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Box Girder Cross Beams: The Hidden Links That Keep Twin-Box Bridges Square
2026-10-19 19:26:45
Box Girder Cross Beams: The Hidden Links That Keep Twin-Box Bridges Square
Cross beams (diaphragm beams) connect adjacent box girders, transfer lateral loads and keep the bridge deck from twisting. Below is G-class midspan crossbeam from an actual twin-box bridge drawing.
Basic Specs
| Item | Value |
|---|---|
| Member ID | G-class midspan crossbeam |
| Location | Between twin box girders |
| Material | Q355MD |
| Beam type | Welded H-section |
| Total length | ~2,400 mm |
| Total depth | ~1,000 mm (972 + 2×14) |
| Unit weight | ~739.4 kg |
| Paint area | 14.8 m² |
Member Schedule
表格
| Part | Spec | Qty | Unit Wt | Total Wt |
|---|---|---|---|---|
| Web N21 | 2400×972×12 mm | 2 pcs | 219.7 kg | 439.5 kg |
| Top flange N22 | 280×14×2103 mm | 2 pcs | 64.7 kg | 129.4 kg |
| Bottom flange N23 | 280×14×2103 mm | 2 pcs | 64.7 kg | 129.4 kg |
| Stiffener N24 | 120×10×800 mm | 4 pcs | 7.5 kg | 30.1 kg |
| Subtotal | — | — | — | 728.5 kg |
| Welds (1.5%) | — | — | — | 10.9 kg |
| Total | — | — | — | 739.4 kg |
How It Works
Cross beams connect twin box girders to form a unified deck:
- Distributes load — lateral loads from one box transfer to the other
- Prevents distortion — keeps both boxes aligned, prevents relative twisting
- Transverse bracing — acts as transverse bracing for the whole deck
- Diaphragm effect — stiffens the deck against uneven loading
Advantages
- Unified deck action — twin boxes work as one through crossbeams
- Lateral load distribution — wind, seismic and eccentric loads spread evenly
- Welded H-section efficiency — 1 m deep beam is strong for its weight
- Q355MD toughness — low-temperature impact toughness for bridge use
- Bolted to boxes — field bolted, no welding on main boxes
Disadvantages / Limitations
- Adds weight — crossbeams add dead load to the bridge
- Harder to inspect — connections between boxes are hard to reach
- Fatigue risk at connections — cyclic traffic loads stress bolted joints
- Spacing optimized by analysis — too far apart means deck deflection
- Additional fabrication — each crossbeam is a separate welded unit
Typical Applications
- Twin-box girder bridges — two side-by-side box girders
- Wide highway bridges — deck width > 12 m needs twin boxes
- Curved bridges — crossbeams maintain box alignment on curves
- Urban elevated roads — wide elevated decks with twin boxes
- Orthotropic deck bridges — crossbeams stiffen the deck plate
Fabrication Notes
- Web and flanges CNC cut, shop-welded into H-section
- End plates welded for bolted connection to box girder
- Stiffeners welded at shear points
- Blast cleaned and painted, 14.8 m² surface per beam
Box girder crossbeam — 2,400 mm long, 1,000 mm deep welded H-section, Q355MD, 739 kg per unit, bolted between twin box girders for transverse deck stability.
**WhatsApp**: +86‑13889862778, +86‑18242444407
**Email**: eddie@zdsteels.com, wells@zdsteels.com
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