Fire Provisions - Annex C

Assumptions

• Based on EN 1992-1-2:2004/A1:2019 Annex C - Buckling of columns under fire conditions
• Only for European preferences (EN 1992-1-2:2004/A1:2019)
• Only for columns with rectangular cross section in braced or unbraced structures
• Annex applicable for f_ck <= 50 MPa
• Assumptions required due to limitations in the tables provided in Annex C:

• The smaller cross section dimension "b" must be within the range of 20 cm <= b <= 60 cm
• The load ratio (n__fi) - should be between 0,2 and 0,6
• The modified mechanical reinforcement ratio should be in the range 0,1 <=
• The axis distance of the main bars must follow the limitation of 25 mm <= a <= 85 mm
• The modified, total first order eccentricity of the normal force needs to be within the range of 20 mm <= e_N <= 100 mm

Calculations

h, b - dimensions of column cross section b<=h

A_c - cross sectional area of column, A_c = bxh

n__fi - load ratio

ω - modified mechanical reinforcement ratio

e_N - modified, total first order eccentricity of the normal force, N_Ed,fi

a - axis distance of the main bars

N_Ed,fi - design axial load in the fire condition

M_0Ed,fi - design first order moment in the fire condition

Maximum permissible slenderness ratio under fire conditions (λ_fi,max) is calculated based on the tables contained in Annex C of EN 1992-1-2:2004/A1:2019. It based on the modified mechanical reinforcement ratio (ω), load ratio (n__fi), modified, total first order eccentricity of the normal force (e_N), axis distance of the main bars (a), and dimension of column cross section. The reference dimension for the cross section in the tables is always the smaller cross section dimension "b".

The values of maximum permissible slenderness ratio under fire conditions ( λ _fi,max ) from the tables in Annex C are linearly interpolated.

Verification Flow

After the calculations, in the "Column-note" section, there is information about whether the verification results are marked as "OK" or "NOT OK", along with a set of warnings that are displayed if the code requirements have not been met.

The "NOT OK" status is displayed when any warning occurs. This means that specific code requirements have not been met, and the user-defined values cannot be used in the calculations. In such circumstances, the nearest limit values from the tables in Annex C are used for the calculations, and all steps are performed based on these values, and shown in the note. This kind of note can be helpful during the process of correcting the adopted model by allowing the user to check and compare specific results. In order to achieve an "OK" status, the model should be revised by the user according to the instructions provided by the warnings.

Column Note

The calculation results of fire analysis can be checked in the "Column-note" tab, paragraph 2.5. All reported warnings are displayed, along with a quick response indicating whether the verification results are correct or incorrect. Additionally, the calculation results for individual steps, as well as the final calculations and comparisons with the limit values specified in the standard, can be reviewed.

Example of Column Note for RSA version 2025:

Additionally, warnings are displayed also in separate dialog window, which is shown immediately after calculations.