How to calculate steel section factor (Hp/A)

Definition

The section factor is the heated perimeter Hp divided by the cross-section area A, expressed in m⁻¹ (EN 1993-1-2 uses Am/V – the same ratio per unit length). A high section factor means a thin, light member that heats quickly; a low one means a heavy member that heats slowly.

• Europe: Hp/A or Am/V, m⁻¹
• Australia (AS 4100): exposed surface area to mass ratio ksm, m²/t
• United States: W/D – weight per foot (lb/ft) divided by heated perimeter (in)

Heated perimeter

Worked example – beam, 3 sides

Universal beam 310UB40.4: d = 304 mm, b = 165 mm, tw = 6.1 mm, mass 40.4 kg/m.

1. Hp = 2 × 304 + 3 × 165 − 2 × 6.1 = 1,091 mm
2. A = 40.4 ÷ 0.00785 = 5,146 mm² (from mass, ρ = 7,850 kg/m³)
3. Hp/A = 1,091 ÷ 5,146 × 1,000 ≈ 212 m⁻¹

Using the tabulated gross area instead of the mass-derived area changes the result slightly; use the method your product's assessment specifies.

Worked example – CHS column

CHS 168.3 × 6.4: Hp = π × 168.3 = 529 mm; A = π × 6.4 × (168.3 − 6.4) = 3,255 mm²; Hp/A ≈ 162 m⁻¹. Mass = 25.6 kg/m, so ksm ≈ 0.529 ÷ 0.0256 ≈ 20.7 m²/t and W/D ≈ 17.2 lb/ft ÷ 20.8 in ≈ 0.83.

Why it matters

Loading tables give the required coating thickness against section factor. Halving the section factor – by using a heavier section – can sharply reduce the coating needed, which is one of the main levers of value engineering.

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