As a professional bridge construction contractor, we understand that the construction of steel box girders is a complex systems engineering project. Its core lies in combining factory-pre-fabricated high-precision segments into a complete structure with the designed alignment on-site through safe and precise techniques. The following are the main construction methods and key process points for steel box girder bridges.
Main Construction Methods
Depending on the bridge span, terrain conditions, traffic situation, and equipment capabilities, the installation of steel box girders primarily employs the following methods:
Scaffolding Method (In-Situ Assembly Method)
This is the most traditional yet straightforward and reliable method.
Process Principle: Erect full scaffolding or temporary supports (typically composed of Bailey beams, steel pipe piles, etc.) below the bridge position to provide continuous support for the segments. The segments transported to the site are directly hoisted by large cranes to preinstall positions on top of the supports for alignment, adjustment, welding, and connection until a complete bridge span is formed.
Applicable Scenarios: Suitable for situations where the foundation conditions below the bridge are good, the height is relatively low (usually below 50 meters), and does not affect ground traffic or water flow. Commonly used for urban viaducts or overpass bridges.
Advantages: Simple and intuitive construction, mature technology, easy control of the girder alignment, high safety factor.
Disadvantages: Large amount of scaffolding work, high cost, long cycle, significant occupation of space below the bridge, poor environmental friendliness.
Incremental Launching Method (Push Launching Method)
This is an advanced mechanized construction method, particularly suitable for bridges crossing busy traffic lines, deep valleys, or rivers.
Process Principle: Set up a prefabrication and assembly platform behind the abutment at one end of the bridge. Segments are successively hoisted and welded into continuous sections. Then, launching devices (such as jacks, sliding blocks) are installed on the piers (abutments). Through alternating lifting and pushing, the entire steel box girder is incrementally launched forward section by section, like “squeezing toothpaste,” until it reaches the designed bridge position.
Applicable Scenarios: Straight bridges or bridges with gentle curves with constant or slightly variable cross-sections. Ideal for crossing highways, railways, and navigable rivers, achieving “construction without affecting traffic.”
Advantages: Minimal impact on the environment below the bridge, no need for large lifting equipment and extensive scaffolding, unparalleled advantage for cross-line construction.
Disadvantages: High requirements for equipment automation, need for precise alignment control and guidance devices, continuous changes in the internal forces of the girder during construction require the entire process monitoring and computational analysis. The design, guided by standards like AS5100, must carefully consider these transient stress states during launching.
Cantilever Assembly Method
This is the primary method for constructing long-span steel box girder bridges, especially the girders of cable-stayed bridges and suspension bridges.
Process Principle: Starting from the completed pier (tower), construction extends symmetrically to both sides. A large deck-mounted girder-erecting machine, fixed on the already installed segments, is used to hoist the next segment. Once the new segment is positioned, it is immediately temporarily connected and welded to the existing segment, gradually “growing” the new bridge body, like extending arms.
Applicable Scenarios: Long-span bridges (e.g., cross-sea, cross-river bridges), projects where there are no support conditions below the bridge or where hydrological/geological conditions are complex.
Advantages: Completely unaffected by navigation or traffic below the bridge, no need for extensive scaffolding, suitable for any height.
Disadvantages: Highest technical difficulty. The structure is in a cantilever state during construction, requiring extremely strict control of wind stability and stress. Detailed construction simulation calculations and real-time monitoring are essential to ensure symmetrical construction to avoid overturning. The fatigue load models in AS5100 are crucial for designing the connections and welds in these cantilevered structures, which are subject to cyclic stresses during the construction phase.
Whole-Span Hoisting Method
This is the fastest construction method when conditions permit.
Process Principle: The entire span or large segments of the steel box girder are completely assembled in a factory or riverside prefabrication yard. Then, using ultra-large floating cranes or gantry cranes, the entire structure is hoisted into place on the piers.
Applicable Scenarios: Projects with moderate spans, access to large waterways for floating cranes, or availability of prefabrication sites near the bridge location.
Advantages: Extremely fast construction speed, shortest on-site operation time, highest engineering quality (most welding work is completed in the factory).
Disadvantages: Limited by lifting capacity (although floating crane capacity now exceeds 10,000 tons), stringent requirements for transportation and hoisting routes, high cost.
Core Construction Process Flow and Control Points
Regardless of the installation method, the core process of on-site connection is the same, mainly including the following steps:
Segment Transportation and Delivery
Segments are transported to the bridge site by ship or heavy transport vehicle. They must be strictly secured during transportation to prevent collision and deformation. Upon arrival, the segment numbering must be checked, and the coating inspected for damage.
