Section Series and What the Letters Mean
Hot-rolled H sections are grouped by the ratio of depth to flange width. The wide series, designated HW, has a depth roughly equal to the flange width and is used for columns and for compression members, because the wide flange gives a good radius of gyration about both axes. The medium series, HM, sits between the two, while the narrow series, HN covers beams with a flange width of roughly half the depth, which is efficient for bending. The section tables in GB/T 11263 and JIS G3192 use the same logic, so an HW 200x200 ordered to either standard describes a closely comparable profile.
Universal rolling produces parallel flanges with a constant thickness across the width, which simplifies bolted end connections and welded stiffener detailing compared with tapered-flange sections.
Common Sizes and Mass per Metre
| Section (mm) | Web thickness (mm) | Flange thickness (mm) | Mass (kg/m) |
|---|---|---|---|
| 100 x 100 | 6 | 8 | 16.9 |
| 125 x 125 | 6.5 | 9 | 23.6 |
| 150 x 150 | 7 | 10 | 31.1 |
| 200 x 200 | 8 | 12 | 49.9 |
| 250 x 250 | 9 | 14 | 71.8 |
| 300 x 300 | 10 | 15 | 93.0 |
| 350 x 350 | 12 | 19 | 134.9 |
| 400 x 400 | 13 | 21 | 171.7 |
| 148 x 100 | 6 | 9 | 20.7 |
| 194 x 150 | 6 | 9 | 29.9 |
The values above are nominal, taken from the section tables; mill rolling tolerances allow small variations in web and flange thickness, and the actual mass of a delivered bundle is normally checked against the theoretical figure at the agreed tolerance.
Grade Requirements Compared
| Grade | Standard | Min. yield (MPa) | Tensile strength (MPa) |
|---|---|---|---|
| A36 | ASTM A36/A36M | 250 | 400-550 |
| A572 Gr.50 | ASTM A572/A572M | 345 | 450 minimum |
| Q235B | GB/T 700-2006 | 235 | 370-500 |
| Q345B / Q355B | GB/T 1591-2018 | 355 | 470-630 |
| SS400 | JIS G3101 | 245 | 400-510 |
| S355JR | EN 10025-2 | 355 | 470-630 |
A572 Gr.50 is a high-strength low-alloy grade with a 345 MPa minimum yield and a 450 MPa minimum tensile strength, often specified where a lighter beam is needed for the same moment capacity. Q345B and its successor designation Q355B deliver a similar 355 MPa minimum yield. The choice between them is normally dictated by the design code and by whether the fabricator's welding procedures are already qualified for the grade.
Finishes, Stock and Ordering Data
H sections are supplied as-rolled, lightly oiled for transit, or hot-dip galvanized to ISO 1461. Galvanizing is the usual choice for exposed frames, but it requires that the fabricator's drain and vent holes are cut before the coating line, so hole positions should be settled at the drawing stage rather than after fabrication. Where a painted finish is required over galvanizing, an epoxy primer formulated for zinc is used to build the system.
A complete enquiry should state: section designation and series, steel grade, quantity in pieces or tonnes, length and length tolerance, end preparation, whether drilling or camber is required, coating requirement, and the inspection certificate type. Stock availability for the common sizes listed above is normally in 6 m and 12 m lengths, with cut lengths produced to order.
Frequently Asked Questions
Q: Is A572 Gr.50 a direct replacement for A36?
A: A572 Gr.50 is stronger, with a 345 MPa minimum yield against 250 MPa for A36, and 450 MPa minimum tensile strength against a 400-550 MPa range. It is not a drop-in replacement without verification, because higher strength often comes with a higher yield-to-tensile ratio and different toughness characteristics, and welding procedures qualified for A36 may need re-qualification.
Q: What is the difference between HW, HM and HN sections?
A: The letters describe the width-to-depth relationship. HW sections have a flange width close to the section depth and suit columns, HN sections have a narrower flange for bending efficiency, and HM sections fall between the two. The mass per metre and the section modulus about the weak axis differ accordingly, so the series must be stated on the order.
Q: Can galvanized H beams be welded on site?
A: Welding through a galvanized coating produces zinc fume and a porous weld, and the coating in and around the joint must be removed first. After welding, the affected zone is repaired with a zinc-rich coating in accordance with the repair provisions of ISO 1461. Field welding of galvanized sections is best avoided at the design stage.
Q: How much does rolling tolerance affect the mass of a delivered beam?
A: Rolling tolerances on web and flange thickness are typically a few percent either side of nominal, which translates into a comparable variation in mass per metre. Buyers ordering large tonnages usually require invoicing on actual weighed mass to avoid a systematic difference on a long order.
Q: What length tolerance is normal for hot-rolled H beams?
A: Mill lengths are normally supplied in the 6 m to 12 m range with a positive cut tolerance, and saw-cut fixed lengths are usually held to 0/+5 mm. Tighter tolerances are achievable with cold sawing but should be stated before the order is rolled, because they affect how the mill plans its cuts.
Q: Which grade should be used for a low-temperature application?
A: For service below 0 C, grades with a specified impact class are required. S355J2 to EN 10025-2 or Q355D to GB/T 1591-2018 are common choices, and ASTM A572 material should be ordered with a supplementary impact test requirement. A36 in its basic form carries no low-temperature impact guarantee.