What Is a Steel Box Girder?
A steel box girder is a structural member with a closed, hollow rectangular or trapezoidal cross-section, fabricated by welding steel plates into a box formed by a top flange, a bottom flange and two webs, with internal diaphragms and longitudinal stiffeners. Because the section is closed, it resists torsion far more effectively than an open I-section of similar weight, which makes it the standard main girder for curved alignments, long-span continuous bridges and cable-supported bridges.
Key components
Top flange: carries compression and, in deck-type girders, supports the roadway.
Bottom flange: carries tension.
Webs: vertical or inclined plates that carry shear between the flanges.
Internal diaphragms and cross-frames: maintain the cross-section shape at intervals and distribute concentrated loads such as bearing reactions and cable anchorages.
Longitudinal stiffeners: flat ribs or closed U-ribs that control local buckling of the thin plates; where the top flange is the traffic deck, the stiffened plate system is called an orthotropic steel deck.
Common cross-section forms
Single-box single-cell: one closed cell; good torsional stiffness for medium-width decks.
Single-box twin-cell or multi-cell: internal webs distribute load across wider decks, typical of highway main lines.
Multi-box (twin or multiple boxes): separate boxes side by side for extra-wide or combined highway-railway bridges; slotted twin-box sections are also used to improve aerodynamic stability on very long spans.
Why Steel Box Girders Suit Long-Span Bridges
High strength-to-weight ratio: steel offers one of the highest strength-to-self-weight ratios of any bridge material. On long spans the superstructure's own weight dominates the total load, so a lighter girder reduces pylon, cable and foundation demand and can extend the feasible main span.
Exceptional torsional stiffness: the closed cell resists twisting by shear flow around the full perimeter, controlling distortion under eccentric traffic loads and wind.
Aerodynamic profile: the flat, streamlined shape reduces drag and raises the critical wind speed for flutter, which is essential for cable-stayed and suspension bridges in coastal or typhoon-prone regions.
Shop prefabrication: segments are fabricated, trial-assembled and match-marked in the factory, which shortens the on-site erection window compared with cast-in-place concrete construction.
Typical Fabrication Specification
The table below is a starting point for a fabrication inquiry. Final plate sizes, steel grades and details are fixed by the project's structural design under the governing code.
| Parameter | Typical range / options | Standard / note |
|---|---|---|
| Girder type | Single-box or multi-box; constant or variable depth | Per structural design |
| Span | Continuous girders from about 30-200 m; stiffening girders for cable-stayed and suspension bridges above 200 m | No single global definition of long-span; the applicable design code governs |
| Steel grades | Q345q, Q370q, Q420q; S355J2, S355NL; ASTM A709 Grade 50 / 50W; JIS SM490C | GB/T 714; EN 10025-2/-3; ASTM A709 / AASHTO M270; JIS G 3106 |
| Plate thickness | Flange plates and web plates sized by calculation; thick plates require through-thickness (Z-direction) properties per GB/T 5313 | Set by structural calculation |
| Surface preparation | Blast cleaning to Sa 2.5 | ISO 8501-1 |
| Coating system | Multi-layer system selected by corrosivity category, e.g., zinc-rich primer, epoxy intermediate coat, polyurethane topcoat; or hot-dip galvanizing | ISO 12944 categories C2-C5-M |
| Connections | Shop welding (SAW / FCW) plus high-strength bolted field splices | Welding quality per EN 1090, ISO 3834, GB 50661 |
Erection Methods for Long-Span Steel Box Girders
Segmental crane erection: segments are lifted from barges or the ground by floating or land cranes and spliced in place, typical for medium spans and accessible sites.
Incremental launching: segments are assembled on the shore or abutment and pushed across the piers with launching noses, suited to continuous girders over valleys or water where crane access is limited.
Cantilever (balanced) erection: segments are placed alternately from the towers or piers outward, the standard method for cable-stayed and long-span continuous bridges; the girder acts as a cantilever until closure.
Full-span lifting: where transport permits, whole spans or large units are lifted into position, minimizing on-site splicing.
Whatever the method, trial assembly, match-marking and an erection manual prepared by the fabricator reduce site risk and shorten the construction window.
Selection Checklist for Buyers
Provide the bridge layout: span arrangement, deck width, alignment (straight or curved) and the governing design code of the project country.
Define the steel grades, including impact (Charpy) requirements for the service temperature.
State the corrosion environment and select the coating system to the corresponding ISO 12944 corrosivity category.
Confirm segment length and weight limits based on transport corridors and available lifting equipment.
Agree on the erection method so that fabrication details (lifting points, temporary bracing, splice locations) match the site plan.
Specify the QC documentation: material certificates, weld inspection (NDT) reports, dimensional reports, coating thickness reports and shop pre-assembly reports.
Request a Steel Box Girder Fabrication Quote
FAQ
What is the difference between a steel box girder and a plate girder?
A plate girder has an open I-shaped section made of web and flange plates. A box girder closes the section with a bottom flange, forming a hollow cell. The closed cell gives the box girder much higher torsional stiffness, which is why box girders are used for curved alignments, wide decks and long cable-supported spans.
Why are steel box girders preferred for spans above 200 m?
Beyond roughly 200 m, the self-weight of a concrete superstructure becomes a large share of the total load and penalizes foundations, pylons and cables. A steel box girder is much lighter, provides high torsional stiffness, and can be streamlined for wind, which together make longer spans feasible.
What is an orthotropic steel deck?
An orthotropic steel deck is a thin steel plate stiffened by closely spaced longitudinal ribs (typically closed U-ribs) and transverse bulkheads, acting as the top flange of the girder and the roadway at the same time. It saves significant dead weight on long spans but requires careful fatigue detailing of the rib-to-deck welds.
Which steel grades are used for steel box girders?
Common grades are Chinese bridge steels Q345q, Q370q and Q420q to GB/T 714, European grades S355J2 and S355NL to EN 10025, and US grades ASTM A709 Grade 50 / 50W. Weathering bridge steels may be used in coastal environments to reduce coating maintenance.
How are box girder segments connected on site?
Segments are joined by full-penetration shop-style welds at the flange and web splices, or by high-strength bolted splices, depending on the design and the code. Matching surfaces are prepared in the factory, segments are match-marked after trial assembly, and erection follows the fabricator's erection manual.
What anti-corrosion systems are used for marine environments?
For coastal and marine sites, surfaces are blast cleaned to Sa 2.5 per ISO 8501-1 and coated with a system selected for the ISO 12944 corrosivity category, for example zinc-rich primer, epoxy intermediate coat and polyurethane topcoat. Internal dehumidification systems may also be specified for closed box interiors.