Določitev debeline nenosilnega sloja za lesene nosilce, ki so izpostavljeni gorivno nadzorovanim parametričnim požarom

Fire resistance is one of the essential requirements to be fulflled in the design of timber structures. For this purpose, a reduced cross-section method is given in the European standards, Eurocodes. The method is based on the assumption that an initial, rectangular timber cross-section exposed to fre conditions reduces to an efective cross-section, which has material properties as at a room temperature. The reduced part of the cross-section with no resistance is determined by a sum of two parameters, namely a charring depth and a thickness of zero-strength layer. Eurocodes give a value of the latter only for the standard fre exposure, which is only one of the fre curves proposed in the same standards. Therefore, the present paper examines the thickness of zero-strength layer in case of 46 diferent fuel-controlled parametric fre exposures applied to a timber beam from three sides. A four-phase numerical analysis is applied for this purpose that includes the use of a hygrothermal model and a mechanical model to determine temperatures of timber over the cross-section and the mechanical resistance of timber beam in fre conditions, respectively. The results show that the thickness of zero-strength layer takes the values between 7.9 and 18.4 mm for the fuel-controlled parametric fre exposures. Since it is clearly dependent on the parameters describing the parametric fre curve, fve equations are proposed that can be used for determination of the thickness of zero-strength layer in case of parametric fre exposures