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What it calculates
- The cross-section class in fire, with ε = 0.85 · √(235 / fy).
- Flexural buckling about both axes, with χfi recalculated at each temperature, up to the critical temperature.
- The buckling length ℓfi from the support conditions (braced or unbraced frames, truss members) or entered directly.
- The fire resistance time, the class achieved and the protection thickness for the target class.
Input data
- The section (I/H, CHS, SHS, RHS) and the steel grade.
- The compression force in the fire situation Nfi,Ed.
- The member length and the support conditions, about each axis.
- The exposure, the fire curve and the protection.
Example
HEB 300 column in S355, storey height 4.00 m, braced frame, compression Nfi,Ed = 2000 kN, exposed on four sides, ISO 834 curve. The full calculation, step by step, is in the HEB 300 worked example.
| Quantity | Value |
|---|---|
| ℓfi (intermediate storey) | 0.5 · L = 2.00 m |
| λ̄y / λ̄z (at 20 °C) | 0.201 / 0.345 |
| θa,cr (z-z axis governs) | 592.6 °C |
| Unprotected | 19.2 min, class R 15 |
| Gypsum boards, box protection (λp = 0.20 W/mK), for R 90 | 15.0 mm |
How it is calculated
For a column in compression the simple μ0 formula does not apply, because of buckling. The critical temperature is found iteratively: at each temperature the slenderness and the factor χfi are recalculated, as described in fire design of steel columns.
Information material summarising EN 1993-1-2. The results must be checked by a designer; they do not replace the text of the standard, the National Annex or verification by a qualified engineer.