The fabrication and installation of long-span steel box girders in urban areas are complex and challenging. Any negligence or improper operation during construction may lead to serious consequences. Therefore, in the construction of long-span steel box girders, it is essential to establish a strong awareness of safety and quality. Construction personnel must strictly comply with management specifications and refer to the specific conditions of the project to rigorously control each construction process, ensuring the quality and efficiency of the project. This article provides an in-depth analysis of the core construction difficulties in the fabrication of long-span highway bridge steel box girders, offering references for subsequent technical optimization and engineering practice.

1 Overview of Long-Span Steel Box Girders

1.1 Concept

Steel girders, also known as steel box girders, are a common structural form in long-span bridge engineering. They are closed structural systems organically connected by top plates, diaphragms, bottom plates, webs, and stiffeners through full welding technology, as shown in Figure 1. Among them, the top plate forms an orthotropic deck by combining longitudinal stiffeners and cover plates, which exhibits excellent load-bearing and fatigue resistance; diaphragms and stiffeners work together to enhance the overall stiffness of the box girder and suppress local deformation. In practical construction, the rational application of long-span steel box girders not only optimizes the mechanical properties of bridge structures but also adapts to various advanced construction technologies such as incremental launching, pulling, and cantilever erection. It is particularly suitable for complex construction scenarios such as urban overpasses and river-crossing/sea-crossing bridges, and has become one of the core structural forms for main girders of modern long-span highway bridges.

1.2 Correlation Between Application Advantages and Construction Complexity

Long-span steel box girders are widely used in long-span scenarios such as crossing rivers, bays, and urban core areas due to their prominent advantages of high strength, large stiffness, light weight, and strong integrity. They can effectively reduce the workload of bridge foundations and shorten the construction period. However, these advantages also determine the complexity of their construction: the lightweight design imposes extremely high requirements on the processing accuracy of components; the long-span characteristic significantly increases the difficulty of alignment control and stability guarantee during installation; and the full-welded structure sets strict standards for welding quality and residual stress control. Various factors collectively contribute to the high difficulty of fabricating long-span steel box girders.

2 Construction Difficulties in the Fabrication of Long-Span Highway Bridge Steel Box Girders

2.1 Large Construction Volume and High Control Requirements

Long-span highway bridge steel box girder projects are often large-scale. Taking a common urban circular pedestrian overpass as an example, the construction volume of only the circular main girder and connecting main girder reaches 149.684 m³, and the entire process from component fabrication, transportation to on-site installation must be completed within a strictly limited construction period, resulting in tight schedules and heavy tasks. More importantly, the weight per unit length of steel box girders can reach 5 t/m, with significant self-weight load. Such a large weight acting on the underlying concrete pile foundations is prone to causing uneven settlement of the foundations.In addition, long-span bridge construction has a long cycle, and there are significant differences in load distribution and environmental conditions among different construction stages. Coupled with the uniqueness of geological and hydrological conditions at various bridge locations, the settlement of piers varies. If such uneven settlement exceeds the allowable range, it will not only damage the designed alignment of the steel box girder but also generate additional stress inside the structure, thereby affecting the stability of the connection nodes between piers and the box girder, and even posing a fatal threat to the safety of the entire bridge structural system. Meanwhile, as the main load-bearing structure, the construction quality of steel box girders is directly related to the long-term load-bearing capacity and durability of the bridge. Every indicator, from material performance and welding quality to alignment accuracy and anti-corrosion effect, must strictly comply with relevant specifications such as the Technical Specifications for Highway Bridge Construction (JTG/TF50-2011), involving multiple control dimensions and high standards.

2.2 Large Number of Prefabricated Components and Heavy Transportation Volume

The structural integrity of long-span steel box girders relies on the precise coordination of various components. There is a wide variety of component types, including face plates, bottom plates, webs, ribs, diaphragms, stiffeners, etc., and the dimensions, shapes, and mechanical properties of different components vary significantly. Due to the high sensitivity of the overall structure of steel box girders to assembly accuracy, the processing accuracy requirements for various components are extremely strict. For example, the dimensional deviation of the connection between webs and top plates must be controlled within 1 mm; otherwise, it will lead to excessive splicing gaps of components, affecting welding quality and structural tightness, and thus posing potential safety hazards.After prefabrication in the factory, components need to be transported to the construction site in batches for assembly, which also faces many challenges: on the one hand, the transportation volume of components is large. A single segment of a large steel box girder can weigh hundreds of tons and exceed ten meters in length, placing extremely high requirements on the bearing capacity of transportation equipment and the traffic conditions of transportation routes; on the other hand, the protection requirements for components are strict. Once key parts such as the anti-corrosion coating and welding grooves on the surface of steel box girder components are damaged during transportation, it will not only increase the cost of later repairs but also affect the construction progress and project quality. Therefore, it is necessary to use professional mechanized processing equipment to ensure component accuracy, fix components with customized tooling fixtures to prevent transportation deformation, and adopt multiple protective measures such as wrapping, anti-collision, and shock absorption during transportation to achieve full-process standardized operations from processing to transportation, so as to address this difficulty.

