FAQ
What is the buckling curve selection procedure?
EN 1993-1-1 Table 6.2 assigns curves based on section type, h/b ratio, flange thickness, and whether the section is rolled or welded. Rolled IPE/HEA with h/b > 1.2 and t_f ≤ 40 mm: y-axis → curve a, z-axis → curve b. Welded I-sections with t_f ≤ 40 mm: y-axis → curve b, z-axis → curve c. Hollow sections use curves a0 (cold-formed) or a (hot-finished). Each curve carries an imperfection factor α from Table 6.1: a0=0.13, a=0.21, b=0.34, c=0.49, d=0.76.
How is non-dimensional slenderness calculated?
λ̄ = √(A·f_y / N_cr) where N_cr = π²·E·I / L_cr². For Class 1–3 sections, the gross area A is used. For Class 4 slender sections, the effective area A_eff should be substituted — this calculator uses gross area (conservative for Classes 1–3, not valid for Class 4). E = 210 000 MPa. When λ̄ ≤ 0.2, or N_Ed/N_cr ≤ 0.04, buckling effects may be neglected per §6.3.1.2(4).
What is the χ (chi) reduction factor?
χ = 1 / (Φ + √(Φ² − λ̄²)) ≤ 1.0, where Φ = 0.5·[1 + α·(λ̄ − 0.2) + λ̄²]. It ranges from χ = 1.0 for stocky sections (λ̄ ≤ 0.2) down to χ → 0 for very slender members. The buckling resistance is then N_b,Rd = χ·A·f_y / γ_M1.
What is the effective buckling length?
L_cr = k·L where k is the effective length factor and L is the system length. For pin-pin (both ends restrained against translation, free to rotate): k = 1.0. For fixed-fixed (both ends fully restrained): k = 0.5. For fixed-pin (one end fixed, one pinned): k = 0.7. In a sway frame, L_cr can exceed L significantly — consult EN 1993-1-1 §5.2.2 and the design model. Enter L_cr directly in the tool.
Why check both axes separately?
Because Iy and Iz differ substantially for most I-sections, and the effective lengths can also differ (e.g. lateral restraint at intermediate floor levels reduces L_cr,z but not L_cr,y). The governing N_b,Rd is the smaller of the two axial resistances. For a stocky UC with equal lengths, z-axis nearly always governs because Iz << Iy.
Does this cover combined axial + bending (beam-column)?
No. Equation 6.49 (this calculator) covers pure compression only. For beam-columns under combined N + M, use the interaction equations §6.3.3 (Eq 6.61 + 6.62) with k_yy, k_yz, k_zy, k_zz interaction factors from Annex A or B. FrameAI full pipeline checks §6.3.3 automatically as part of the member verification pass.