Steel Building Calculator — Portal Frame in 30 Seconds

A steel building calculator turns the four numbers a designer starts with — the span (clear width), the length (number and spacing of frames), the eave height and the roof pitch — into a real structure with real quantities. This one generates a symmetric gable portal frame, the workhorse of industrial sheds, warehouses, barns, hangars and agricultural buildings worldwide, and returns the answers an early-stage estimate actually needs: how much wind the roof sees, how big the rafter and the column have to be, and how many kilograms of steel the whole building weighs.

Almost every "steel building" tool you find online is a lead-generation form: you type your dimensions and a salesperson emails you a quote. There is no engineering behind the screen. This calculator is the opposite — it is a calculable model. It computes the velocity pressure from the wind code, the design bending moments from statics, screens the real steel-profile catalog to pick the lightest section that passes, and adds up the weight. The number is on screen in seconds, with no login, no email wall and no watermark, and every value is a closed-form quantity you can check by hand (the worked example below does exactly that).

Use it to sanity-check a supplier's tonnage, to compare two geometries (is a 10 m or a 12 m bay lighter per m²?), to teach how a portal frame carries load, or as the first 30 seconds of a design you then finish in the full 3D editor. It is a pre-dimensioning screening tool, and it says so — but a screening built on the same standards (NBR 6123 for wind, NBR 8800 / AISC 360 for the member resistance) that the final design uses.

1. Set the geometry. Enter the span (transversal width between columns), the bay spacing (distance between portal frames), the number of frames, the eave height (column height) and the roof pitch in degrees. The transversal elevation redraws live — with the span, eave, ridge and rise dimension lines, the pinned bases and two ghosted frames that imply the building running back.
2. Set the loads. The dead load is the permanent roof weight (sheeting + purlins + services), the live/imposed load is the maintenance or roof imposed action — both in kN/m² of roof plan. Enter the steel yield strength fy (250 MPa for common mild steel, 345/350 for high-strength).
3. Set the wind. Type the basic wind speed V₀ (the NBR 6123 30-year, 3-second gust for your site — 30–45 m/s covers most of Brazil) and pick the terrain roughness category (II = open country is the default). The calculator computes S2 at the mean roof height and the velocity pressure q = 0.613·Vk².
4. Read the results — they are already there. The KPI strip shows the span × length, the ridge height, the wind pressure, the sized rafter and column with their utilizations, and the total steel weight in kg and kg/m². The breakdown tables give the design actions per frame and the member take-off.
5. Share or continue. Copy link to this building creates a permalink that reproduces your exact model for a colleague. Open the full warehouse in the 3D editor to generate the same geometry as an editable CalcSteel project and run the complete NBR 8800 / AISC 360 design — load combinations, buckling, second-order, purlins, bracing and connections.

The SI ⇄ imperial toggle converts the geometry (m ↔ ft) and the weights (kg ↔ lb, kg/m² ↔ lb/ft²); the code-defined inputs (loads in kN/m², wind in m/s) stay in the metric units the NBR standards are written in.

The calculator runs a transparent, hand-checkable model — no black box:

Geometry. From the span L, eave H and pitch θ: the rise is (L/2)·tan θ, the ridge height is H + rise, and each rafter is (L/2)/cos θ long. The building length is (frames − 1)·bay and the plan footprint is L × length.

Wind — the real NBR 6123 engine (same as the editor). This is the key differentiator: the calculator does not re-implement a weak wind formula. It calls the exact CalcSteel wind engine — computeVelocityPressure() for the velocity pressure and lookupWallCp() / lookupRoofCp() for the external pressure coefficients — feeding it a bundled snapshot of the NBR 6123 standard the 3D editor loads from the API. The engine evaluates Vk = V0·S1·S2·S3 with S2 = b·(z/10)^p at the mean roof height z, using the tabulated Class-B roughness parameters b, p of the selected terrain category (I → V). Topography S1 and the statistical factor S3 (occupancy group 2) are taken as 1.0 for the screening — override them in the full analysis. The velocity pressure is q = 0.613·Vk² (Pa, shown in kN/m²). The windward-wall line load on a column is w = Cpe·q·s, where the windward-wall Cpe = +0.7 comes straight from the NBR 6123 external-pressure table (not a guessed value), and s is the bay spacing. The roof windward/leeward Cpe (e.g. −0.6 / −0.4 at a 10° pitch) come from the same tabulated duopitch coefficients.

