When designing a steel beam, two sections with the same steel grade can behave very differently. One beam may reach full plastic capacity. Another may start buckling locally long before that. The reason is cross-section classification.
In Eurocode design, the cross-section class determines how much of the material strength can be used in calculations.
What is Cross-Section Classification?
Steel plates in a cross-section can buckle locally before the full material strength is reached.
This behavior depends primarily on plate slenderness. Thin plates buckle earlier, while thicker plates can sustain higher stress before instability occurs.
Eurocode accounts for this using four cross-section classes.
Class 1: The section can reach the full plastic resistance and has sufficient rotation capacity to form a plastic hinge.
Class 2: The section can reach plastic resistance, but the rotation capacity is limited.
Class 3: Local buckling occurs before plastic redistribution is possible. The section can only reach elastic bending resistance.
Class 4: Local buckling occurs before the elastic stress distribution is reached, meaning the effective cross-section must be reduced.
Local Buckling and Plate Slenderness
A steel cross-section consists of plates: flanges and webs.
If these plates are too slender, compressive stress can cause local buckling before the steel reaches yield stress.
This limits how much of the theoretical capacity the section can develop.
Cross-section classification therefore acts as a simplified stability check that determines which resistance model is valid.
Why Cross-Section Classification Matters
The cross-section class directly affects the moment capacity of a beam. To illustrate this, consider an I-beam subjected to bending. As the bending moment increases, the stress distribution evolves:
The cross-section class determines which of these stages the section can reach.
Example
Consider a statically indeterminate beam analyzed in PolyBeam. The geometry and steel grade remain the same. Only the cross-section class changes.
Class 4 cross-section
Local buckling occurs before the elastic resistance is reached.
Point load capacity: 60.5kN
Class 3 cross-section
The beam can reach the full elastic bending resistance.
Point load capacity: 71.5kN
This corresponds to roughly 18% higher capacity compared with the Class 4 section.
Class 2 cross-section
The cross-section can develop plastic resistance.
Point load capacity: 80kN
This provides an additional 12% increase compared with Class 3.
Class 1 cross-section
The section can form plastic hinges, allowing moment redistribution in statically indeterminate structures.
Point load capacity: 102kN
This is 28% higher capacity than Class 2. The advantage of plastic hinges becomes more significant as the degree of static indeterminacy increases.
Cross-Section Classification in Daily Structural Design
In everyday structural design tasks, cross-section classification influences several key decisions:
- which resistance formulas are valid
- whether plastic analysis can be used
- whether effective widths must be calculated
- how much capacity a beam can provide
Correct classification therefore affects both safety and economy in steel design.
A clear understanding of the behavior behind the classes helps engineers choose sections more efficiently and avoid unnecessary over-dimensioning.
Cross-Section Classification in PolyStruc
In PolyStruc calculation tools, the cross-section class is determined automatically according to Eurocode rules. This ensures that:
- the correct resistance model is used
- local buckling is accounted for correctly
- the resulting documentation remains transparent and reviewable
The goal is not to add complexity, but to make the structural behavior behind the calculation easy to understand in daily design work.
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