Steel Section Check Calculator

What it checks

This tool verifies a steel member to Eurocode 3 (EN 1993-1-1). It classifies the cross-section, computes the section resistances of clause 6.2 - axial, shear, bending about both axes, and the combined interaction - and then the buckling resistances of clause 6.3: flexural buckling, torsional and flexural-torsional buckling, and lateral-torsional buckling. It reports the governing utilisation across all of them.

Section resistance and buckling resistance are different questions. A member can be comfortably within its cross-section capacity and still fail, because buckling depends on length and restraint rather than on the section alone. The tool reports both so the distinction is visible.

Classification decides which resistance you are allowed to use

Cross-section class is not a description, it is a permission. Class 1 can form a plastic hinge and rotate, so plastic design is allowed. Class 2 can reach the plastic moment but not rotate. Class 3 can only reach first yield, so the elastic modulus must be used. Class 4 cannot even reach first yield before a plate element buckles locally, so an effective cross-section with reduced properties is required.

The practical consequence is a step change, not a gradual one: a section that slips from class 2 to class 3 loses the difference between the plastic and elastic modulus - typically 10 to 15 per cent for an I-section - for no change in geometry other than crossing a limit. Class depends on the steel grade too, through epsilon, so the same section is a different class in S355 than in S275.

Lateral-torsional buckling is usually what governs a beam

An unrestrained beam in bending does not fail by reaching its moment capacity - it fails by twisting and displacing sideways at a lower moment. The reduction depends on the unrestrained length, the shape of the bending moment diagram through the C1 factor, and the section geometry.

The unrestrained length is the input that matters most and the one most often entered carelessly. It is the distance between points of effective lateral restraint, not the span: a beam with the compression flange restrained by a floor slab may have almost no unrestrained length at all, while the same beam during construction, before the slab is cast, has the full span. Those are two different design situations for the same member.

Formula reference

Material parameter

$\varepsilon = \sqrt{\dfrac{235}{f_y}}$

Axial resistance, class 1-3

$N_{pl,Rd} = \dfrac{A f_y}{\gamma_{M0}}$

Bending, class 1-2

$M_{pl,Rd} = \dfrac{W_{pl} f_y}{\gamma_{M0}}$

Bending, class 3

$M_{el,Rd} = \dfrac{W_{el} f_y}{\gamma_{M0}}$

Shear resistance

$V_{pl,Rd} = \dfrac{A_v f_y}{\sqrt3\,\gamma_{M0}}$

Non-dimensional slenderness

$\bar\lambda = \sqrt{\dfrac{A f_y}{N_{cr}}}$

Flexural buckling

$N_{b,Rd} = \dfrac{\chi A f_y}{\gamma_{M1}}$

Lateral-torsional buckling

$M_{b,Rd} = \dfrac{\chi_{LT} W_y f_y}{\gamma_{M1}}$

Assumptions and limits

The buckling checks depend entirely on the effective lengths and restraint positions you enter. Those are engineering judgements about how the member is actually held, not properties the tool can derive - an optimistic effective length produces an optimistic and unsafe answer that looks identical to a correct one.

Class 4 sections require effective cross-section properties to EN 1993-1-5. Where the tool reports class 4, treat the result as an indication and confirm the effective-section calculation, since the reduction depends on the stress distribution as well as the geometry.

Static persistent design situation with a uniform member. Tapered and haunched members, members with web openings, fatigue, fire resistance and seismic design are outside the scope, as is any second-order analysis of the frame the member sits in.

FAQ