Segment Positioning and Attitude Adjustment
This is a key step to ensure the final bridge alignment. Using measuring equipment such as total stations and GPS, and adjustment devices like jacks, the three-dimensional coordinates (planar position, elevation) and attitude (inclination) of the segment are precisely adjusted to achieve a perfect match with the port of the already installed segment. Matching accuracy is usually required to be controlled at the millimeter level.
Temporary Matching and Fixation
Once the segment attitude is accurately adjusted, temporary matching pieces (high-strength bolts or drift pins) are immediately used to firmly connect the new and old segments, forming a stable temporary structure in preparation for subsequent welding. This process must be rapid and accurate to prevent positional deviation due to wind or temperature changes.
Site Welding
This is the most critical process determining the overall quality and durability of the steel box girder.
Process Requirements: Must be performed by certified welders according to the pre-qualified Welding Procedure Specification (WPS).
Welding Sequence: Follow the principle of “inside first then outside, bottom first then top, symmetrical welding” to minimize welding stress and deformation. The typical sequence is: bridge bottom plate → inclined bottom plate → longitudinal diaphragm → bridge deck plate → wind fairing.
Weld Class: The main load-bearing welds (e.g., top plate, bottom plate butt welds) require full penetration primary welds, which need 100% non-destructive testing (NDT). The welding procedures and welder qualifications must often comply with stringent requirements outlined in standards like AS5100, which specifies categories based on fatigue performance.
Non-Destructive Testing (NDT)
All major site welds must undergo non-destructive testing by a qualified third-party inspection unit 24 hours after welding completion. Common methods include:
Ultrasonic Testing (UT): Mainly used for internal defect detection, the most common method for steel bridges.
Radiographic Testing (RT): Used for sampling important welds, can intuitively display defect morphology.
Magnetic Particle Testing (MT)/ Penetrant Testing (PT): Used for surface and near-surface defect detection.
Defective welds must be gouged out and rewelded, then retested until they qualify.
Coating Repair and Sealing
Coating damaged areas caused by on-site welding and hoisting must undergo strict surface treatment and coating repair. The process must be consistent with the original design (e.g., blast cleaning to Sa2.5 grade, spraying primer, intermediate coat, and top coat layer by layer) to ensure the continuity and completeness of the entire steel structure’s anti-corrosion system. The coating system specification itself is often defined by the design standard, such as the protective treatment requirements in AS5100.
Closure (Cantilever Closing Segment)
Closure is the final key process in cantilever or incremental launching construction, with the highest technical difficulty.
Closure Gap Preparation: The variation of the closure gap width with temperature needs continuous monitoring to determine the benchmark closure temperature (usually chosen during the night or early morning when temperatures are stable).
Closure Segment Installation: Within the determined temperature window, the pre-prepared closure segment is quickly hoisted into place and temporarily fixed and welded. Sometimes jacking devices are used to actively adjust the closure gap spacing to optimize structural internal forces.
Control Target: Achieve “stress-free closure,” meaning the internal force state of the girder after closure closely matches the design ideal state. The design, according to AS5100, must ensure the final structure can withstand all permanent, live, wind, and other loads in this configured state.
Construction Control and Safety Assurance
Monitoring and Measurement: Runs throughout the process. The girder alignment, elevation, stress, and temperature are continuously monitored. Data is compared with theoretical calculated values to guide the next construction step, realizing informationized construction.
Wind Resistance Measures: During long-span cantilever construction, temporary wind-resistant cables or other aerodynamic measures need to be installed to ensure construction safety under wind loads. AS5100 provides detailed wind load calculations for both completed structures and during construction phases, which guides these temporary works design.
Safety Protection: Special safety plans must be developed forhigh-altitude work, over-water work, large-scale hoisting, etc., including edge protection, fire prevention (welding sparks), fall prevention measures, etc.
In summary, steel box girder construction is a concentration of technology, management, and experience. Selecting the appropriate construction method and strictly controlling every process from positioning and matching to welding and coating are the cornerstones for ensuring the successful final completion of the bridge and its safe operation for a century. The integration of design standards like AS5100 (Australian Bridge Design Code) is crucial, as it provides comprehensive specifications for loads (including traffic, wind, earthquake), materials, design principles (limit states design), fatigue assessment, fabrication, and erection, ensuring the final structure meets all safety and serviceability requirements.
Post time: Sep-18-2025