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

Technical Article

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Using 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.

The fire

The fire is an action on the structure and therefore regulated with all boundary conditions in EN 1991-1-2. This section also includes all nominal temperature-time curves and simple natural fire models that can act on a structure.

Nominal time-temperature curves

Over the years, many fire tests have been carried out to evaluate the behavior of structural components in case of fire. As part of these tests, it was possible to determine the development of a fire over time as well as the temperature depending on the fire load and the components and materials. These temperature-time curves are only partially applicable because they do not include any generation phase or cooling phase. Based on the tests and for a standard evaluation, a temperature-time law based on international agreements was defined. DIN EN 1991-1-2 contains three fire curve concepts that can be used for the simplified method according to DIN EN 1993-1-2.

Figure 01 - Fire Curve Concepts According to DIN EN 1991-1-2

Standard temperature curve (ETK):
Θ g = 20 + 345 ∙ log 10 (8 ∙ t +1)
α c = 25 ∙ [W/(m ∙ K)]

External fire curve:
Θ g = 660 ∙ (1 - 0.687 ∙ e 0.32 ∙ t - 0.313 ∙ e -3.8 ∙ t ) + 20
α c = 25 ∙ [W/(m ∙ K)]

Hydrocarbon fire curve:
Θ g = 1.060 ∙ (1 - 0.325 ∙ e -0.167 ∙ t - 0.675 ∙ e -2.5 ∙ t ) + 20
α c = 50 ∙ [W/(m ∙ K)]

In addition, there is a parametrized fire resistance curve, which is, however, intended for the use of the general fire resistance design according to DIN EN 1993-1-2. In this case, the ETK is the most used fire curve, since most experimental analyzes for fire protection materials are carried out after this fire curve. The external fire curve, on the other hand, is not very important because the temperature of this curve rises to 660 ° C and is therefore not suitable for longer fire durations with fire protection material. The hydrocarbon fire curve is similar to a tunnel fire curve because the temperature of this curve rises to 1350 ° C and the rise is also steeper than for all other fire curves.

Parametric fire exposure

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 structural component during the entire fire phase including the cooling phase or within a required resistance period. In EN 1991-1-2, a so-called parametric temperature-time curve is given in Annex A. 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. This curve can be used to completely describe a possible fire scenario, which means: from the development phase to the full fire phase to the decay phase.

Figure 02 - Parametric Temperature-Time Curves According to EN 1991-1-2/NA

The curve sections are limited by distinctive points, thus giving the course of the heat release rate. 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 with the equations in Annex A.

Reference

[1] Eurocode 1: Actions on structures - Part 1-2: General actions - Fire actions on structures; EN 1991-1-2: 2002 + AC: 2009
[2] National Annex - Nationally determined parameters - Eurocode 1: Actions on structures - Part 1-2: General actions - Fire actions on structures; DIN EN 1991-1-2/NA: 2015-09
[3] Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings; EN 1993-1-1: 2005 + AC: 2009
[4] National Annex - Nationally determined parameters - Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings; DIN EN 1993-1-1/NA: 2015-08
[5] Eurocode 3: Design of steel structures - Part 1-2: General rules - Structural fire design; EN 1993-1-2: 2005 + AC: 2009
[6] National Annex - Nationally determined parameters - Eurocode 3: Design of steel structures - Part 1-2: General rules - Structural fire design; DIN EN 1993-1-2/NA: 2010-12

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