Extension Steel Calculator — Beam Sizing & Padstones

This calculator estimates structural steel beam requirements using simplified loading analysis per current UK structural standards (BS EN 1990, BS EN 1991-1-1 and BS EN 1993-1-1, each with its UK National Annex — the Eurocodes that replaced the withdrawn BS 6399 / BS 5950).

Loading calculation

Characteristic dead loads (structure self-weight and finishes) and imposed loads (people, furniture, roof maintenance) are combined at the ultimate limit state per BS EN 1990 equation 6.10: 1.35 × dead + 1.5 × imposed. Load per metre on the beam = the area load × the tributary width it collects.

Beam selection

The ULS bending moment of a simply supported beam under a uniform load is M = wL²/8. The calculator picks the smallest Universal Beam (S275) whose design bending resistance Mc,Rd = 0.275 × Wpl,y (kNm, with Wpl,y in cm³) is at least M. It then checks a simple uniform-load deflection δ = 5wL⁴/384EI (E = 210 kN/mm²) against span/360 for imposed load (plastered/brittle finishes) and span/250 for total load, using the unfactored service loads, and steps up to a stiffer section if either limit is exceeded.

What this does NOT check

The indicative size assumes the beam is fully laterally restrained. It does not assess lateral-torsional buckling, web bearing/buckling, shear, connection design, notching, fire protection or any bespoke point loads — any of which can govern and require a larger section. The padstone figure is an indicative masonry bearing-stress check, not a verified padstone design. A structural engineer MUST carry out the full design and Building Control must approve it.