What Is a Welded Steel Box Girder?
A welded steel box girder is a closed-section beam built by welding steel plates into a rectangular or trapezoidal cell. The top plate carries the deck or applied load, the web plates take shear, and the bottom plate completes the compression and tension couple under bending. Because the section is closed, off-centre loads set up torsion, and the closed cell resists that torsion with far less added material than an open section would need. This is the same structural family as the concrete box girder used on modern highway viaducts; the welded steel version is lighter, can be fabricated in segments, and suits projects where erection speed, long spans or curved alignments matter.
Main Components
| Component | Function | Typical form |
|---|---|---|
| Top flange / deck plate | Carries direct loads; part of the bending couple | Flat plate, often stiffened; orthotropic deck with longitudinal ribs where the deck is the top plate |
| Web plates | Transfer shear between flanges; define the section depth | Vertical or inclined plates; thickness set by shear and buckling checks |
| Bottom flange | Completes the bending couple under positive moment | Flat plate, wider or thicker in hogging zones of continuous girders |
| Diaphragms | Keep the cell cross-section stable; distribute concentrated loads | Solid plates or braced frames at intervals along the span |
| Longitudinal and transverse stiffeners | Prevent local buckling of thin panels | Flat bars, bulb flats or angles welded to the web and flanges |
Cross-Section Types
Based on the number of cells, steel box girders are classified into single-box single-cell, single-box multi-cell and multi-box types.
| Type | Cross-section | Where it is used |
|---|---|---|
| Single-box single-cell | One closed cell | Narrow decks, short-to-medium spans, urban ramps, pedestrian bridges |
| Single-box multi-cell | One box split by internal webs (twin-cell, triple-cell) | Wide highway decks, elevated expressways, continuous and rigid-frame bridges |
| Multi-box | Two or more separate boxes under one deck | Ultra-wide mainlines, interchange ramps, long spans with separate carriageways |
The cell layout is a structural choice driven by deck width and plate slenderness: wider decks push toward multi-cell or multi-box arrangements to control the distance between webs and the deck plate.
Design Loads and Governing Codes
A box girder is designed for dead load, traffic live load, wind, seismic action and fatigue, using one governing design code. The table lists the principal codes used in international bridge projects and what each one governs.
| Code / Standard | Issuing body | Scope |
|---|---|---|
| AASHTO LRFD Bridge Design Specifications | American Association of State Highway and Transportation Officials (AASHTO) | US and North American highway bridges; load and resistance factor design; HL-93 vehicular live load; Article 6.10 steel girder provisions |
| EN 1993-2 with EN 1991 actions | European Committee for Standardization (CEN) | Steel bridges in the EU and associated states; stability, fatigue and plate buckling per EN 1993-1-5; execution per EN 1090-2 |
| JTG D64-2015 | Ministry of Transport, PRC | Highway steel bridges in China; commonly used as a reference for export projects in Southeast Asia |
| GB/T 714 | Standardization Administration of China (SAC) | Bridge steel grades Q345q, Q370q, Q420q and others: chemical composition, mechanical properties and impact toughness |
| AWS D1.5 Bridge Welding Code | American Welding Society (AWS) | Weld procedures, welder qualification, weld acceptance criteria and NDT requirements for bridge steel |
| ISO 8501-1 | International Organization for Standardization (ISO) | Surface preparation grades for coating; Sa 2.5 denotes very thorough blast cleaning |
Plate Thickness, Welds and NDT
There is no standard plate thickness for a box girder. Web thickness is set by shear strength and local buckling limits, for example AASHTO LRFD Article 6.10 or EN 1993-1-5; flange thickness is set by bending, compression and fatigue checks. The correct values for a specific bridge come from the design calculation and are confirmed in the fabrication drawing set.
Full-penetration groove welds join the flange to the web and connect girder segment splices where the joint is in tension or fatigue-critical. Fillet or partial-penetration welds attach stiffeners, diaphragms and non-critical details. Weld quality is accepted per AWS D1.5 or EN ISO 5817, with ultrasonic testing (UT) for full-penetration butt splices, magnetic particle testing (MT) for surface crack inspection, and radiographic testing (RT) for complex deep weld joints.
Advantages, Limitations and Main Applications
| Advantages | Limitations |
|---|---|
| High torsional stiffness without external bracing, suited to curved and eccentrically loaded bridges | More plate and weld length than an I-girder, so higher fabrication cost per tonne |
| Lighter than a concrete box of equal capacity, reducing pier and foundation loads | Closed cells limit access for inspection; internal access hatches must be planned |
| Long-span capability with fewer intermediate piers | Transport and lifting of large segments require planning and permits |
| Factory prefabrication: fast site erection, minimal wet work and less traffic disruption | Fatigue-sensitive welded details require strict weld quality control |
Main applications include highway bridges and urban viaducts, curved bridges and interchange ramps, cable-stayed and suspension bridge main girders, pedestrian bridges and light-rail viaducts, and heavy industrial structures such as crane runway girders and long-span roof frames.
Fabrication Sequence
CNC plate cutting and component machining: top and bottom plates, webs, diaphragms and stiffeners are cut from mill-certified plate, with splice notches and weld grooves pre-machined.
Groove milling and main welding: long primary seams are welded with automatic submerged arc welding (SAW) for stable penetration; complex joints use qualified manual or semi-automatic procedures.
Assembly on jigs: components are set on dedicated assembly jigs and checked for flatness, verticality and internal spacing before tack welding.
Post-weld straightening, NDT, shot blasting, coating and pre-assembly: full dimensional inspection and NDT come first, then blast cleaning to Sa 2.5, zinc-rich primer and finish coats; complete segments can be trial-assembled in the shop before dispatch.
Frequently Asked Questions
What design loads must a welded steel box girder resist? box girder is checked for dead load, traffic live load, wind, seismic action and fatigue. In North America the design follows AASHTO LRFD with the HL-93 vehicular live load; in Europe, EN 1991 actions combined with EN 1993-2; in China, JTG D64-2015. The governing load case is confirmed in the design report before fabrication.
How thick should the plates of a box girder be? There is no standard value. Web plates are sized from shear and local buckling checks, for example AASHTO LRFD Article 6.10 or EN 1993-1-5, and flange plates from bending and compression checks. The correct values for a specific bridge come from calculation, so the supplier should provide the design report.
What weld classes are used on welded steel box girders? Full-penetration groove welds join the flange to the web and segment splices; fillet or partial-penetration welds attach stiffeners and diaphragms. Fabrication follows AWS D1.5 for bridges or EN ISO 5817 quality levels, with NDT at the ratio required by the governing code and project specification.
What limits the transport of a box girder segment? Road width, length, height and axle-load limits set the maximum segment size. Large girders are split into modular segments at the design stage, with splice locations marked for field bolting or welding. Oversized-load permits should be checked with the logistics provider.
Box girder or I-girder, which costs less? For the same span, an I-girder is cheaper to fabricate per tonne because it uses less plate and less weld length. A box girder costs more in material and welding but resists torsion and lateral loads without a bracing system, which often saves on wide or curved decks and on substructure work. The comparison should be based on total installed cost, not unit price.
How long can a steel box girder span? Welded box girders cover a wide range from short urban ramps to long continuous highway crossings, and even longer main girders in cable-stayed or extradosed bridges. The feasible span for a project is established by a design and fabrication study; the supplier should return a feasibility check with section sizes for the stated span and loading. Browse our full range of steel columns and beams for bridge and building structures.