Structural Fire Design According to EN 1993-1-2 (Parametric Fire Exposure)

Technical Article

With RF-/STEEL EC3, you can apply nominal temperature-time curves 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. This steel temperature can be calculated using the parametric temperature-time curve, as described in the Annex to EN 1992‑1‑2. The different fire exposures are explained in this article.

Parametric Fire Exposure

If parametric fire exposure is used as a fire scenario, the load reduction effect of the structural component must be ensured. No failure of the component should occur during the fire phase, including the cooling phase, or within the required fire resistance time.

Annex A of EN 1991‑1‑2 provides a parametric temperature-time curve. This fire scenario is no longer permitted in Germany as there is the binding National Annex to EN 1991‑1‑2 that must be applied.

This scenario was replaced by design fire, which allows for a complete description of a possible fire scenario, that is, from the development phase over the compartment fire phase to the decay phase. The curve sections are limited by distinctive points that result in the distribution of the rate of heat release. When determining temperature values, it is necessary to distinguish between ventilation controlled fires and fuel controlled fires. Moreover, the application of this natural fire model is limited. It applies to surface areas with an area of up to 400 m² and a height of up to 6 m. In the case of the ventilation controlled design fires, the characteristic value of the maximum rate of heat release can be calculated using the equations provided in Annex A.

${\overset.{\mathrm Q}}_{\max,\mathrm v,\mathrm k}\;=\;1.21\;\cdot\;{\mathrm A}_{\mathrm W}\;\cdot\;\sqrt{{\mathrm h}_{\mathrm W}}$

For fuel controlled fires, the characteristic value of the maximum heat release rate may be calculated according to the following equation.

${\overset.{\mathrm Q}}_{\max,\mathrm f,\mathrm k}\;=\;0.25\;\cdot\;{\mathrm A}_{\mathrm f}$

The design value of the maximum heat release rate results from:

${\overset.{\mathrm Q}}_{\max,\mathrm d}\;=\;{\mathrm\gamma}_{\mathrm{fi},\overset.{\mathrm Q}}\;\cdot\;\min\left\{\begin{array}{l}{\overset.{\mathrm Q}}_{\max,\mathrm v,\mathrm k}\\{\overset.{\mathrm Q}}_{\max,\mathrm f,\mathrm k}\end{array}\right.$

Figure 01 - Temperature-Time Curve Diagram According to the Simplified Natural Fire Model

EN 1991-1-2 indicates the formulas to determine the fire room temperature. They can be found in Annex AA.

Combining the Parametric Fire Exposure with EN 1993-1-2

By means of this fire room temperature, it is now possible to determine the temperature of the cross-sections by combining the determined fire room temperatures with the function given in EN 1993-1-2 to determine the radiation temperature. Simply use the individual temperature values of the fire room in the calculation.

Please note that ΔΘ (temperature gradient) must not become negative. Therefore, it is limited to a maximum of 0. The reason for this is the cooling phase, which cannot be considered with the simplified method. By means of this trick, you can also consider real fires, which allows for a more economical design of structural components.

Keywords

Fire Fire exposure Fire design

Reference

[1]   EN 1993-1-2 (2005): Eurocode 3: Design of steel structures - Part 1-2: General rules - Structural fire design [Authority: The European Union Per Regulation 305/2011, Directive 98/34/EC, Directive 2004/18/EC]

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