, for LVL, the one-dimensional charring rate β0 = 0,65mm/min can be used for an extended fire exposure. The results were similar in both face side and edge side exposure specimens. This gives the necessary information and confidence for fire designers in assessing d,fi = mod,fi 20 M,fi (6.2) (EC5 2.1) m,d,fi = mod,fi ∙ fi ∙ m,k M,fi =1,0∙ 1,1∙ 36 m Nm2 1,0 = 39,6 mNm2 162 LVL Handbook Europe
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