Q355NE for Steel Box Girders: Why the Grade and the Test Matter

English Version

Title: Q355NE Box Girder: −40°C, 34 J Impact, No Hot-Rolled Coils Allowed

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Overview

A steel box girder is only as good as the plate it is made from. On a bridge, the steel has to hold a 2,200 mm deep box together in winter cold, under traffic fatigue, for a design life of a century. That is why the specification does not just say "Q355" — it says Q355NE, with impact tests at −40°C and a ban on hot-rolled coil plate.

The drawings show the box section, the bottom plate layout, and the material specification that governs both.

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Material Specification (from Drawing)

表格

Item	Requirement
Plate steel	Q355NE, per GB/T 1591 (low-alloy high-strength structural steel)
Plate source restriction	Hot-rolled coil and opened coil plate not permitted
Charpy impact (plate ≥ 6 mm)	−40°C, ≥ 34 J
Charpy impact (weld metal + HAZ)	−40°C, ≥ 34 J
Cold bend test	180°; d = 2a for t ≤ 16 mm, d = 3a for t > 16 mm
Bend acceptance	No cracking, fracture, or lamination
Tie rods / pins / nuts	45CrNiMo
High-strength bolts	GB/T 1228-2006
Nuts / washers	GB/T 1229-2006 / GB/T 1230-2006
Shear studs	Cylindrical headed studs, GB/T 10433-2002
Support zone infill	C30 concrete or C30 iron-sand concrete

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Section Data (from Drawing)

表格

Item	Value
Box width	17,800 mm
Box height (centre cell)	2,200 mm
Cells	Three-cell box
Web slope	1:38 inclined webs
Cross fall	2%
Span (first span)	29,940 mm
Deck plate width	11,157 mm
Plate zones	2,578 / 3,700 / 800 / 2,433 mm

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Why Q355NE and Not Plain Q355

"N" means normalized or TMCP delivery. The steel is delivered in a controlled-rolling or normalized condition, which gives a fine grain structure and consistent toughness — not the variable properties of hot-rolled coil.

"E" means −40°C impact tested. The letter grade sets the Charpy test temperature. Q355E is tested at −40°C; the 34 J requirement applies to the plate, the weld metal, and the heat-affected zone. A box girder in a cold-climate bridge needs all three.

No hot-rolled coil. Coil plate has residual rolling stress and poorer through-thickness toughness. Bridge box girders use plate rolled on a plate mill, then cut and welded — not coil that has been unrolled and flattened.

45CrNiMo for pins and tie rods. These are the small, highly stressed parts — the connection pins and rods in the box. Alloy steel, not mild steel.

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Advantages

1. Toughness guaranteed at −40°C — plate, weld, and HAZ all tested; brittle fracture risk is controlled, not assumed.
2. Consistent properties — normalized/TMCP delivery avoids the scatter of coil plate; welding procedures are more reliable.
3. Cold bend test backs the toughness — 2a/3a bend at 180° without cracking proves ductility, not just strength.
4. Matched connection materials — GB/T 1228/1229/1230 bolts and 45CrNiMo pins are specified as a system, not bought ad hoc.
5. Composite action at supports — C30 iron-sand concrete infill plus GB/T 10433 shear studs stiffens the support zones where girder forces peak.

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Disadvantages / Limitations

1. Higher plate cost — Q355NE with −40°C impact and plate-mill rolling costs more than ordinary Q355B.
2. Procurement discipline required — the "no coil plate" rule must be enforced at the mill and in the shop; substituting coil plate is a real risk.
3. More testing — Charpy tests on plate, weld, and HAZ, plus bend tests, add inspection cost and schedule time.
4. Welding procedure is stricter — low-temperature toughness in the HAZ demands controlled heat input and preheat; not every shop is set up for it.
5. Infill concrete complicates fabrication — C30 iron-sand concrete and shear studs at the supports add a composite stage inside the box.

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Typical Use Cases

1. Highway bridge box girders — 29.94 m spans with three-cell sections, as in this drawing.
2. Cold-climate bridges — projects where winter service temperature drops below −30°C; Q355E is the minimum grade.
3. Rail and road composite bridges — support zones filled with C30 iron-sand concrete and studded for composite action.
4. Long-span urban viaducts — 17.8 m wide box sections carrying multi-lane traffic with 2% cross fall.
5. Rehabilitation of existing bridges — replacement box segments specified to the same Q355NE and impact requirements.

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Fabrication Notes

• Order plate as Q355NE per GB/T 1591; require mill certificates with −40°C Charpy results.
• Reject hot-rolled coil and opened coil plate at goods receipt — check the mill's delivery condition, not just the grade stamp.
• Charpy test each plate ≥ 6 mm and each weld procedure; acceptance −40°C, ≥ 34 J for plate, weld, and HAZ.
• Bend test: 180°, d = 2a for t ≤ 16 mm and 3a for t > 16 mm; reject any cracking, fracture, or lamination.
• Use GB/T 1228/1229/1230 bolts and 45CrNiMo pins; do not substitute unlisted fasteners.
• Support zones: place GB/T 10433 cylindrical headed studs on bottom plate, webs, diaphragms, and end plates before pouring C30 iron-sand concrete.

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Bottom Line

Q355NE is not a marketing grade — it is a specification with three hard requirements: plate-mill steel, −40°C impact of 34 J on plate/weld/HAZ, and a 180° bend that proves ductility. Add 45CrNiMo pins, GB/T 1228 bolts, and C30 iron-sand infill at the supports, and the box girder is built to survive cold, fatigue, and time. Skip the coil-plate ban or the impact tests, and the grade is just a stamp on a drawing.

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