Why columns are checked differently
A column in compression usually fails in fire by buckling before the cross-section reaches its resistance. As temperature rises, both the yield strength and the modulus of elasticity decrease, and the modulus decreases faster. That is why the critical temperature of a column follows from the buckling check (EN 1993-1-2, cl. 4.2.3.2). The simplified formula does not apply to columns.
Buckling resistance in fire
The slenderness at temperature θ is obtained from the slenderness at normal temperature:
Unlike the normal-temperature design, a single buckling curve is used in fire regardless of the section type, through the factor α above.
Reduction factors ky,θ and kE,θ
A few values from EN 1993-1-2, Table 3.1 (carbon steel):
| θa [°C] | ky,θ (yield strength) | kE,θ (modulus of elasticity) |
|---|---|---|
| 400 | 1.00 | 0.70 |
| 500 | 0.78 | 0.60 |
| 600 | 0.47 | 0.31 |
| 700 | 0.23 | 0.13 |
The critical temperature of the column is the temperature at which Nb,fi,t,Rd drops to the compression force Nfi,Ed. It is found iteratively, for each buckling axis, and the lowest value governs.
Buckling length ℓfi
- In general, ℓfi is determined as at normal temperature, from the support conditions.
- In braced frames where each storey is a separate fire compartment, the heated column remains elastically restrained by the cool parts above and below. The standard then allows ℓfi = 0.5 · L for intermediate storeys and ℓfi = 0.7 · L for the top storey.
- In sway frames, ℓfi is determined from the stiffnesses of the columns and beams meeting at the joints.
Cross-section class in fire
Classification follows EN 1993-1-1, but with a reduced ε (cl. 4.2.2), which accounts for the different reduction of strength and stiffness:
Sections that become Class 4 need a separate calculation (or the simplified critical temperature of 350 °C).
Informative material summarising the provisions of EN 1993-1-2. It does not replace the text of the standard, the National Annex or verification by a qualified engineer.