UPN and UPE Channels: Differences, Shear Centre and Use in Pairs

The UPN and UPE ranges look interchangeable in an elevation drawing: both are U-shaped hot-rolled channels with the same logic of nominal depths. Up close they belong to two generations of rolling practice. The UPN is the classic European standard channel: its inner flange faces are inclined, at 8% up to UPN 300 and at 5% for the four largest sizes, UPN 320 to 400; the flange is thick at the root and thins towards a rounded toe, and the root radii are generous. The UPE is the modern parallel-flange channel: inner and outer flange faces are parallel, the toe is square-cut, and the section is proportioned more efficiently for bending about the strong axis.

For engineers this means two things. Structurally, a UPE of the same nominal depth typically gives you comparable or better bending performance at lower weight, because more of the material sits in the flanges where it works hardest. Practically, the flange geometry changes how you bolt to the section, and that is often the deciding factor on site.

The tables below compare the two families size by size: overall depth and flange width (h×b), web and flange thickness (t_w/t_f), the centroid offset e_y measured from the back of the web, and the weight per metre G. Note how the centroid of every channel sits close to the web: the flanges pull it only part-way across the width. That asymmetry is the root of everything discussed in the next section.

Individual profile pages such as UPN 200 and UPE 200 list the full set of section properties, including torsion and warping constants that matter for the twisting behaviour described below.

A channel is singly symmetric: it has a horizontal axis of symmetry but no vertical one. Its shear centre (the point through which a transverse load must act to produce bending without twisting) does not coincide with the centroid. For a channel it lies outside the section altogether, on the far side of the web from the flange toes.

The consequence is easy to underestimate. Apply a vertical load through the centroid, or even through the plane of the web, and the load is still eccentric to the shear centre. The eccentricity times the load is a torque, and the channel responds by twisting as it bends. Because an open thin-walled section has very little torsional stiffness, even a modest eccentricity produces visible rotation, warping stresses in the flanges and, in slender members, an earlier onset of lateral-torsional buckling than the bending check alone would suggest.

This behaviour is a property of the shape, not of the standard: UPN and UPE suffer from it equally. The qualitative rule to remember is simple: a single channel loaded anywhere except through its shear centre wants to rotate, and something in the detailing has to either remove that torque or resist it.

There are three honest ways to deal with the offset shear centre:

1. Bring the load to the shear centre. Occasionally practical (a bracket or cleat projecting behind the web so the reaction line passes through or near the shear centre), but it costs fabrication and is sensitive to tolerances.
2. Restrain the rotation. The most common solution in practice. A purlin with roof sheeting screwed to its top flange, a stringer welded to a landing plate, or a channel bolted back-of-web to a rigid support is continuously or discretely held against twist. The torque still exists; it is simply carried by the restraint, and the connection must be detailed for it.
3. Use channels in pairs. Two channels placed back-to-back (webs together, with spacer plates or a gusset between them) form a doubly symmetric built-up section whose shear centre coincides with its centroid. The twisting problem disappears by symmetry, and the pair also gives a clean two-shear-plane bolted connection through the webs. Toe-to-toe pairs forming a closed box go further still, gaining real torsional stiffness, at the cost of an interior that is hard to inspect and protect. In both arrangements the interconnections (battens, stitch bolts or welds) must be close enough that the two channels genuinely act as one member; design codes give spacing rules for built-up compression and bending members.

Back-to-back pairs are the standard answer for hangers, trimmers, bracing members and column lacing, and they are why channels remain popular despite the awkward shear centre of the single section.

The flange geometry decides the hardware. On a UPN the inner flange face slopes at 8% up to UPN 300 and at 5% for UPN 320 to 400, so a nut or bolt head seated directly on it would bear on one edge only. Bolted flange connections therefore need taper washers (square washers machined with the matching slope), fitted the right way round so the bearing face ends up square to the bolt axis. Forget them, or fit them rotated, and the bolt is bent in service and the preload is unreliable.

A UPE has parallel flanges: standard flat washers seat correctly, ordinary cleated connections work without special hardware, and the flat inner face makes it easier to land fittings, fasteners for decking, or cold-formed accessories. The back of the web is flat on both families, so web-bolted connections (including the back-to-back pairs above) are equally straightforward for either. On drawings it pays to flag UPN flange bolting explicitly: a missing taper washer is one of the most common channel detailing errors found in inspections.

Channels appear wherever a flat back or an open face is useful: purlins and side rails on light industrial roofs, stair stringers where the web forms a natural closure to the flight, trimmers framing floor and roof openings, edge members of platforms and walkways, skid and machine frames, and lintels paired around existing masonry.

The split between the families in practice is largely historical. UPN stock has been rolled for over a century, so renovation and refurbishment work constantly meets existing UPN members, and matching like with like keeps connections, alignments and visual continuity simple. For new work, UPE is usually the default: better strength-to-weight, parallel flanges that simplify every bolted detail, and no taper washers to specify or lose. Availability varies by market, so it is worth confirming which range your supplier actually stocks before the connection design is frozen.