Why Six Grades Cover Most Structural Projects
H beams are hot rolled I-shaped sections with parallel flanges, chosen for their high section modulus relative to weight. The grade determines strength, weldability, impact performance and the regional code under which the section is accepted. Q235 and Q345 belong to the Chinese GB system, A36 to the ASTM system, S235JR and S355JR to the European EN 10025-2 system, and SS400 to the Japanese JIS G3101 system. Understanding where each one sits makes it possible to compare bids that quote different designations for the same structure.
Mechanical Properties Side by Side
| Grade | Standard | Minimum yield strength | Tensile strength | Typical elongation |
|---|---|---|---|---|
| Q235B | GB/T 700 | 235 MPa | 375–500 MPa | 26 % |
| Q345B | GB/T 1591 (now Q355) | 345 MPa | 470–630 MPa | 21 % |
| A36 | ASTM A36/A36M | 250 MPa | 400–550 MPa | 20–23 % |
| S235JR | EN 10025-2 | 235 MPa | 360–510 MPa | 26 % |
| S355JR | EN 10025-2 | 355 MPa | 470–630 MPa | 22 % |
| SS400 | JIS G3101 | 245 MPa | 400–510 MPa | 17–21 % |
Q345 has been renamed Q355 in GB/T 1591 with the same nominal yield strength of 355 MPa; drawings issued before that revision still show Q345, and the substitution is direct. Values above apply to the normal thickness range up to 16 mm, which covers the majority of building sections. Yield strength falls slightly as flange thickness increases, so the thickness-dependent limits in the standard always govern.
Chemistry and Weldability
Q235 and S235JR are low-carbon steels with carbon held to about 0.17–0.20 %, which gives excellent weldability with ordinary electrode and wire classifications and no mandatory preheating at normal plate and flange thicknesses. A36 sits in the same low-carbon family with a carbon limit of 0.26 % and a manganese range of 0.80–1.20 %. SS400 behaves similarly and is widely welded without special measures. Q345 and S355JR are higher-strength steels, either by increased manganese or by micro-alloying with elements such as niobium and vanadium, and they carry a steady carbon equivalent that calls for controlled heat input, dry electrodes and, in thick sections, preheating or post-weld treatment to avoid hydrogen-assisted cold cracking. In every case, weldability also depends on section thickness and restraint, not on grade alone.
Size Range and Section Design
Standard type: height from 100 mm to 900 mm, flange width typically from 100 mm to 400 mm, web thickness from 6 mm to about 20 mm and flange thickness from 8 mm to 30 mm.
Common sizes: 100×100, 150×150, 200×100, 200×200, 300×150, 300×300, 400×200 and larger built-up or rolled sections.
Dimensions and tolerances: Chinese H beams are covered by GB/T 11263 and the hot rolled section steel standard GB/T 706, European wide flange beams by EN 10365, and Japanese sections by JIS G3192, with the general requirements of ASTM A6/A6M applying to North American shapes.
Delivery: standard lengths of 6 m, 9 m and 12 m, with cutting to fixed length available before shipment.
Choosing a Grade for the Application
Q235, S235JR and SS400 suit low-rise frames, warehouses, floor beams, purlins and general fabrication where loads are moderate and economy matters. A36 performs the same role in North American practice and is widely used for beams, channels, angles and connections in buildings and bridges. Q345 and S355JR are selected for large-span roofs, heavy industrial buildings, crane runway beams, transmission towers and bridge girders, where the higher yield strength allows a lighter section and therefore a smaller self-weight and reduced foundation load.
Interchangeability and Verification
Grades are not freely interchangeable. Replacing a higher-strength grade with a lower-strength one changes deflection, load capacity and safety factors, and is not acceptable in a design that was checked against the higher grade. Design codes also tie acceptance to specific standards, so an A36 beam may not satisfy a European verification without conversion. Each delivery should be supported by a mill test certificate giving heat number, chemical analysis and tensile results, plus dimensional records showing height, flange width, web thickness, flange thickness and straightness within the limits of the applicable section standard.
Frequently Asked Questions
Q: What is the practical difference between Q235 and Q345 H beams?
The difference is strength. Q235 offers a minimum yield strength of 235 MPa and is used for ordinary frames and lighter loads, while Q345 reaches 345 MPa and is chosen where bending moments and deflections are large, such as long-span girders or heavily loaded industrial floors. The higher grade usually allows a shallower or thinner section for the same duty, which can offset its higher price per tonne. Deflection limits and connection design still have to be checked against the lighter section.
Q: Are A36 and S235JR H beams equivalent?
They are close but not identical. A36 has a minimum yield strength of 250 MPa and a tensile range of 400–550 MPa, while S235JR has a minimum yield of 235 MPa and a tensile range of 360–510 MPa. Both weld easily and both are used for general structures. The difference is the governing standard and the certification trail, not the fabrication behaviour, so substitution in either direction needs the approval of the designer.
Q: Why is S355JR preferred for bridge and crane structures?
Its 355 MPa yield strength carries more load in the same depth, and its guaranteed 27 J impact energy at +20 °C gives the toughness needed where dynamic or repeated loading occurs, as in crane runways and highway bridge girders. The lower self-weight that comes with a smaller section also reduces the load that the substructure has to carry. Welding procedures must be qualified for the higher strength and the carbon equivalent involved.
Q: Do these H beams need surface protection?
Yes, unless the environment is dry and controlled. Uncoated beams form mill scale during rolling, which gives limited temporary protection, but moisture, chlorides and industrial pollutants will start corrosion at edges and connection details. Hot-dip galvanizing to EN ISO 1461 is the usual choice for outdoor and coastal work, while blast cleaning followed by an epoxy or polyurethane paint system is used where colour and long-term appearance matter. Fire protection is a separate requirement in buildings.
Q: Can H beams be supplied cut to length and with holes or plates?
Beams are normally delivered in standard 6 m, 9 m or 12 m lengths and can be sawn to fixed length to reduce site waste. Cutting tolerance is wider than the tolerance on the rolled section, so the finished length and its tolerance should be stated on the order. Drilling, notching and welded end plates are workshop operations that are usually performed after delivery, since they affect the coating system and must be dimensioned against the final connection design.
Q: What documents should accompany an H beam delivery?
Expect a mill test certificate to EN 10204 3.1 for every heat, showing chemical composition and mechanical test results including yield strength, tensile strength, elongation and impact energy where specified. Dimensional inspection records should confirm height, flange width, web and flange thickness, length and straightness. For galvanized or coated sections, a coating certificate stating the standard applied and the measured thickness completes the set. These records allow the section to be traced back to the producing heat if a query arises later.