Web Stiffener Effect on Buckling with pyCUFSM

About this calculation

An intermediate stiffener rolled into the web of a cold-formed section costs almost nothing in material yet can lift the local buckling load substantially, because it halves the flat width that has to buckle. This page compares a plain web against a stiffened one and sweeps the stiffener depth to find the point where the benefit stops.

How it works

Local buckling of a plate is governed by its flat width, and severely so: the critical stress varies with the inverse square of that width. A web 200 mm deep and 1.5 mm thick is very slender and buckles at a low stress. Fold a small groove into the middle of it and the web is no longer one 200 mm plate but two plates of roughly half the depth, each of which buckles at around four times the stress of the original.

That is a very large return for a fold that adds a few percent to the material. It is why almost every deep cold-formed purlin and stud on the market has a stiffener rolled into the web.

The benefit is not unlimited. The stiffener only divides the web if it is stiff enough to stay put while the sub-panels buckle around it. Too shallow and the whole web still buckles as one plate, with the stiffener simply riding along. The transition between those two behaviours is sharp, and it is exactly what the sweep below locates.

A stiffener also changes distortional buckling, usually a little, and barely touches global buckling, which does not care about the web's internal geometry.

What the analysis runs

This page runs a real finite strip analysis (pyCUFSM) in the browser, so it handles arbitrary geometry entered as a table rather than a fixed standard case, and returns buckling diagrams and contour plots alongside the numbers.

Limits and caveats

• Only local buckling is swept here. A web stiffener also alters distortional buckling and changes the section properties, so the governing mode of the finished section may not be local at all. Check the stiffened section as a whole before treating the gain reported here as a capacity increase.
• The stiffener is idealised as a sharp V folded from the centreline polyline. Real rolled stiffeners have corner radii and are often trapezoidal rather than triangular, both of which change the stiffness somewhat. A very deep or very narrow stiffener can also become a slender element in its own right and buckle locally; this simple geometry will report that behaviour, but it needs judgement to interpret.
• AS/NZS 4600 and AISI S100 impose limits on intermediate stiffeners for a section to be prequalified for the Direct Strength Method, covering the stiffener's proportions and the sub-panel widths. Confirm your geometry sits within those limits before relying on DSM.

Frequently asked questions

Why does a small web stiffener make such a large difference?

Local buckling stress varies with the inverse square of the flat width. A stiffener rolled into the centre of the web splits one deep plate into two plates of roughly half the depth, so each sub-panel buckles at around four times the stress of the original single plate. That gain comes from geometry, not from added material.

Is there a point where a deeper stiffener stops helping?

Yes. Once the stiffener is stiff enough to stay in place while the sub-panels buckle around it, making it deeper adds little further benefit, and a very deep or very narrow stiffener can itself become slender and buckle locally. The sweep on this page locates the sharp transition between the whole web buckling as one plate and the web buckling as two sub-panels.

Does the stiffener improve distortional and global buckling too?

It changes distortional buckling, usually a little, and barely affects global buckling, which does not depend on the web's internal geometry. Because the governing mode of the finished section may not be local, check the stiffened section as a whole rather than reading the local gain here as a straight capacity increase.

What analysis engine does this page use?

It runs a real finite strip analysis with pyCUFSM directly in the page. You enter arbitrary geometry as a table, and it returns the signature curve, buckling mode diagrams and contour plots alongside the critical loads.

Can I use the reported gain directly for design to AS/NZS 4600 or AISI S100?

Not without checking. Both standards impose limits on intermediate stiffener proportions and sub-panel widths for a section to be prequalified for the Direct Strength Method. The stiffener here is idealised as a sharp V without corner radii, so confirm your real geometry sits within the code limits before relying on these numbers for capacity.