Module 05
Steel sections and cross-section classification
Why a thin plate gives way before the steel it is made of ever reaches yield — and why a section does not have a class until you say what it is carrying.
What this module covers
- Explain why a compression plate can buckle below the yield stress
- Say why an internal element is far better off than an outstand, and by how much
- Compute plate slenderness and relate it to the class limits it stands in for
- Classify a section under a stated action, plate by plate
- Explain why a higher steel grade makes local buckling MORE likely
- State what each class permits, and what a Class 4 section loses
Lessons
A section is not a solid shape. It is an assembly of thin plates, and a thin plate in compression has its own opinion about how much stress it will take.
Start lesson →Classification is not a property of a section. It is a property of a section carrying a particular thing, and the same beam can be Class 1 in bending and Class 4 in compression.
Start lesson →
Module checkpoint
Check what you have taken in
6 questions
Question 1
A plate 400 mm wide and 15 mm thick is an internal element in uniform compression. What is σcr, in N/mm²? Take π²E/[12(1−ν²)] = 189 800 and kσ = 4.0.
Question 2
What is ε for S460 steel?
Question 3
A web has c/t = 55 and is in S275 (ε = 0.924). The Class 3 limit for an internal element in COMPRESSION is 42ε. Does it pass? Give the limit.
Question 4
A plate has λ̄p = 1.20 and is in uniform compression (ψ = 1). What is the effective-width factor ρ?
Question 5
By what factor is an internal element's critical stress greater than an outstand's, all else equal?
Question 6
An I section in major-axis bending has a shape factor of 1.13. If it is downgraded from Class 2 to Class 3, by what percentage does its bending resistance fall?