2.3 Complex On-Site Installation Conditions and High Risk Coefficient

The on-site installation of long-span steel box girders is the core link of the entire construction process and a key area for risk prevention and control. Firstly, the bridge site environment is often complex and diverse. River-crossing and sea-crossing bridges are affected by natural environments such as wind, waves, tides, and strong corrosion, while urban bridges are restricted by surrounding buildings, transportation routes, underground pipelines, etc., resulting in limited hoisting space and narrow construction sites, which brings great challenges to the positioning of large hoisting equipment and the control of operating radius.Secondly, the selection and implementation of installation technology are difficult. Common installation technologies for long-span steel box girders, such as cantilever erection, incremental launching, and pulling, have extremely high requirements for the performance of construction equipment, the professional skills of construction personnel, and the coordinated cooperation of various processes. Taking cantilever erection as an example, it is necessary to precisely control the hoisting and positioning of each segment to ensure that the misalignment and axis deviation of segment joints meet the standards. At the same time, it is necessary to real-time monitor the deflection and stress changes of the cantilever structure to prevent safety accidents caused by structural instability. In addition, the system conversion link during installation is highly risky. The transition from the temporary support state to the permanent load-bearing state requires step-by-step unloading and precise regulation. Any improper operation may lead to the imbalance of structural internal force redistribution, affecting the bridge alignment and structural safety.

2.4 Difficulties in Welding Quality Control and Susceptibility to Defects

Steel box girders adopt a full-welded structure, and welding quality directly determines the integrity and safety of the structure. The welding difficulties of long-span steel box girders are mainly reflected in three aspects: first, the large welding workload. The welds are long and complex in type, including butt welds, fillet welds, penetration welds, etc., and the proportion of thick plate welding (plate thickness ≥ 30 mm) is relatively high, which increases the difficulty and workload of welding operations; second, the difficulty in controlling welding deformation. Steel box girder components are large in size and uneven in stiffness, and the residual stress generated during welding is prone to causing angular deformation, bending deformation, and other problems of components, affecting the structural assembly accuracy; third, the susceptibility to welding defects. In the complex outdoor environment, factors such as temperature, humidity, and wind speed will affect welding quality, which may lead to defects such as porosity, slag inclusion, incomplete fusion, and cold cracks. Especially when low-hydrogen welding materials are not used properly or post-weld hydrogen relief treatment is inadequate, the risk of cold cracks increases significantly. If these defects are not detected in a timely manner, they will seriously affect the load-bearing capacity and durability of the welds.

2.5 High Environmental Adaptability Requirements and Difficulties in Durability Guarantee

Long-span highway bridges are mostly located in complex natural environments. As exposed structures, steel box girders need to withstand various effects such as sun exposure, rain, temperature changes, and atmospheric corrosion for a long time, which puts forward high requirements for their durability. In high-temperature and high-humidity areas, the surface of steel box girders is prone to rust; in coastal areas, salt spray corrosion will accelerate steel corrosion and coating aging; in cold areas, freeze-thaw cycles may cause cracking at the concrete bonding parts. These environmental factors will affect the service life of steel box girders.According to specification requirements, the service life of the surface coating of steel box girders exposed to the atmosphere must be maintained for more than 25 years, and the humidity inside the box girders must be controlled above 50%, which places strict requirements on anti-corrosion coating construction, box girder sealing treatment, and other work. However, in actual construction, problems are likely to occur in links such as surface derusting before coating construction, coating thickness control, and coating quality at welds. If the surface derusting does not meet the Sa2.5 level standard or the coating thickness is uneven, the protective effect of the coating will be reduced, and the durability life of the steel box girder will be shortened. At the same time, if the internal drainage and ventilation design of the steel box girder is unreasonable, a humid environment is likely to form, accelerating the corrosion of the internal structure and further increasing the difficulty of durability guarantee.

Conclusion

In summary, the fabrication of long-span highway bridge steel box girders faces multiple construction difficulties, such as large construction volume, high control requirements, difficulties in transporting prefabricated components, high risks in on-site installation, strict welding quality control, and challenges in durability guarantee. These difficulties are intertwined, placing extremely high requirements on construction technology, management level, and resource allocation. Effectively overcoming these difficulties is directly related to the quality, safety, progress, and cost of bridge projects.To address the above challenges, a series of targeted key technologies have emerged. In the next article, we will focus on the core technologies for the fabrication of long-span highway bridge steel box girders, detailing key contents such as steel box girder pre-assembly technology, high-precision manufacturing processes, cantilever erection and pulling installation technologies, welding deformation control technologies, and durability protection technologies. We aim to provide practical technical solutions for engineering practice and promote the high-quality development of long-span steel box girder bridge construction.