29.94 m Span, 37.9 m Wide: Multi-Cell Steel Box Girder

English Version

Title: 29.94 m Span, 37.9 m Wide: Steel Box Girder That Crosses a Live Highway

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Overview

A steel box girder is the go-to choice when a bridge has to cross something that cannot be closed — a motorway, a canal, a rail line. The girder is fabricated off site, lifted in a few nights, and the traffic below never stops for long.

This is the first span of such a bridge: 29.94 m, 37.9 m wide, multi-cell boxes under each half, 2.2 m deep at the supports and 2.0 m at mid-span.

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Geometry (from Drawing)

 

Item	Value
Span	29,940 mm
Drawn length	30,000 mm (incl. 60 mm at each end)
Girder depth	2,200 mm at supports / 2,000 mm at mid-span
Overall width	37,900 mm
Half width	17,000 mm each side
Cross fall	1.5%
Vent holes	φ80
Drainage holes	φ50

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Deck Layout (from Drawing)

 

Item	Value
Footway	2,000 mm
Cycle lane	3,500 mm
Motor lanes	3,500 / 3,250 / 3,250 mm per half
Wearing course	9 cm asphalt concrete
Waterproofing	membrane between asphalt and concrete
Leveling layer	8 cm C50 concrete

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Box Section and Segments

表格

Item	Value
Section	Multi-cell box, three cells per half plus cantilever
Webs	Inclined
Segment chain	60 / 590 / 1,210 / 2,000 / 4,000 / 2,500 / 12,540 / 6,000 / 1,100 mm
Bearings	Bearing center lines marked at both ends, with bearing pads

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Substructure (from Drawing)

 

Item	Value
Cap beam	200 mm deep, 215 mm cantilever each side
Column	Tapered (vase-shaped) pier
Piles	D1.0 / D1.2, spaced 300 mm apart in the group
Site	Bridge crosses the Shenyang–Dalian expressway and the Bayi canal

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Advantages

Erection without closing the road — segments are lifted at night; the expressway below stays open.

Light for its span — a 29.94 m steel box weighs a fraction of a concrete girder, so smaller cranes and lighter temporary works.

Multi-cell stiffness — three cells per half resist torsion and local buckling; the deck does not need a thick slab.

Shallow depth — 2.0–2.2 m for a 30 m span leaves room for clearance and approach grades.

Factory quality — welding and coating are done indoors, inspected before the girder leaves the shop.

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Disadvantages / Limitations

Welding volume is huge — every cell, diaphragm, and stiffener is welded; distortion control is a full-time job.

Fatigue at details — box girders live with cyclic traffic load; weld details at diaphragms and stiffeners need careful design.

Coating maintenance — a box interior is hard to inspect and repaint; closed cells need dehumidification or sealed access.

Erection needs road closure windows — even night lifts need permits, cranes, and traffic management.

Cost — steel box girders are expensive per tonne and per square metre compared with a concrete deck.

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Typical Use Cases

Expressway overpasses — crossing a live motorway, as in this drawing.

Canal and river crossings — spans where a concrete girder would need too much temporary support.

Urban viaducts — wide decks (37.9 m here) over traffic that cannot stop.

Rail and metro overpasses — short erection windows, high precision.

Bridge replacement — new steel span lifted into an existing alignment.

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Fabrication and Erection Notes

Fabricate the box in cells: bottom plate, webs, diaphragms, top plate; check camber before closing the top.

Vent holes φ80 and drain holes φ50 per drawing; keep them clear and locate them at low points.

Coat the interior before closing; seal cells and record access points.

Leveling layer: 8 cm C50 over the steel deck, then waterproofing, then 9 cm asphalt — cure before paving.

Bearings: set pads on the bearing center lines at both ends; check the 2,200 mm end depth against the support.

Erection: night lifts with a road closure permit; check the cross fall (1.5%) before releasing the crane.

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Bottom Line

29.94 m of span, 37.9 m of width, three cells per half, 2.0–2.2 m deep — this is a box girder built to cross a highway without stopping it. The steel does the work; the 8 cm C50 layer, the waterproofing, and the 9 cm asphalt keep the deck serviceable. Fabricate it straight, coat it inside, and lift it at night

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