6. PERFORMANCE OF LVL IN FIRE Figure 6.4. Left: One-dimensional charring of panel or wide cross section when fire exposure is below on one side, Right: Charring depth dchar,0 for one-dimensional charring and notional charring depth dchar,n which takes into account the rounding of corners. a zero strength layer. The remaining residual cross section beneath the zero strength layer on exposed sides is defined as the effective cross section. The effective cross section is calculated by subtracting the effective charring depth def from the initial cross section, see Figure 6.5 and equation (6.1). def=dchar,n + k0 ∙ d0 (6.1) (EC5 4.1) where d0 is 7 mm; dchar,n is the notional design charring depth, see equation (6.4); and k0 is in the case of unprotected surfaces t/20, when t< 20 min and 1,0 when t>20 min. In the case of protected surfaces the value of k0 is given in Figure 6.6. For protected surfaces with a start of charring time of tch > 20 minutes, k0 is assumed to vary linearly from 0 to 1 during the time interval from t = 0 to t = tch, see Figure 6.6 (b). For protected surfaces with tch ≤ 20 minutes k0 is t / 20. Figure 6.5. Definition of residual cross section and effective cross section. 1. Initial surface of member, 2. Border of residual cross section, 3. Border of effective cross section. Figure 6.6. (a) Variation of k0 for unprotected members and protected members where tch ≤ 20 minutes and (b) for protected members where tch > 20 minutes. LVL Handbook Europe 161
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