Timber is a natural structural material, which has good mechanical properties compared to its weight. Additionally, it has good characteristics in a case of fire exposure since the charring layer of the cross-section protects the inner timber from sharp temperature increase. In design practice, simple methods for fire resistance of timber structures are commonly used according to standard SIST EN 1995-1-2:2004 [9]. However, the latest research show, that this methods can be non-conservative and using them do not guarantee the adequate fire safety of timber structures in the case of natural fire such is a parametric fire curve given by SIST EN 1991-1-2:2004 [8]. The reason is the thickness of zero strength layer, which is calibrated to standard fire exposure, and is, according to standard SIST EN 1995-1-2:2004 [9], 7 mm. When using simple design method [9], the same thickness of zero strength layer is also taken in the case of a parametric fire curve. However, based on the advanced thermal and mechanical analysis, it was discovered in this master thesis, that the values of zero strenght layer are usually greater than 7 mm in the case when the parametric fire curve is considered. This means that according to simple method [9], the fire resistance for the parametric fire curve is overestimated and consequently the method is non-conservative.
In this master's thesis a timber beam exposed to various parametric fires is analysed. The temperature distribution across the cross-section was determined by advanced heat-moisture analysis. Further on, the load-bearing capacity of timber beam was determined based on two methods – simple and advanced method. According to the simple method, the load-bearing capacity was determined based on the effective cross-section, where isotherm 300 °C is implemented to estimate the char layer thickness,. The main criterion for determining thickness of the zero strength layer d0 was that the bending capacity determined by the simple design method at the end of charring is less or equal than the bending capacity according to advanced method.
In the thesis new values for the thickness of the zero strength layer were given. It was also discovered,that the thickness of the zero strength layer is not constant during the fire exposure, as it is the case for standard fire exposure. Furthermore, some disadvatanges of the reduced cross section method were disclosed. One of them is time when charring and calculation end. According to the simple design method, the end of calculation is associated with the end of charring, since afterwards the load-bearing capacity does not decreases anymore. In reality the temperature inside the cross-section still rises also after charring end, and consequently the load-bearing capacity of the timber beam decreases.
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