How this estimate is made

• Real designs, not ratios. Behind the tool are – complete buildings, each modelled in 3D and designed member by member with finite-element analysis: frames, gable posts, eave struts and bracing, under dead, snow, roof live, wind, temperature and seismic load combinations with second-order effects. Your input is interpolated between the nearest designs.
• Economical frames. Rafters carry haunches cut from the same section (10% of the span) and the lightest passing hot-rolled section is chosen for every rafter and column, checked to AISC 360.
• Purlins and side rails are continuous cold-formed C sections at 1.5 m and 2.0 m spacing, sized from a manufacturer’s published limit-state capacity tables including deflection limits.
• Connections are measured, not guessed. Plates, haunches, stiffeners and base plates add – of the steel weight, measured on real fabrication models of built portal frame buildings.
• Calibrated against published designs. The same engine was run on a set of published standard portal frame halls (12–20 m span, two snow loads, complete designs with steel quantities). Its fully optimized designs came out – of the published tonnage, so the estimate adds that margin back to the frames, posts, bracing and struts. The low end of the range is the fully optimized design.
• Checked against a published table. The rafter and column sizes were cross-checked against a European pre-design table for portal frames; where the table ignores wind and sway, the designs here are heavier on tall frames, as they should be.

This version assumes a pitched roof of 6°, frames at 6 m, basic wind speed 28 m/s in open terrain, a high seismic zone (S_DS ≈ 0.94 g), steel S275 for frames, no crane and no mezzanine. Cranes, other spacings, lower wind or seismic demand, and built-up tapered frames are coming next. For a project outside these limits, ask for a real number below.