Design actions per interior frame. The frame carries a tributary width equal to the bay spacing. The gravity line load on the rafter (horizontal projection) is factored to ULS as w = 1.4·dead + 1.5·live times s (NBR 8800 normal combination). Wind is factored by 1.4. The screening design forces are:

• Rafter: the free bending moment Md = w·L²/8 — the simply-supported envelope, a safe upper bound for a rigid frame with haunches.
• Column: axial N = w·L/2 (half the frame's vertical load) plus a wind bending moment Md = w·H²/8 (pinned base, restrained at the eave by the roof plane).

Member sizing. The whole catalog of doubly-symmetric I/H (W-series) profiles is screened, lightest first, using the NBR 8800 resistance fRd = fy/1.10 (γa1). The rafter is the lightest section whose elastic moment capacity Mrd = Sx·fRd covers Md. The column is the lightest section that satisfies the AISC 360 H1-1 beam-column interaction N/Nrd + (8/9)·M/Mrd ≤ 1 (with Nrd = A·fRd). Section properties (Sx, A) are computed from each profile's nominal plate dimensions.

Weight take-off. Primary steel = 2 columns of height H and 2 rafters of the sloped length, per frame. A secondary allowance of +45 % over the primary weight accounts for purlins, girts and bracing — typical for light pitched-portal buildings. The intensity is the total steel divided by the plan footprint. These are estimating figures; the exact tonnage comes from the detailed 3D model.

Search "steel building calculator" and you get two kinds of results: quote generators (dimensions in, a salesperson out) and cost estimators (dollars per square foot with zero structural content). Neither tells you a single engineering number. This tool is a category of one:

• Real structural output, instantly. Wind pressure, design moments, the actual profile designations for the rafter and the column, utilizations, and the steel weight in kg and kg/m² — visible on mount, before you touch anything.
• The lightest section that passes, screened live against the real 454+ I/H profile catalog — not a fixed "we use W250" assumption.
• The real wind engine, not a toy formula. The velocity pressure and the wall/roof pressure coefficients come from the same NBR 6123 engine the CalcSteel 3D editor runs (computeVelocityPressure + lookupWallCp/lookupRoofCp), fed the tabulated NBR standard — no re-derived approximations. NBR 8800 (with an AISC 360 cross-check) governs the member resistance.
• A drafting-board sketch with cotas — span, eave, ridge and rise dimensions, pinned bases, the projected gravity load and the wind pressure annotated, the sized profiles labelled on the members — not a marketing render.
• Shareable and free. A permalink rebuilds your exact building for a colleague; nothing is paywalled or watermarked.
• One click to the real thing. Open the full warehouse in the 3D editor generates the same geometry as an editable model and runs the complete NBR 8800 / AISC 360 verification — combinations, buckling, second-order, connections. The 30-second estimate and the final design are the same platform.

Read these before you trust the number for anything beyond a screening:

• Symmetric gable portal frame, prismatic members (no haunch modelled — real portals haunch the eave, which lets a lighter rafter carry more, so the rafter here is slightly conservative).
• Elastic section modulus Sx for the bending resistance (no plastic Mp, no compact-section bonus).
• Buckling is not checked. Lateral-torsional buckling of the rafter and flexural/torsional buckling of the column can govern real members — the screening uses the gross-section resistance fRd = fy/1.10 and Nrd = A·fRd. The full check runs in the 3D editor.
• One governing combination per member (gravity for the rafter, gravity axial + wind moment for the column). Real design envelopes several combinations, including wind uplift that can reverse the rafter moment.
• Wind runs the real NBR 6123 engine for q and the Cpe tables, but the screening fixes S1 = S3 = 1.0 (flat ground, group-2 occupancy) and loads a single frame from the windward-wall Cpe. Internal pressure (Cpi), the per-zone roof distribution and other wind directions — all available in the engine — are not enveloped here; the full analysis in the 3D editor applies them.
• Secondary steel (purlins, girts, bracing) is a +45 % allowance, not a member-by-member design.
• No second-order (P-Δ) effects, no serviceability drift limit, no connection or base-plate design, no foundation.

In short: a fast, standards-based pre-dimensioning that lands within engineering range for tonnage and member size — and a clear on-ramp to the full model where every one of these limits is lifted.