KB 001613 | Fire resistance analysis by means of a parametric temperature-time curve according to...

Subject:

Fire resistance analysis by means of a parametric temperature-time curve according to DIN EN 1991-1-2



Comment:

With RF-/STEEL EC3, nominal temperature-time curves can be used in RFEM or RSTAB. The standard time-temperature curve (ETK), the external fire curve and the hydrocarbon fire curve are implemented. Moreover, the program provides the option to directly specify the final temperature of steel.



Description:

This steel temperature can be calculated, for example, with a parametric temperature-time curve, as it is available in the Annex of DIN EN 1991-1-2. In the following, the calculation using the parametric temperature-time curve is explained and applied.



If a parametric fire exposure is used as a fire scenario, the load-bearing effect of the structural component must be ensured. There must be no failure of the component during the entire fire phase, including the cooling phase or within a required resistance period. Annex A of EN 1991-1-2 provides a so-called parametric temperature-time curve. This fire scenario is no longer allowed in Germany because there is a national annex to EN-1991-1-2, which is mandatory. This scenario was replaced by a design fire. With this curve, a possible fire scenario can be completely described, from the development phase to the full fire phase to the decay phase. The curve sections are limited by distinctive points, thus giving the course of the heat release rate. However, when determining the temperature values, it is necessary to distinguish between ventilation controlled fires and fire load controlled fires. Moreover, this natural fire model has only limited application. It applies to base areas with up to 400 m² and a height of up to 6 m. For ventilation-controlled design fires, the characteristic value of the maximum heat release rate may be calculated according to the following equations.



To calculate the temperature in the steel cross-section, you can use, for example, Microsoft Excel. Under Downloads, there is an Excel macro that can calculate the temperature. The calculated value can then be used directly in the module.



Calculation process



The program first calculates the coating area, the opening factor and the individual thermal penetration coefficients from the initial values, which are then averaged. Now, a distinction is made as to whether it is a ventilation controlled or fire load controlled fire. The program makes this distinction of fire type independently by comparing Q -max, v and Q -max, f . In the next step, the individual time periods and the corresponding temperatures are calculated using a reference fire load density q = 1,300 MJ/m². After calculating the reference temperature ranges, a comparison with the available fire load and the reference fire load follows in order to calculate the really available time ranges and the corresponding temperatures. The individual points are calculated by means of a loop. In addition, the standard temperature curve (ETK) is included in the program to obtain an immediate comparison with the natural fire scenario.



Results from the calculation



The calculation shows that the natural fire proceeds less than the ETK, although a very high fire load density was selected for the natural fire model. The heating of the fire chamber is also not as fast as the ETK, which is closer to reality. The duration of a natural fire scenario is usually significantly longer than required because it always includes a cooling-down phase. This cooling down phase includes the burning-out of the fire compartment and can be significantly reduced, for example, by extinguishing processes.



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Keywords:

#FireProtectionDesign #FireCurve #FireResistance



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