Widening-Ready Steel Box Girder With Bolt-On Flanges to EC3, AWS D1.5 & AS

Steel box girder bridges are closed thin-walled structures composed of a top slab, bottom slab, web, and longitudinal and transverse stiffeners. Unlike traditional I-girder composite beams, the box-shaped section has extremely high torsional stiffness and distortion resistance, making it particularly suitable for curved bridges, eccentric loads, and wide deck conditions. Its hollow interior naturally provides space for internal prestressing, dehumidification systems, or maintenance passages. The products cover constant-section continuous box girders, variable-section cantilevered box girders, and steel–concrete composite box girders, with spans ranging from 40 to 150 meters, making them the preferred choice for urban viaducts, interchange ramps, and overcrossing bridges.

1.Application Scenario

2.Product Highlights

l Steel-Concrete Composite Deck: The deck is cast with 10–15 cm of concrete contributing to compression, reducing steel usage by 20%, while eliminating fatigue issues of orthotropic decks.

l Parabolic Variation in Variable-Height Bottom Plate: The support beam height is 1.8–2.5 times the midspan, where steel concentration for maximum bending moment occurs, saving 15% of steel.

l Optimized Spacing of Transverse Plates: Parametric FEA is used to determine the optimal spacing (4–6 m), with distortion deformation controlled within ±5% of the bridge deck transverse slope.

l Post-Buckling Strength Utilization: When the web height-to-thickness ratio reaches 250, the post-buckling tensile field effect is allowed, reducing the amount of stiffening ribs for the web.

l Wind Vent Used as Maintenance Route: The triangular wind vent on the flange exterior is hollow inside, serving both as a passageway for maintenance personnel and as a cable tray.

l Sacrificial Anode Cathodic Protection (Underwater Section): Zinc alloy anode blocks are attached to beams near supports and piers, achieving maintenance-free protection for a century.

3.Product Materials

ü Deck/Bottom Plate/Web (Primary Load-Bearing Steel Plate)

European Standards S460M/S550M / American Standard A709 Gr.70W / Australian Standard AS 3597 Grade 500, plate thickness 12–60 mm, Z35 requirement (tensile in the thickness direction).

ü Longitudinal Stiffening Ribs (U-Ribs/Plate Ribs)

U-Ribs (6–8 mm thick, cold-formed), material matches the main steel plate; holes at the open end reduce stress concentration.

ü Transverse Plates (Solid-Web/Truss Type)

Material: S355J2 N, plate thickness 10–16 mm, equipped with manholes (ø600 mm) and utility ducts.

ü Shear Connectors (Composite Deck)

Welded studs (ø19–22 mm, ML15), withstand 45° post-weld bending without cracking, used with epoxy-coated reinforcement for rust protection.

ü Internal Prestressing Strands (Optional)

High-strength low-relaxation steel strands (1860 MPa), with PE sheathing and special anchors, arranged within thickened areas of the bottom plate or web.

ü Dehumidification System (Rust Protection Inside Box)

Rotary dehumidifiers (dew point ≤ -15°C), air supply ducts and humidity sensors maintain relative humidity inside the box ≤40%.

4.Design Features

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

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

Ø Orthotropic Plate Fatigue: U-rib to top plate welds are classified according to FAT 90–125, and verified for 2 million cycles of equivalent stress range.

Ø Transverse Bridge Anti-Overturning: Curve box girders supported on a single bearing need to verify anti-overturning stability coefficient ≥ 2.5, with anti-climbing devices or mass concrete added.

Ø Installation Stage Stability: During pushing construction, the arrangement of guide beams and temporary piers must ensure that there is no bottom plate buckling in the negative moment region at the maximum cantilever state.

Ø Integrated Maintenance Passage: Set non-slip grating walkways on the bottom plate inside the box, and provide ladders and safety rope anchor points on the web.

5.Core Advantages

⭐Absolute Advantage in Torsional Stiffness: The torsional constant (J) of a closed section is more than 1000 times that of an open section, eliminating the need for double supports for curved bridges.

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

⭐Fully Prefabricated and Rapid Installation: Standard segments (12–20m) are manufactured entirely in the factory, and on-site single crane can directly place the girder, installing up to 20 meters per hour.

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

⭐Clean Bridge Deck Layout: Flange plates can directly serve as utility supports, with no exposed facilities on the deck and a smooth, aesthetically pleasing underside.

⭐Post-Prestressing Strengthening: Reserved prestressing ducts inside the box allow for external tendons to be added later to handle increased loads or prestress loss.

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 plate buckling and the torsional resistance potential of closed sections. We do not just deliver box-shaped components—we deliver transverse diaphragm arrangements refined through detailed distortion analysis, effective flange widths reduced by shear lag effects, and torsional stability redundancy perfectly matched to curved alignments.

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