Effective Class 2 Web — Recover the Plastic Moment (Eurocode 3)

❓ What is Effective Class 2 Web?

Effective Class 2 Web is a bending resistance enhancement intoruced in Eurocode 3 for steel I section, where the flange is classified as Class 1 or 2 while the web is considered as Class 3. Details about section classification, please found in previous article.

Since section classification considering individual plate's class and take the worst case for calculation. While normally in this case, the section is classified as Class 3, the corresponding bending resistance will be calcualted based on elastic modulus W_el, which the different in capacity may be typically 10-15% between W_pl and W_el.

However, as the flange can be perfectly plastic (Class 1 or 2) but whose web just identified into Class 3 — such as a welded girder with a deep thin web, or a hot-rolled. It will be a waste in the bending resitance. Therefore, Eurocode 3 has a specific clause for this case: effective Class 2 cross-section and effective plastic modulus.

EN 1993-1-1 Cl.6.2.2.4(1)

flowchart TD
  A("Classify flanges and web") --> B{"Web <b>Class 3</b>?"}
  B -- no --> R("Use W<sub>pl</sub> or W<sub>el</sub> as classified")
  B -- yes --> C{"Flanges <b>Class 1 or 2</b>?"}
  C -- no --> D("<b>Class 3</b> → elastic W<sub>el</sub>")
  C -- yes --> E("<b>Effective Class 2</b> (Cl. 5.5.2(11))")
  E --> G("W<sub>pl,eff</sub>  → M<sub>pl,eff,Rd</sub>")

📐 The effective web — 20ε·t_w and neglected ineffective web

The rule is purely geometrical introduced by Eurocode 3EN 1993-1-1 Cl.6.3.

With use of class 2 effective web, the web in compression are divided into 3 parts as below (also shown in diagram):

• top 20ε·t_w from the compression flange
• middle neglectable web area between two 20ε·t_w stripes
• bottom 20ε·t_w from the plastic neutral axis

where ε is the grade factor:

🛡️ Formulas for Effective Plastic Modulus, W_pl,eff

To calcualte the effective plastic modulus:

1. first calcualted the shifted plastic neutral axis (due to neglected area) by equaling compression and tension area (the web length below plastic Nautral axis is 2 * 20 εt_w).

2. Then calculate the corresponding plastic modulus

hence the bending moment resistance. EN 1993-1-1 Cl.6.2.2.4(1)

🛡️ The bending resistance of Effective Class 2 Section

With W_pl,eff in hand, the bending resistance is the ordinary plastic formula — just with the effective modulus:

For a section with a Class 3 web and Class 1/2 flanges, it now have two possible bending resistances:

Since W_pl,eff > W_el, the effective Class 2 method typically recovers 80–95% of the gap between the elastic and plastic bending resistance. Details worked example can be found in Work Example of Bending Resistance of Effective Class 2 Web for a Steel I-Section (Eurocode 3)

🏗️ Why the neglected part give better section?

In a plastic stress block the web's contribution to the moment is very less — the flanges, far from the neutral axis, provided more. And the strip you discard sits in the middle of the compression zone, next to the neutral axis, where the lever arm is small.

So by removing part of the compression web strip, W_pl,eff ends up close to the full W_pl — it recover almost all of the plastic capacity, paying only a small fraction for the web's slenderness.

📝Summary & Key Takeaways

1. Effective Class 2 is a bending resistance enhancement concept.
2. It can be used when an I-section has Class 1 or 2 flanges but a Class 3 web.
3. By neglecting part of the web, an effective plastic modulus can be used to calculate the moment resistance instead of the elastic modulus, which typically enhances bending resistance by 5–15% for the same section.

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