Course · lesson 3 of 6

Cold-formed sections

Why thin steel behaves differently, how the Direct Strength Method of SANS 10162-2 deals with it, and how to check struts, built-up pairs, beams, webs and screws with the STATIX cold-formed member calculator.

1. Learning outcomes

After this lesson you can:

explain local, distortional and global buckling in a thin section;
say how the Direct Strength Method differs from effective widths;
check a cold-formed strut, braced and unbraced;
check a back-to-back pair and its connector spacing;
check a beam with and without restraint, web crippling and screws in outline.

2. Before you start

Lesson 2 (purlins: the uplift rules are there, not repeated here) and Steel Design 1, lesson 2 (columns).
The free cold-formed member calculator.
A pocket calculator and about 50 minutes.

3. Why thin steel is different

Why thin steel is different. A hot-rolled section’s plates are stocky and mostly reach yield. A cold-formed section is folded from sheet 1 to 3 mm thick, so its flat parts are slender: they buckle at stresses below yield, and the section can still carry more after they do.

Three modes. Local buckling ripples the flat plates in short waves; distortional buckling swings a flange and its lip about the corner over a longer wave; global buckling bends or twists the whole member over its length. Each has its own elastic buckling load.

Two routes in SANS 10162-2. The effective width method reduces each plate on its own; the Direct Strength Method (Section 7) takes the elastic buckling loads of the whole section and reads the strength off curves. The STATIX cold-formed calculators use the Direct Strength Method throughout.

The strength curves (7.2.1, 7.2.2). For each mode the slenderness is the square root of the yield (or global) capacity over the elastic buckling load. Below a limit the full capacity counts; above it the strength falls, but more slowly than the elastic curve, because thin plates carry load after they buckle. The smallest of global, local and distortional governs.

Pre-qualified sections. The resistance factors of the Direct Strength Method hold for sections within the limits of Tables 7.1.1 and 7.1.2; outside them, and for built-up pairs, which the tables do not list, the calculator drops to the rational-analysis value 0.80.

4. Compression members

Global buckling of a channel. A lipped channel can buckle about its weak axis or in a flexural-torsional mode, because its shear centre lies outside the web. The calculator works out both from the three effective lengths (x, y and torsion) and uses the smaller.

Bracing a strut. A brace at mid-length (a girt, a fly brace or a tie) halves the weak-axis and torsional lengths, and the global buckling load rises about four times. Then local buckling usually takes over.

Built-up sections. Two channels back to back act together only if they are joined often enough. The calculator offers back-to-back and toe-to-toe pairs, raises the slenderness for the connector spacing a (with r_i of one channel) and checks that a/r_i stays within half the pair’s slenderness.

5. Bending, webs and connections

Bending with and without restraint. A beam whose compression flange is held along its length reaches the section strength (local or distortional buckling); one free over a length L_b buckles laterally and torsionally first. The purlin rules of 3.3.3.4 are in lesson 2.

Web crippling. A thin web under a reaction or a point load folds locally. SANS 10162-2 gives the capacity from tested coefficients for each case (end or interior, one or two flanges loaded, fastened or not) and the bearing length; 3.3.7 combines it with the moment. The calculator checks it when you give the reaction and bearing length.

Connections in thin steel, in outline. Self-drilling screws are checked for tilting and hole bearing in the thinner sheet, end distance, and pull-out and pull-over in tension, and the screw itself must be stronger by a margin (the maker’s tested values). The calculator has these screw checks; bolted and welded thin-steel joints follow Section 5 and are not covered in this course.