Old vs New Steel Profile Designations: ČSN/DIN → EN

Anyone assessing or renovating a twentieth-century building in Central Europe will sooner or later hold a drawing that reads I 200, U 180 or L 50×50×5. These labels come from national standards that predate the harmonised European system: in Czechoslovakia the ČSN 42 55xx series of dimension standards, in Germany the DIN 1025 and DIN 1026 series. The steel is still in the building and still carrying load. Only the naming system around it has been replaced.

Reading such a drawing correctly is not a formality. Expand an old label into the wrong modern family and every downstream check (bending, shear, deflection, buckling) runs on the wrong section properties. The reassuring part: for most hot-rolled profiles the mapping is one-to-one and the geometry itself never changed. You only need to know which current family the old name points to, and where the exceptions hide.

The most common and most dangerous mistake when interpreting legacy drawings is expanding a bare letter into the wrong parallel-flange family.

• Old „I“ is IPN, not IPE. An entry such as I 200 denotes the narrow-flange beam with tapered inner flange faces, today's IPN family. The parallel-flange IPE is a different section with a different flange width, web thickness and second moments of area at the same nominal depth.
• Old „U“ is UPN, not UPE. U 180 on a legacy drawing means the tapered-flange channel, today's UPN. The parallel-flange UPE arrived later and is not dimensionally interchangeable.

A practical clue on site: the tapered flanges of IPN and UPN historically required wedge-shaped taper washers in bolted connections. If you find them, you are almost certainly looking at the older families. When in doubt, feel the flange: constant thickness suggests IPE or UPE, thickness growing towards the web suggests IPN or UPN.

For the rolled families the transition to European standards was largely administrative. The dimension tables of ČSN 42 5550 and DIN 1025-1 for the I-beam, and of ČSN 42 5570 and DIN 1026-1 for the channel, were carried over into the European catalogue essentially unchanged; dimensions and masses of I, H and U sections are now collected in EN 10365, with rolling tolerances in EN 10034 and EN 10279. In other words, an IPN from a current mill table is geometrically the same section your drawing calls I.

The table is a curated overview of the cases most frequent on Czech and Slovak drawings. Regional variants existed, so treat any unusual or clearly non-standard label as a prompt for a site measurement, not a guess.

Angles are the friendly case: the logic of the designation (leg sizes and thickness) survived the change of standards intact. An old L 50×50×5 maps directly to a current equal angle, and unequal legs work the same way. Dimensions are now standardised in EN 10056-1; the current equal angle and unequal angle tables tell you whether a size is still rolled. On older Czech drawings the angle may also appear with the ∟ symbol instead of the letter L.

Hollow sections need more care. Czech drawings label a square tube TR 4HR (trubka čtyřhranná, square tube), typically followed by the outer size and wall thickness. Today that corresponds to the SHS family, but the modern catalogue splits it in two: hot-finished sections to EN 10210 and cold-formed sections to EN 10219, which differ in corner radii and therefore in section properties. On an existing structure the corner geometry is often the only visual way to tell them apart, and it matters for any reuse calculation.

Old Czechoslovak drawings specify steel by the ČSN numeric grade system: most commonly 11 373 or 11 375 for ordinary structural steel and 11 523 for the higher-strength grade. Established practice maps 11 373 to today's S235 and 11 523 to S355, and as a first orientation in a survey this is a reasonable starting point.

It is not a substitute for verification. For any load-bearing assessment or reuse of existing steel, the actual material properties must be confirmed from original documentation (mill certificates, acceptance tests) or by testing: tensile tests on extracted samples, hardness measurements, chemical analysis. Historic steels can differ from their modern equivalent in yield strength scatter, ductility, toughness and above all weldability; rimmed steels from older production may be unsuitable for welded strengthening altogether. Standards for the assessment of existing structures expect exactly this verification step: the mapping tells you what to expect, not what to assume in the calculation.

A reliable workflow for translating a legacy drawing into a current model:

1. Read the label in context. The era of the drawing and the standard references in its title block usually tell you whether you are in ČSN or DIN territory.
2. Map the family, not just the size. Using the table above: I → IPN, U → UPN, L → angles, TR 4HR → SHS.
3. Measure on site. Depth, flange width, flange and web thickness, and whether the flange is tapered. Record corrosion losses at the same time.
4. Compare with current tables. If the measured dimensions match a row of the current family table, you can take the full set of section properties from it with confidence. If they match no row, the member may be a withdrawn size or a non-standard rolling. Then work from the measured geometry.

Every family page on this site carries the complete dimension and property table of the current EN series, so the comparison takes minutes rather than an archive visit.