In GOST, hollow sections are split between two dimension standards: GOST 8639-82 for square, GOST 8645-68 for rectangular; the technical requirements for both are set by GOST 13663-86, and for building structures GOST 30245-2003 on cold-formed closed welded sections applies. In China all of this is one standard, GB/T 6728-2017, cold-formed hollow sections for structures. The European EN 10219-1 and the American ASTM A500 describe the same product: a welded tube formed from strip into a square or rectangle.
How to write the size
The section size is three numbers: side a, side b and wall thickness t, all in millimetres: 60×40×3. A square has two equal sides, but both are written: 100×100×5. Length is separate, usually 6 or 12 m. The outside corner radius is set by the standard from the thickness: under EN 10219-1 for walls up to 6 mm it is 1.6 to 2.4 t, for 6–10 mm 2.0 to 3.0 t; GOST 8639 and GB/T 6728 set the radius by their own tables. The radius affects the mass and how the section sits in a gusset plate, so when joining to an existing structure it is written into the specification.
Where the 2.86 in the mass formula comes from
The mass of a metre of section is the cross-section area multiplied by density. The section is the perimeter multiplied by the thickness minus the four rounded corners. For an outside radius of 1.5 t and an inside radius of 0.5 t the corner correction equals (6 − π) · t² = 2.858 t², and after multiplying by 7.85 / 1 000 the formula folds into 0.0157 · t · (a + b − 2.86 · t). GOST 8639 and GB/T 6728 compute their table masses on the same principle, so the mass in the catalogue, the mass in a GB price list and the mass under GOST agree within rounding.
Formula
m = 0.0157 · t · (a + b − 2.86 · t)
m is the mass of 1 m in kg; a and b are the sides, t the wall thickness, all in mm.
| a×b×t, mm | Mass, kg/m | Metres per tonne | 6 m lengths per tonne |
|---|---|---|---|
| 40×40×2 | Mass, kg/m2.33 | Metres per tonne428.7 | 6 m lengths per tonne71 |
| 40×40×3 | Mass, kg/m3.36 | Metres per tonne297.3 | 6 m lengths per tonne49 |
| 50×50×3 | Mass, kg/m4.31 | Metres per tonne232.2 | 6 m lengths per tonne38 |
| 60×40×3 | Mass, kg/m4.31 | Metres per tonne232.2 | 6 m lengths per tonne38 |
| 80×80×4 | Mass, kg/m9.33 | Metres per tonne107.2 | 6 m lengths per tonne17 |
| 100×100×5 | Mass, kg/m14.6 | Metres per tonne68.6 | 6 m lengths per tonne11 |
| 120×120×5 | Mass, kg/m17.7 | Metres per tonne56.4 | 6 m lengths per tonne9 |
| 150×150×6 | Mass, kg/m26.6 | Metres per tonne37.5 | 6 m lengths per tonne6 |
The 60×40×3 and 50×50×3 sections weigh the same: their perimeters are equal, and the mass depends only on perimeter and thickness. This is a convenient way to check a mill’s price list: two lines with an equal sum of sides and equal wall must show one mass.
Steel grade and the letter H
A section is formed from strip, and its steel grade is that of the strip: St3sp or 09G2S under GOST, Q235B or Q355B under GB. EN 10219-1 adds the letter H to the grade, S235JRH, S355J2H, meaning hollow, and it carries its own test requirements for the finished section rather than the original strip. ASTM A500 names no steel grades at all: Grade B and Grade C are yield strength levels of the finished section, 315 and 345 MPa for square and rectangular shapes. So the specification “section 100×100×5, steel St3sp” translates for a Chinese mill as “GB/T 6728, Q235B” with a note that the yield strength is confirmed by the certificate for the section.
Grade equivalents
| GOST | GB (China) | ASTM | EN | Note |
|---|---|---|---|---|
| 09G2S | GB (China)Q355B | ASTMA572 Gr.50 | ENS355J2H | NoteLow-alloy steel, nearest equivalents. Before GB/T 1591-2018 the grade was designated Q345B; A572 is a standard for rolled products, not for pipe; S355J2H is a hollow section grade under EN 10219 |
| St3sp | GB (China)Q235B | ASTMA500 Gr.B | ENS235JRH | NoteHollow sections. A500 specifies a higher yield strength than Q235B; structural design uses the standard stated in the specification |
Equivalents are the closest match, not identical: chemistry and tolerances differ between standards. The exact grade and standard are written into the contract.
Theoretical weight at a steel density of 7.85 g/cm³, calculated by the same function as the catalogue tables and the calculator. The actual weight of the batch is by scale and mill certificate.
