Self-Launching Variable-Depth Steel Box Girder to EN 1993, ASTM A709 & Australian AS 4100

A steel box girder bridge is a closed thin-walled structure composed of a top flange, bottom flange, web plates, and longitudinal and transverse stiffeners. Unlike traditional I-girder composite beams, the box section has extremely high torsional rigidity and deformation resistance, making it particularly suitable for curved bridges, eccentric loads, and wide bridge decks. Its internal hollow characteristic naturally provides space for internal prestressing, dehumidification systems, or maintenance passages. The products include constant-section continuous box girders, variable-section cantilevered box girders, and steel–concrete composite box girders, with spans ranging from 40 to 150 meters.

1.Scenario Category

2.Product Highlights

l Steel-Concrete Composite Deck: The deck is cast with 10–15 cm of concrete participating in compression, reducing steel usage by 20% and eliminating fatigue issues in orthotropic plates.

l Variable-Height Bottom Plate Parabolic Variation: The support beam height is 1.8–2.5 times the mid-span, concentrating the maximum bending moment in the steel and saving 15% of steel.

l Optimized Spacing of Transverse Partitions: Parametric FEA determines the optimal spacing (4–6 m), controlling distortion deformation within ±5% of the bridge deck cross slope.

l Post-Buckling Strength Utilization: When the web's height-to-thickness ratio reaches 250, post-buckling tension field effects are allowed, reducing the amount of web stiffeners needed.

l Wind Vents Also Serve as Maintenance Passage: The triangular wind vents on the outside of the flanges are hollow inside, serving as passages for maintenance personnel and cable trays.

l Sacrificial Anode Cathodic Protection: Zinc alloy anode blocks are installed on beam sections near supports and piers, achieving maintenance-free protection for a hundred years.

3.Product Materials

The core load-bearing components of our box girder steel bridge are all made of high-performance special steel for bridges that meet the world's mainstream bridge standards, and the materials are precisely selected according to the stress characteristics of the box section. The core material system is as follows:

ü Low alloy high strength special steel plate for bridges: Adopting mainstream special steel plates for bridges such as A709M Grade 345W and Q345qD, with yield strength ≥ 345MPa, excellent bending resistance, compression resistance, welding performance and low temperature impact toughness, it is the core main material for the top plate, bottom plate and web of the box girder, which can perfectly adapt to the complex stress requirements of the box section.

ü High weathering atmospheric corrosion resistant steel plate: Adopting weathering steel plates such as Corten A and Q355NH, a stable and dense passivated rust layer is formed on the surface through alloying design, realizing long-term coating-free anti-corrosion, which is especially suitable for strong corrosion environments such as coastal, high humidity and industrial pollution, and greatly reduces the later anti-corrosion maintenance cost of the box girder.

ü Stainless steel composite anti-corrosion plate: For extreme environments with high salt spray and strong corrosion, 304/316 stainless steel and carbon steel composite plates are used, which take into account the high strength of carbon steel and the excellent anti-corrosion performance of stainless steel, which can realize anti-corrosion maintenance-free for the whole life cycle of the box girder and greatly improve the service life of the structure.

ü High performance welding and fastening materials: All box girder welding adopts low hydrogen high strength welding wire and electrode that meet AWS and EN standards to ensure that the mechanical properties of the welded joint are completely matched with the base metal; segment splicing adopts 10.9 grade high strength bolts that meet ASTM A325 and EN 14399 standards to ensure the rigid connection and uniform stress of the splicing joints.

4.Design Features

Ø Special Control of Distortion Effects: Install intermediate diaphragms or anti-distortion struts to keep distortion and warping stress within allowable limits.

Ø Consideration of Shear Hysteresis Effects: For wide box girders (width-to-span ratio > 0.3), use effective flange width reduction, with a reduction factor of 0.6–0.9.

Ø Orthotropic Plate Fatigue: Welds between U-ribs and top plate are classified as FAT 90–125, verified for 2 million cycles of equivalent stress amplitude.

Ø Transverse Bridge Anti-Overturning: Curved box girders supported on a single bearing must be checked to ensure an anti-overturning stability coefficient ≥ 2.5; climbing devices or mass concrete can be provided.

Ø Installation Phase Stability: During pushing construction, the layout of guide beams and temporary piers ensures that the maximum cantilever condition does not cause bottom plate buckling.

Ø Integrated Maintenance Access: Anti-slip grating walkways are provided on the bottom plate inside the box, and ladders and safety rope anchor points are set on the web.

5.Core Advantages

⭐Absolute Torsional Stiffness Advantage: Closed-section torsional constant (J) is more than 1000 times that of open sections; curved bridges do not require dual supports.

⭐Minimum Structural Height: For the same span, steel box girder height is only 1/2–2/3 of that of concrete box girders, saving clearance under the bridge.

⭐Fully Prefabricated and Rapid Installation: Standard segments (12–20m) are manufactured integrally in the factory; a single crane can directly place the girder, installing 20 meters per hour.

⭐Significant Reduction in Wind Load: Streamlined wind fairings can reduce the drag coefficient (Cd) from 1.8 to 0.6, decreasing wind-induced vibration.

⭐Built-in Fire Protection: Poor air circulation inside the box leads to slow temperature rise during a fire; optional fireproof coating is available (FRL 90/90/90).

⭐Future Prestress Reinforcement: Prestress ducts are reserved inside the box, allowing for future external tendons to handle increased loads.

6.Global Standard

7.Choose us  

Choosing our steel box girder bridges means you are choosing a technical path that maximizes the strength of thin plates after buckling and the torsional resistance potential of closed sections. We don’t just deliver box-shaped components—we deliver carefully calculated diaphragm layouts for distortion deformation, effective flange widths reduced by shear lag effects, and torsional stability redundancy perfectly matched to curved alignments.