How to design a steel column

A short, stocky steel column fails by squashing: the whole section yields. A long, slender one fails much earlier by buckling — it bows sideways and collapses at a load that can be a fraction of the squash load. Column design is mostly about working out how much buckling reduces the strength, and then adding the effect of any bending.

#Euler buckling and slenderness

An ideal pin-ended strut buckles at the Euler load:

Real columns are never perfectly straight and contain residual stresses from rolling or welding, so they fail below both the squash load and N_cr. The codes capture this with buckling curves that give a reduction factor χ (EC3) or a critical stress F_cr (AISC) as a function of the non-dimensional slenderness:

EC3 has five curves (a₀, a, b, c, d) chosen by section shape, axis, flange thickness and steel grade; AISC uses a single curve:

#Effective length

The buckling length depends on how the ends are held and on what braces the column between floors:

A column buckles about each axis separately. Bracing or beams framing in at mid-height halve the length about one axis but not necessarily about the other — check both, with the weak axis usually critical.

#Worked example: HEB 240 column (Eurocode 3)

A 4.0 m column in a braced frame, pinned at both ends about both axes. S355, HEB 240: A = 106.0 cm², i_y = 10.31 cm, i_z = 6.08 cm, W_pl,y = 1053 cm³. Design forces: N_Ed = 1500 kN and, from beam end moments, M_y,Ed = 60 kN·m at the top, zero at the bottom.

#Axial buckling

λ₁ = 93.9 × √(235/355) = 76.4. HEB 240 has h/b = 1.0 and t_f = 17 mm: curve b about y–y (α = 0.34), curve c about z–z (α = 0.49).

Axial alone: 1500 / 2348 = 0.64 about the weak axis.

#Bending and lateral-torsional buckling

M_y,Rk = 1053 × 10³ × 355 = 373.8 kN·m. With L = 4.0 m and a triangular moment (C₁ ≈ 1.77), M_cr ≈ 1526 kN·m, so λ̄_LT = √(373.8/1526) = 0.495, curve a → χ_LT = 0.926 and M_b,Rd = 346.1 kN·m.

#Interaction, EN 1993-1-1 6.3.3

For a triangular moment, C_my = C_mLT = 0.6. Annex B gives k_yy = 0.684 and k_zy = 0.843.

The weak-axis interaction governs at 0.79: the HEB 240 works with some margin.

#AISC 360 equivalent

AISC uses a simpler interaction, H1-1:

The second-order effects (P-δ, P-Δ) that EC3 folds into k_ij are handled in AISC by the direct analysis method or by B₁/B₂ amplifiers on the moments.

#Design it in Struxt3D

1. Open a framed model such as the Multi-storey frame or Portal frame shed. Columns are their own group, so they get their own section.
2. Choose the column section in Materials › Sections. H-sections (HEB, W14) suit columns: their stiffness about the two axes is closer.
3. Check how the column is held. Bracing and beams that frame into a column split its buckling length. In the Full interface, a member's Ends (rigid, pinned, pin start, pin end) set how moments pass into it — see Supports and releases.
4. Run the design (F6). Each column is checked to the code with flexural and torsional buckling, LTB and the combined axial–bending interaction, for every strength combination.
5. Inspect the worst column. Click it: the inspector shows the governing check, clause and combination. The Calculation report lists the buckling lengths L_y, L_z, L_b it used and the full check.
6. Size automatically. With Size members automatically on, Struxt3D picks the lightest passing column section per group.

#Common mistakes

• Checking only the strong axis. The weak axis nearly always buckles first unless it is braced.
• Using K = 1 for sway columns. Unbraced portal and moment frames buckle sideways as a whole.
• Ignoring the moments from eccentric beam connections — even "pinned" beams load a column off its axis.
• Upsizing instead of bracing. Halving the buckling length quadruples N_cr.