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Steel Box Girders: The Closed-Section Bridge That Spans Further
2026-10-19 18:59:50
Steel Box Girders: The Closed-Section Bridge That Spans Further
Steel box girders are hollow box-shaped superstructure members used in medium-to-long span bridges. Below is data from an actual twin-cell steel box girder bridge drawing.
Basic Specs
| Item | Value |
|---|---|
| Bridge type | Twin-cell steel box girder |
| Deck width | 10,300 mm (5.3 + 5.0 m) |
| Girder depth | 1,600 mm |
| Cantilever | 1,500 mm each side |
| Slope | 1% |
| Steel grade | Q355MD |
| Steel weight | ~41,137 kg (41 t) |
| Deck concrete | C50, 812.45 m³ |
| Prestressing | 24,448 kg strand |
Section Details
The box section consists of:
- Top flange: steel top plate with C50 concrete deck composite
- Bottom flange: steel bottom plate
- Webs: two inclined internal webs + two outer webs
- Cell: twin-cell (two boxes side by side)
- Vents & drains: ventilation holes + drain holes in each cell
Advantages
- Torsional rigidity — closed box section resists torsion far better than I-beams
- Aerodynamic shape — streamlined box resists wind flutter on long spans
- Lightweight per span — box section uses steel efficiently for longer spans
- Composite action — concrete deck works with steel box, higher load capacity
- Clean underside — smooth box bottom looks better than open trusses
Disadvantages / Limitations
- More complex fabrication — box section needs welding inside and out
- Harder to inspect — interior of box needs access hatches for inspection
- Higher cost than I-beams — box girders cost more per meter than plate girders
- Needs drainage & ventilation — enclosed cells need vents and drains
- Transport limits — wide 10 m box sections need special transport
Typical Applications
- Highway bridges — medium-span highway overpasses and viaducts
- Urban elevated roads — city elevated expressways
- Pedestrian bridges — long-span pedestrian bridges
- Curved bridges — horizontally curved bridges need torsional stiffness
- Railway bridges — high-speed rail bridges with dynamic loads
Fabrication Notes
- Steel plates Q355MD, shop-welded into box segments
- Full penetration welds on main web-flange joints
- External blast cleaned and painted; internal coated with anti-corrosion
- Segments transported to site, spliced with bolts/welding
Steel box girder bridge — 10.3 m wide, 1.6 m deep, twin-cell Q355MD steel box, composite with C50 concrete deck, 41 t steel per span, built for highway and urban bridges.
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