Steel Beam Sizing: A Practical Guide for Homeowners and Builders

Steel beam sizing combines load calculation, span analysis, and serviceability checks. While the final calculation belongs to a chartered engineer, understanding the principles helps you sanity-check designs and avoid expensive mistakes.

What controls beam size

• Span: distance between supports
• Load: what the beam carries (roof, floor, wall, etc.)
• Deflection: how much the beam can sag
• Strength: bending and shear capacity
• Lateral stability: resistance to twisting

Typical loads

• Tiled roof: 0.85 kN/m² dead + 0.6 kN/m² imposed
• Domestic floor: 0.4 kN/m² dead + 1.5 kN/m² imposed
• Brick external wall: 2.2 kN/m² per storey
• Plasterboard partition: 0.4 kN/m

Common South African beam sections

Universal Beams (UB) — wide flanges, deep webs, used as bending members:

• 152×89×16 UB
• 178×102×19 UB
• 203×102×23 UB
• 254×102×28 UB
• 305×127×37 UB
• 356×171×45 UB and larger

Universal Columns (UC) — square-ish, used vertically or as short stocky beams.

Indicative spans for residential

For a typical wall-supporting beam in a domestic context:

• 3m clear span: 152×89 UB or 178×102 UB
• 4m: 203×102 UB
• 5m: 254×102 UB
• 6m: 305×127 UB
• 7m+: 356×171 UB and above

Heavier loads (e.g. supporting roof AND wall above) push sizes up.

Deflection check

Beams that pass strength can still fail serviceability. Eurocode limits typical deflection to span/360 for finishes — for a 4m beam, max deflection ~11mm. Aesthetic crack avoidance often pushes design to span/500.

Padstones and end bearing

The beam's end load concentrates at the support. Without a padstone, the supporting masonry crushes. Padstone size depends on beam load and masonry strength — typically 100mm to 215mm long, full beam width, 100mm+ deep.

Fire protection

Internal residential steel typically requires 30–60 minute fire protection (intumescent paint or boarding). Loft conversion beams often need 60 minutes.

MCFAR sizes steel beams for renovations and extensions across South Africa.

Frequently Asked Questions