1. Why Bridge Steel Must Be Controlled
Bridge members are subjected to repeated traffic loads, wind, temperature changes and, in some regions, seismic action. The steel must therefore combine adequate strength with guaranteed toughness and good weldability. Angles used in bridge trusses, cross-frames, bracing and gusset connections are specified with tight limits on chemical composition and mechanical properties, and each heat is tested and documented before delivery.
2. Classic European Grades: St37-2 and St52
St37-2 is the former DIN designation for a mild structural steel with a minimum yield strength of 235 MPa, now replaced in EN 10025 by S235JR. St52 is the former designation of the 355 MPa yield strength grade, now supplied as S355JR or S355J2. Both names remain common in specifications written for older plants and in markets where DIN practice is still followed, and the current EN grades are considered the direct equivalents.
3. High-Strength Bridge Grades: Q420 and Q460
Q420 and Q460 are high-strength low-alloy grades defined in GB/T 1591, with minimum yield strengths of 420 MPa and 460 MPa respectively for thin material. They are used in long-span bridge trusses and girders where the higher strength reduces the self-weight of the structure and the size of the members. Their chemical design includes micro-alloying with vanadium, niobium or titanium, and the welding procedure must be qualified to prevent cold cracking in the heat-affected zone.
4. Angle Sections in Bridge Construction
Angles appear throughout steel bridges: as diagonal web members in trusses, as lateral bracing between girders, as cross-frame members and as edge stiffeners. Paired angles back-to-back form chords and struts, while single angles connect gusset plates with bolted or welded joints. The selection of leg size and thickness follows the member force, the slenderness limits of the design code and the detailing rules for gusset connections.
5. Testing and Quality Assurance for Bridge Steel
Bridge steel is delivered with mill certificates covering the chemical composition, tensile properties and Charpy impact energy of each heat. Depending on the specification, additional testing includes ultrasonic examination, through-thickness property verification and weldability tests. Traceability is maintained by stamping or marking each angle with the heat number, so that every member can be traced back to its test results.
Frequently Asked Questions
Q: What is St37-2 equivalent to in EN 10025?
St37-2 corresponds to S235JR, with a minimum yield strength of 235 MPa and guaranteed impact energy.
Q: What is the yield strength of St52?
St52 has a minimum yield strength of 355 MPa and is supplied today as S355JR or S355J2 depending on the toughness requirement.
Q: When are Q420 and Q460 used?
They are used in long-span bridges and heavy structures where the higher strength reduces member sizes and self-weight.
Q: Are high-strength angles harder to weld?
They require qualified welding procedures and often preheat, because the higher alloy content increases the risk of hydrogen cracking.
Q: Why are angles used in bridge trusses?
Angles are efficient for bracing and web members because the L-shape carries load well in two directions and simplifies bolted connections at gusset plates.
Q: What documentation is required for bridge steel?
Mill certificates with heat traceability, chemical composition, tensile and impact test results are standard, with additional NDT where the specification requires it.