Steel Beam Calculator (RSJ) — Domestic Openings

This calculator estimates a suitable Universal Beam (UB) section for a domestic wall opening based on the span, load type and wall construction. It uses simplified loading assumptions derived from BS EN 1991-1-1 (Eurocode 1 — Actions on structures).

Loading assumptions

• Timber floor: 0.5 kN/m² dead + 1.5 kN/m² imposed = 2.0 kN/m²
• Concrete floor: 3.5 kN/m² dead + 1.5 kN/m² imposed = 5.0 kN/m²
• Roof (tiles): 1.0 kN/m² dead + 0.6 kN/m² imposed = 1.6 kN/m²
• Wall self-weight: ~10.8 kN/m per storey (cavity brick + dense block + plaster at 2.4 m storey height; load-bearing walls only)
• Tributary width: 4.0 m assumed floor span onto beam

Beam selection

Loads are factored to the ultimate limit state using the partial factors from BS EN 1990 (γ_G = 1.35 on permanent/dead, γ_Q = 1.5 on variable/imposed), then the applied bending moment is calculated as M = wL²/8 for a simply supported beam, using the effective span (clear span + bearing length). The smallest UB section whose moment capacity M_c,Rd = W_pl × f_y (S275, f_y = 275 N/mm²) exceeds the applied moment is the starting point.

Deflection (serviceability) check

The selected section is also checked for deflection δ = 5wL⁴/(384EI) using the unfactored (characteristic) loads and E = 210,000 N/mm². The beam is sized up until it satisfies both span/360 under imposed load (to protect brittle finishes such as plaster, per the UK National Annex to BS EN 1993-1-1 Table NA.2) and span/250 under total load. For UK domestic beams, deflection frequently governs over bending strength at longer spans, so the result shows which check governs.

Important disclaimer

This calculator is for estimation only. A qualified structural engineer must produce the final beam specification and calculations. Building Control approval is required before any structural work begins.