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RF-CONCRETE Members Version 5

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3.6.9 Fire Resistance

Fire Resistance

The final tab of window 1.6 is available if at least one load case or load combination is selected for the fire resistance design in the 1.1 General Data window (see chapter 3.1.4). The fire protection design is performed as per the simplified calculation method according to EN 1992-1-2, clause 4.2 (see chapter 2.3).

Figure 3.50 Window 1.6 Reinforcement, tab Fire Resistance

In the bottom right corner of this tab, you can find the [Default] button for restoring the initial values.

Data for Fire Resistance Design

Five drop-down lists control the parameters that have a decisive influence on the fire resistance design:

These parameters are described in the theory chapter 2.3.

The Cross-section sides exposed to fire have to be defined as well. If not All sides are affected by charring, clear the corresponding check box (see Figure 3.48). The check boxes around the cross-section symbol to the right become accessible, allowing for specific settings. The directions refer to the local member axes.

In the case of asymmetrical effects of fire, the cross-section is stressed by an additional thermal precamber due to the temperature difference that must be considered in the calculation according to EN 1992-1-2, clause 2.4.2 (4). This thermal precamber affects the load-bearing capacity of structural components such as brackets calculated according to the second-order analysis. The program internally creates a member load as precamber of the cross-section and superimposes it with the design loads.

The check box for Design longitudinal reinforcement for fire resistance controls whether the provided reinforcement also considers effects of fire in addition to the ultimate limit state.

Material Factors in Case of Fire

The two upper input fields control the Partial safety factors for concrete γc and reinforcing steel γs that must be applied for the fire resistance design. The values recommended in EN 1992-1-2, clause 2.3 (2) are preset.

The Reduction factor α used to consider long-term effects on the concrete strength in case of fire can be specified separately for compression and tension loads. The value 1.0 recommended in EN 1992-1-1, clause 3.1.6 is preset in both input fields.

With the option Consider thermal strain of concrete and reinforcement steel, it is possible to consider the difference between the strain of the "hot" reinforcement and the concrete cross-section's regular thermal strain in the form of a pre-compression strain of the rebar. For loading due to temperature, thermal longitudinal strains occur in concrete and reinforcing steel, varying within the cross-section because of the non-uniform temperature distribution. The thermal strains cannot freely arise everywhere in the cross-section as they are influenced by the adjacent areas. Generally, it may be assumed that the cross-sections remain plane. As the thermal strain of the reinforcement in the cross-section's edge area is restricted, the reinforcement is pre-compressed.

The zone method according to EN 1992-1-2 includes only a calculation of structural components, which means the thermal additional strains in the centroid are not taken into account by the standard. However, according to Hosser [12], those thermal strains must not be neglected for calculations according to the second-order analysis. In his approach, the concrete's thermal strain is calculated across the entire concrete cross-section by using the temperature's mean value.

Consider Checks

Annex D to EN 1992-1-2 contains a calculation method for shear and torsional design of structural components exposed to fire. This method can be activated separately for both internal force types.

As this calculation method for shear and torsional design is not allowed in Germany, both options are disabled for a design according to German standards.

[12] Heydel, Günter, Krings, Wolfgang u. Hermann, Horst. Stahlbeton im Hochbau nach EC2: Einführung und Anwendungsbeispiele. Ernst & Sohn Verlag, 1995