Why thin-walled design differs
The starting point
Section titled “The starting point”With a thick steel section the design question is simple: can the material take the stress. Every point of the cross-section works until the yield strength is reached.
With a thin-walled section this is not true. The plate elements — web, flange, lip — are slender: wide relative to their thickness. Under compression they buckle before the material reaches its yield strength.
What that means
Section titled “What that means”A buckled plate element does not stop working, but it takes part in carrying load unevenly: fully near its supported edges, less and less towards the middle.
Design handles this by replacing the real, uneven stress distribution with an effective cross-section: the reduced cross-section that gives the same capacity. Effective cross-section.
The three characteristic stability phenomena
Section titled “The three characteristic stability phenomena”Local buckling
Section titled “Local buckling”The plate element ripples while the corners and edges stay in place. Local buckling.
Distortional buckling
Section titled “Distortional buckling”The flange and lip rotate together and the web distorts. A phenomenon specific to cold-formed, lipped sections. Distortional buckling.
Global, flexural-torsional buckling
Section titled “Global, flexural-torsional buckling”The whole member buckles, and because the cross-section is open and not symmetrical, bending is accompanied by twisting. Flexural-torsional buckling.
These do not exclude one another — they interact, and design handles that too.
The practical consequences
Section titled “The practical consequences”Geometry is worth more than strength
Section titled “Geometry is worth more than strength”Adding a lip or a stiffener improves capacity more than a higher steel grade — because it improves stability rather than raising the limit of the material.
Restraint is a design parameter
Section titled “Restraint is a design parameter”How far a member is braced directly determines its capacity. The lining and the bracing are therefore structural elements, not “extras”. Restraint and bracing points.
Accuracy matters
Section titled “Accuracy matters”A few tenths of a millimetre of deviation in the cross-section changes the effective cross-section, and with it the capacity.
Which standard
Section titled “Which standard”In Europe EN 1993-1-3 gives supplementary rules for cold-formed sections alongside the general rules of EN 1993-1-1. The role of EN 1993-1-3.
The point
Section titled “The point”Designing thin-walled steel is not harder, it is about something else: stability. Understand that, and the rest of the technology’s characteristics follow logically.