The European standard EN 1995-1-2 specifies the method of reduced cross-section for determining the fire resistance of timber elements. This is determined based on the effective cross-section, which is the reduction of the initial dimensions of the cross-section by the char layer depth dchar and zero-strength layer depth d0. For the standard fire curve ISO 834 the zero-strength layer depth is 7 mm. This value is also often used in the case of non-standard (natural) fire curves, which, however, as recent studies show, is not appropriate, as fire resistance of timber elements can be overestimated. Thus, the main purpose of the thesis was to determine the zero-strength layer depth in the case of a natural fire. For this purpose, advanced computational analyses were performed, on the basis of which the zero-strength layer depth was determined. In the thesis, first the advanced calculation models are presented then the input data, the calculation procedure and finally the results are given. The analyzed timber beam was exposed to 42 natural fire curves. Results showed that for all 42 fire curves the value of the d0 is bigger than 7 mm. In addition, we also investigated the relations between the zero-strength layer depth and the natural fire curve parameters. As it turned out, the zero-strength layer depth is most accurately described with the cooling rate, as well as linear combination of the following parameters: heating rate, cooling rate, maximum achieved gas temperature in compartment and time when the temperature in compartment exceeds 220 ⁰C.
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