What is a steel box girder?
Steel box girders are a type of bridge girder, named for the fully sealed hollow box structure and material. They are the most important structural member of steel box girder bridges. There are also many composite box girder types made of concrete and various steel structures. Steel box girders are fabricated in the structure forms of single box, single-cell, multi-cell, double-box, with customized profile shapes of rectangular, trapezoidal, etc.
Steel Material Options: Steel box girders are made from steel plates featuring high-strength and corrosion resistance, such as weathering steels and bridge structure steel. The strength of Q345qD steel plate exceeds 345 MPa and maintains high toughness at -40°C. The 100 mm thick plate has uniform performance and meets the strict requirements of a steel box girder. After precision welding, the steel box girders are widely used in various long-span bridges (highway bridge, railway bridge, cable-stayed bridges), curved structures, and heavy-duty cranes.
What AGICO offers: Besides steel box girders, AGICO also provides full fabrication services of all kinds of large steel structure members in bridge projects, such as large steel box columns, steel pipe piles for bridge foundations, steel truss structures, and embedded steel plate parts: bridge bearing plate, embedded plate for catenary system and wind barrier, T section embedded plate, walkway cross beams, etc.
Contact AGICO now if you need professional bridge steel structure fabrication service!
Steel Box Girder Specification
AGICO supplies various types of steel box girders for bridges, including single and double box girders, single/double and multi-cell girders.; segment box girders, curved box girders, variable section girders, multi-cell box girders, and orthotropic deck integration. We offer one-stop fabrication from plate cutting to full girder assembly.
Steel Box Girder Fabrication Details:
- Girder type: Single box / multi-cell
- Span: 30 m – 200m
- Roadway width: 8 m – 42m
- Material Grade: Bridge structure steel Q345qC/D/E, Q370qE, Q420qE, ASTM A709 Gr.50/Gr.50W, EN 10025 S355J2/S355NL, JIS SM490, etc.
- Load Capacity: Designed to meet AASHTO HL-93 loading
- Surface Treatment: Shot blasting: Sa2.5
- Coating system: Zinc-rich primer, epoxy intermediate, and polyurethane topcoat.
- Joint System: Bolt & Nut fastener.
Note: The above values are typical for reference purposes only—contact us if you need the latest design and engineering specifications.
Steel Box Girder Fabrication Process
The fabrication of steel box girders for a bridge is a complex process involving multiple steps. The following has listed the main technological processes, production equipment, production standards, and key quality control points for producing steel box girders. The preparation step is raw material lofting: verify the steel plate’s composition, specifications, and appearance quality, and correct or clean as necessary. Allow for welding shrinkage during layout, based on construction drawings and process requirements, and allow for machining allowances. Once the material passes inspection, steel box girder fabrication can start.
1. Steel Plate Cutting
2. Straightening & Bending
3. Edge Beveling
4. Assembly
5. Welding
6. Corrosion Protection & Coating
Steel Box Girder Structure & Design
Steel box girders are typically composed of top plates, bottom plates, web plates, transverse ribs, longitudinal ribs, and stiffeners, all connected through full welding. The top plate consists of an orthotropic deck system formed by a deck plate and longitudinal stiffening ribs.
Steel Box Girder Design Example:
The size of processed steel plates varies based on the design of the steel box girder. Typical steel plate thicknesses and dimensions of a steel box girder are as follows:
- Deck plate (top plate): 14 mm
- Longitudinal U-ribs: 8 mm thickness, 300 mm top width, 170 mm bottom width, 280 mm height, spaced at 620 mm
- Bottom plate: 10 mm, with longitudinal U-shaped stiffeners
- Inclined web plate: 14 mm
- Vertical web plate: 9 mm
- Transverse ribs: spaced at 4.0 m, thickness 12 mm
- Girder depth: 2.0 – 3.5 m
This configuration of steel box girder structure represents a commonly adopted structural arrangement in Steel Box Girder design. The designs for each steel box girders are different according to project requirements differ.
Steel Box Girder Production Line
Steel box girder production mainly involves 4 major technologies: CNC plasma/flame cutting, automatic submerged arc welding, large section assembly welding, and deformation control.
Steel box girder production requires 3 types of equipment due to the processing step: 1, steel plate cutting, 2, precision welding, 3, surface treatment.
- Steel Plate Cutting Equipment: CNC flame cutting machine, plasma cutting machine.
- Welding Equipment: manual electric arc welding machine, CO2 welding machine, submerged arc welding machine, straightening machine.
- Coating equipment: sandblasting and spraying equipment.
Steel Box Girder Fabrication Advantages
AGICO is deeply involved in China’s modern infrastructure and industrial upgrading. We also offer affordable steel box girders and technical support for building factories, expanding industrial parks, and connecting infrastructure in Belt and Road countries, helping to boost the local bridge construction economy.
- High Degree of Automation: Steel box girder production lines are usually equipped with automated or semi-automated machinery and equipment, which can significantly improve production efficiency and quality stability.
- Short Production Cycle: Through assembly line operation, the various processes are closely connected, which greatly shortens the production cycle.
- High Product Quality: Advanced processes and equipment are used to ensure that the dimensional accuracy, weld quality and overall performance of the steel box girder meet the design requirements.
- Energy Conservation and Environmental Protection: The production line is designed with energy conservation and environmental protection in mind, using low-energy-consumption, low-emission equipment and technologies to minimize environmental impact.
Why Choose AGICO
AGICO has made a series of inspection and testing measures from the steel material to cutting, welding, surface treatment, we can guarantee the high-quality of steel box girders and columns for your bridge project.
Welding Quality Control
- Non-destructive testing: According to specifications, 100% or sampling non-destructive testing should be performed on the welds: Ultrasonic testing ( UT ) should be prioritized for butt welds of steel box girders. ( I , II ) Class I welds must meet the corresponding grade (e.g., Class I) The UT inspection level for this type of weld is B , and the pass level is I. Level II); some critical welds (such as tension zones) require radiographic testing ( RT ), with a pass level of II. class.
- Flaw detection report: the flaw detection ratio, the detection area, the defect location and the evaluation result must be clear, and the repair record of unqualified welds (repair number, repair process, re-inspection result) must be complete (the same part should not be repaired more than twice ).
- Compliance of welding process: Verify that the welding process is consistent with the evaluation report, such as whether the parameters such as welding current, voltage, welding speed, and preheating temperature meet the WPS requirements, and whether the interpass temperature record for thick plate welding is complete.
Coating Quality Control
- Rust Removal Grade Inspection: The surface rust removal quality of steel box girders must meet the design requirements (e.g., Sa2.5 or St3). This can be achieved by visually inspecting the surface against a standard sample or by using a roughness tester to check the surface roughness after rust removal (usually 50-80μm is required ).
- Coating performance testing
- Coating thickness: The dry film thickness is measured using a magnetic thickness gauge, with measurements taken per 10 square meters . The total thickness must meet the design value, and the thickness deviation of a single layer should not exceed ±10% .
- Coating adhesion: The adhesion level must meet the specifications by cross-cut test or pull-off test (e.g., no peeling of the coating after cross-cut test, pull-off adhesion ≥ 5MPa ).
- Coating appearance: The surface should be uniform and flat, free from defects such as pinholes, bubbles, runs, and peeling, and the color should be consistent with the design color card.
