Sigma sections: when do the extra folds help?

It is worth looking at the cross-section, not just a photo of a long element. The stiffenings are often barely visible in perspective. When ordering, dimensions from the documentation for the specific product must be given. Information about the sigma height alone does not allow the flange widths, the depth of the stiffening or the length of the lips to be reproduced.

An example sigma with a height of 200 mm made from 2 mm sheet has flanges 55 mm wide and lips 15 mm long. In the same geometric set, the bend radius is 3.5 mm. Such data make it possible to check the overall dimensions and the space for fixings straight away. The name sigma 200 alone conveys only part of this information.

A thin, wide wall under load can lose stability locally. The folds affect how it is divided and how it behaves. This is one of the reasons for using more elaborate cold-formed cross-sections. The designer, however, takes into account not only a single wall but also the interaction of the whole profile and the possible displacements of its parts.

The difference therefore cannot be reduced to the statement that sigma is always stronger. Depending on the direction of bending and the support conditions, a different cross-section may be advantageous. Torsion and deflections are also assessed. The right choice follows from the specific loads, not from the number of visible bends.

If one channel is taller, another thicker, and the sigma is made from a different steel, the result of a simple price comparison says little. First the task of the beam must be established. Then the variants that fulfil that task must be checked. Only then can their weight, cost and method of installation be compared in one table.

In offers for cold-formed profiles, pay attention to the full designation, the cross-section drawing and the core thickness. The steel grade is also important. A similar appearance does not guarantee identical properties. When the supplier proposes a substitution, the designer’s confirmation is needed that the new element fits both in terms of calculation and with the planned connections.

The beam interacts with the sheet, board or other element through a specific bearing surface. The flange width affects the space for fixings and the possibility of making the connection in accordance with the documentation. With sigma, one should not focus solely on the web. Both flanges and their edges are equally important for practical application.

Before purchase, it is worth comparing the profile drawing with the cladding detail. The position of the screws, tool access and distance from the bends are checked. A narrow installation zone can make work difficult even when the beam itself meets the requirements. The selection of galvanised profiles should therefore also take account of the installer and the sequence of joining the elements.

Sigma is sometimes supplied with factory-prepared holes. In the stiffened web, it must be precisely indicated on which part of it they are to be located. The drawing should include a dimensional datum and the orientation of the profile. A small offset can mean that the hole falls on a bend or does not match the connecting element.

Additional holes require assessment in the context of the cross-section and the connection. It should not be assumed that a cut-out will be neutral simply because it occupies a small area. This applies in particular to support zones and points of concentrated load transfer. Well-prepared holes reduce working time but do not relieve anyone of the need to check the documentation.

Sigma profiles can perform structural functions in structures with various layouts. The scope of their application follows from the design. It must be established whether the element will be laterally supported, how it will be restrained against rotation and how the load will reach the supports. The assessment of a beam taken from a catalogue does not automatically cover these conditions.

The construction stage must be considered in the same way. Before the cladding is closed, the profile may behave differently from in the finished structure. Temporary restraints and bracing are not decorative extras. Their role should be clearly described. Galvanised profiles retain the advantages of a light material when the whole system has been prepared for the anticipated loads.

It is worth considering sigma when its geometry gives a favourable result for a specific task and does not complicate the connections. Sometimes the saving comes from lower weight, sometimes from matching the height or the repeatability of elements. It must, however, be calculated together with the costs of accessories, holes and installation. A kilogram of steel alone does not describe a complete solution.

Z-sections are convenient in certain lap arrangements, channels in many frames, and omegas in grids and battens. Sigma profiles complement this family. There is no need to replace all the other cross-sections with them. The best result comes from assigning to each type of cold-formed profile the task in which its geometry genuinely brings a benefit.