As mentioned in the previous article, the fabrication and installation of long-span highway bridge steel box girders involve multiple complex construction difficulties, such as large engineering volume, strict precision requirements, high transportation and installation risks, and challenges in welding quality and durability guarantee. These difficulties have put forward severe tests for the smooth progress of bridge projects. To effectively address these challenges and ensure the safety, quality, and efficiency of construction, it is crucial to adopt scientific and targeted core construction technologies. This article will focus on five key technologies in the fabrication and installation of long-span highway bridge steel box girders, including pre-assembly, manufacturing process, cantilever erection, general installation, and pulling installation, elaborating on their operational essentials and application requirements to provide technical support for engineering practice.

1. Steel Box Girder Pre-Assembly

Pre-assembly of steel box girders is mandatory before factory delivery, with the following core purposes:(1) Correction of bridge deck or deck length: The actual dimensions during pre-assembly are adjusted by calculating the length difference between the top and bottom plates of each girder segment.(2) Segment pre-assembly of steel box girders: During pre-assembly, five girder segments must be assembled into an integrated unit. According to drawing requirements, gaps shall be reserved between segments to match the connecting parts of adjacent segments, followed by welding of pre-assembly welds. After passing inspection, the previous steel box girder unit is shipped out, while one segment is retained for pre-assembly with the next unit. The pre-assembly sequence must be consistent with the on-site installation sequence, and no modification of segment numbers is allowed during installation.(3) Correction of total length of steel box girders: After the pre-assembly of each unit, its total length is measured and compared with the theoretical length. Any deviation can be corrected in the next pre-assembly unit to avoid error accumulation.(4) Interface trimming: The dimensional deviation of segment ports is trimmed to ensure calibratability during installation, prior to welding.

2. Manufacturing Process of Steel Box Girders

In the manufacturing process of long-span steel box girders, the standard production flow for components is as follows: blanking → straightening → plate splicing → welding → defect inspection → fitting assembly → final assembly inspection. During the specific design and production phase, strict quality control must be implemented for component manufacturing in accordance with the above standard procedures.Since the steel box girders of viaducts are parallel to the roof panels, the transverse slope is fully formed by rotating the box girders. Steel reinforcement blocks and crossbars shall be fabricated in accordance with drawing sequences, with accurate and reliable relevant data. On-site positioning lines shall be established, and precise assembly shall be carried out in compliance with relevant standards and requirements.To ensure the production quality of long-span box-type steel girders, all manufacturing and assembly work shall refer to the Technical Specifications for Highway Bridge Construction (JTG/TF50-2011) as the reference standard, and illegal construction is strictly prohibited. Particularly during welding, the actual welding quality must meet the Class I and Class II weld quality standards specified in the above specifications. Ultrasonic testing (UT) and X-ray testing (RT) shall be performed promptly after welding.During construction, 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 shall be kept above 50%.

3. Cantilever Erection Construction Technology

There is a wide variety of hoisting equipment, which is mainly located in the median strip of linear bridges. The projection surface of some segments falls within the median strip, making it unfavorable to set up temporary supports. Therefore, during the selection of hoisting schemes, the cantilever erection method can be adopted for certain segments.Before the cantilever erection of steel girders, a simulated installation process can be conducted first. The plate units of the main girder are analyzed and calculated using the MIDAS construction program to verify the feasibility of the construction scheme. During construction, pier-related devices and double-sided auxiliary supports shall be used to increase the length of cantilevers on both sides to a certain extent, and each corner only needs to be lifted once. Cantilever erection construction technology is conducive to ensuring the safety of construction operations.To effectively exert the role of cantilever technology, construction personnel must strictly assemble the steel box girders on the ground in accordance with requirements. The assembly effect of the engineering structure can be effectively guaranteed under the action of ultra-high crossbeams, which plays an important role in construction. Construction units should attach great importance to this technology and ensure its proper implementation. During concrete construction, construction personnel shall erect the main box girders properly.

4. Steel Box Girder Installation

During the installation of steel box girders, the standard erection process is as follows:

1. Survey the construction site to fully understand the on-site topographical information and avoid potential safety hazards during installation.

2. After completing the on-site survey, conduct setting out and install the steel pipe columns.

3. After the installation of steel pipe columns, inspect their bearing capacity and stability to ensure compliance with relevant specification requirements.

4. Hoist the steel box girders and weld them span by span.

5. After welding, once the weld quality meets the standards, paint the steel box girders and finally remove the supports.

Long-span steel box girder construction is usually large-scale and not suitable for on-site fabrication. To ensure construction quality and progress, and reduce construction difficulty, prefabrication is generally carried out in accordance with design requirements. After prefabrication in the factory, the girders are transported to the construction site for installation.During installation, the bearing capacity of hoisting machinery must meet relevant standard requirements, and 200-ton crawler cranes are generally used. In addition, the installation shall be carried out in strict accordance with the standard procedure: transverse frames are installed from the center to both sides, and vertical frames are installed from near to far. Before installation, an environmental survey shall be conducted to understand the local terrain, and a suitable installation scheme shall be selected in combination with the characteristics of the engineering structure to ensure the smooth and safe progress of the installation construction.Furthermore, due to the small hoisting radius of this bridge section, the crane is likely to collide with surrounding walls during hoisting. Therefore, the selection of 200-ton crawler cranes is appropriate.

5. Pulling Installation Construction Technology

During the installation of long-span steel box girders, influencing factors such as the traction system and reaction supports should be fully considered. To ensure the assembly efficiency of long-span steel box girders, targeted assembly construction schemes have been implemented. To smoothly carry out the construction plan, the role of steel boxes in the construction process should be fully exerted.First of all, construction units should select construction personnel with strong professional skills and rich experience to fully engage in the application of pulling construction technology, ensuring the smooth progress of the project. For specific construction operations, construction and technical personnel must meet the following requirements:(1) Conduct a comprehensive inspection before pulling. Set up benchmarks on each temporary pier to better observe settlement, ensure the integrity of the line, and confirm the presence of personnel.(2) Prepare for pulling: Jog the system 2-3 times as required to inspect the relevant hoisting facilities.(3) When measuring resistance, construction personnel need to make appropriate adjustments according to the frictional resistance and calibrate the jacks and oil gauges as required. During concrete construction, if the girder remains stationary even after the maximum pulling force is applied, relevant personnel must conduct an in-depth inspection, and the jacking operation can only be continued after the pulling force is clarified.

To Sum Up

In summary, the five key technologies discussed above—steel box girder pre-assembly, standardized manufacturing process, cantilever erection technology, general installation procedure, and pulling installation technology—are core guarantees for overcoming the construction difficulties of long-span highway bridge steel box girders. Their rational application and strict implementation are essential to ensuring the structural integrity, installation accuracy, and construction safety of steel box girders. However, mastering these technologies alone is not sufficient to achieve the ultimate goal of high-quality project completion; systematic and comprehensive quality control throughout the construction process is equally indispensable. In the next article, we will focus on the construction quality control measures for long-span highway bridge steel box girders, exploring in detail the quality management points covering raw material inspection, process supervision, defect prevention, and acceptance standards, aiming to form a complete technical system from difficulty resolution to quality assurance and further promote the standardized and high-quality development of long-span steel box girder bridge construction.