Angle Section Design Theory - Equal & Unequal Angles (Eurocode 3)
Angle sections (Equal and Unequal Angles) are designed about their principal axes, not the geometric axes you might expect - a key distinction that catches many engineers out.
📐See it applied - full worked exampleA complete EC3 capacity calculation for a representative section, across the full design range.Worked example →Principal axes (u-u, v-v) vs geometric axes
For an equal angle the principal axes are at 45° to the legs; for an unequal angle they are skewed. The minimum radius of gyration (about the weak principal v-v axis) governs buckling - not or . Designing an angle as if it bent about the leg directions over-estimates its buckling strength.
Eccentric connections
Angles are usually connected through one leg only (bolted or welded), so the load is eccentric to the centroid. EN 1993-1-1 (and EN 1993-1-8) provide effective-slenderness rules that account for this eccentricity, effectively treating the single-leg-connected angle as having reduced buckling capacity.
Typical use
Single angles are common in trusses, lattice towers and bracing where the member is primarily axial. Pairs of angles (back-to-back) form a stiffer, more symmetric member for heavier truss chords.
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