Steel member code check
Enter a built-up I-section, the unbraced length and the demands, and get the governing limit state with the utilisation ratio to AISC 360-22 LRFD or EN 1993-1-1. Section properties are derived from the plate dimensions you type, so nothing is pulled from an unverified shapes table, and every substituted formula is printed for review.
| Step | Clause | Value |
|---|---|---|
| Section properties | Geometry | Ag = 15,600 mm² · Sx = 2,362×10³ mm³ · Zx = 2,604×10³ mm³ |
| Plastic moment | AISC 360-22 F2-1 | 345 × 2,604,000 = 898.4 kN·m |
| Bracing limits | AISC 360-22 F2-5 / F2-6 | Lp = 3,219 mm · Lr = 10,287 mm · Lb = 1,000 mm |
| Flexural strength | AISC 360-22 F2-1 (yielding) | Mn = 898.4 kN·m · φMn = 808.5 kN·m |
| Shear strength | AISC 360-22 G2-1 | Aw = 4,000 mm² · Vn = 828 kN · φVn = 745.2 kN |
| Compression strength | AISC 360-22 E3-2 / E3-3 | KL/r = 79 · Fcr = 218.6 MPa · φPn = 3,069.1 kN |
| Serviceability | AISC 360-22 L3 (L/360) | δ = 1.79 mm · allowable = 16.67 mm |
Scope: doubly-symmetric compact I-sections, strong-axis bending, K = 1. Torsion and combined axial + bending are not checked.
Mn = Mp or Cb·(Mp − (Mp − 0.7FySx)(Lb−Lp)/(Lr−Lp)) · AISC F2: yielding below Lp, inelastic LTB between Lp and Lr, elastic LTB beyond.
Vn = 0.6·Fy·Aw·Cv · G2 with Cv = 1 for h/tw below the compact limit.
Fe = π²E/(KL/r)²; Fcr = 0.658^(Fy/Fe)·Fy · E3 inelastic branch; 0.877Fe above 4.71√(E/Fy).
δ = 5wL⁴/(384EI) · L/360 unless changed in the design criteria.
A clear path from inputs to results.
A steel member is designed by the limit state that fails first, not by the one that looks largest. This check derives the section properties from the plates you specify, then runs bending, shear, compression and deflection to AISC 360-22 LRFD or EN 1993-1-1, reports the governing utilisation, and prints every substituted formula so the number can be audited.
How to use
- Enter the section as plate dimensions — overall depth, flange width, flange and web thickness.
- Set the yield stress and the unbraced length; the unbraced length is what drives lateral-torsional buckling.
- Enter the required moment, shear, compression and the service load for deflection.
- Choose AISC 360-22 LRFD or EN 1993-1-1.
- Read the maximum utilisation and the governing limit state, then check the derivation table.
The formula
Reference: AISC 360-22, chapters F (flexure), G (shear), E (compression) and L (serviceability); EN 1993-1-1, 6.2 and 6.3.
Worked example
400×300 built-up I, 20 mm flanges, 10 mm web, Fy 345, Lb 1.0 m
- Section: d = 400 mm, bf = 300 mm, tf = 20 mm, tw = 10 mm.
- Fy = 345 MPa, E = 200 000 MPa, span 6.0 m, unbraced length 1.0 m.
- Demands: Mu = 200 kN·m, Vu = 100 kN, Pu = 500 kN, service load 10 kN/m.
- 1
Ag = 2(300)(20) + (360)(10) = 15 600 mm²; Ix = 4.725×10⁸ mm⁴; Zx = 2.604×10⁶ mm³.
- 2
Mp = Fy·Zx = 345 × 2.604×10⁶ = 898.4 kN·m; φMn = 808.5 kN·m.
- 3
Lp = 1.76(75.97)√(200000/345) = 3 219 mm, which is longer than Lb = 1 000 mm, so the section yields before it buckles laterally.
- 4
Shear: Aw = 4 000 mm², Vn = 0.6(345)(4000) = 828 kN, φVn = 745.2 kN.
- 5
Compression: KL/r = 6000/75.97 = 79.0; Fe = 316.4 MPa; Fcr = 0.658^(345/316.4)(345) = 218.6 MPa; φPn = 3 069 kN.
- 6
Deflection: δ = 5(10)(6000)⁴/(384 × 200000 × 4.725×10⁸) = 1.79 mm against an allowable 16.7 mm.
- 7
Utilisations: bending 0.247, shear 0.134, compression 0.163, deflection 0.107 — bending governs and the member passes.
Maximum utilisation 0.247 in bending · φMn 808.5 kN·m · φPn 3 069 kN · pass.
Interpreting the result
Utilisation is demand divided by capacity, so 0.247 means the member is working at about a quarter of its bending resistance. The governing ratio is the only one that matters for the verdict, but the others tell you what would happen if the load case changed. Watch the unbraced length: as Lb passes Lp the available moment starts to fall, and beyond Lr it falls steeply — the capacity-versus-Lb chart shows exactly where that transition sits for this section.
Assumptions & limitations
- Doubly-symmetric compact I-sections in strong-axis bending. Slender-element local buckling is not evaluated, so check the width-to-thickness limits in AISC B4 or EN 1993-1-1 Table 5.2 before relying on the result.
- Effective length factor K = 1: the unbraced length is taken equal to the member length. Frame sway and end restraint need an appropriate K or a second-order analysis.
- Shear uses Cv = 1, valid while h/tw is below 2.24√(E/Fy). The Eurocode shear area is the standard I-section approximation.
- Combined axial force plus bending interaction (AISC H1, EN 1993-1-1 6.3.3), torsion, connections, fire and seismic detailing are outside this check.
- Section properties are derived from the plate dimensions you enter, not looked up. If you are checking a rolled shape, enter the published Ag, Sx, Zx, ry, rts, J and Cw from your shapes database.
Frequently asked questions
Why does the tool ask for plate dimensions instead of a section name?
Because a shapes database transcribed from memory is a silent-error risk: one wrong plastic modulus changes the answer without changing the page. Deriving the properties from geometry is exact for a built-up section, and an engineer checking a rolled shape can enter the published properties directly.
Why did the bending capacity drop when I increased the unbraced length?
Lateral-torsional buckling. Below Lp the section reaches its plastic moment; between Lp and Lr the available moment falls linearly; beyond Lr it falls with the square of the slenderness. Bracing the compression flange is usually cheaper than increasing the section.
Does it support LRFD and ASD?
It implements LRFD (φ factors) for AISC and the γM0/γM1 partial factors for EN 1993-1-1. ASD is not included; for AISC ASD you would compare service-level demands against Ω-reduced strengths.
What does "governing limit state" mean?
It is the check with the highest demand-to-capacity ratio. A member is sized by its weakest behaviour, so that ratio — not the largest absolute capacity — decides whether the section is adequate.
Is a passing utilisation a design approval?
No. It confirms four limit states for one load case. Connections, stability bracing, combined actions, fatigue, fire and seismic detailing still need a qualified engineer, and local slenderness limits must be checked separately.
Methodology last reviewed 2026-10-20. Results are informational and not financial or engineering advice — see our disclaimer.