Introduction Steel box girder bridges are among the most commonly used for long spans because of the efficiency of design (fewer boxes) and the ease of installation. The modules are mostly manufactured in the factory, and the slabs can be quickly installed upon completion of on-site mounting. However, there are also disadvantages. A partition wall with appropriate spacing and strength is required to maintain the cross-sectional shape of steel box girders. Steel box girders require periodic maintenance painting to protect them from corrosion. And unlike concrete bridges, steel bridges are susceptible to deflection and vibration. The complex partial box girder offers an alternative to steel box girders and improves on their efficiency. It is a composite girder bridge that uses less structural steel materials but has increased structural stiffness by applying a box girder to the support part and an I-shaped steel girder to the central part of the span. The Korea Institute of Civil Engineering and Building Technology (KICT), a government-funded research institute in the Republic of Korea, conducted a joint research project with a local company, Daeyoung Engineering & Steel Industries, to improve the bridge diaphragm for the complex partial box girder technology. The project also sought to apply the technology in Southeast Asian countries. What makes this technology suitable to Southeast Asia? Many countries in Southeast Asia construct bridges with cost and environmental load considerations in mind. This makes steel bridges popular in the region as it offers excellent adaptability. The complex partial box girder is an economical construction method that has been successfully commercialized in the Republic of Korea. It is suitable for medium- to long-span bridges. It can be constructed even when the span exceeds 70 meters, and the cost of the substructure construction can be greatly reduced. The box girder provides torsional stiffness to the support, which allows a longer steel span. The compression stress is absorbed by injecting concrete in the lower area, reducing steel materials by more than 30%. The lightweight superstructure makes it seismic-resistant and helps protect the substructure. What makes it different or better? Photo credit: KICT and Daeyoung Engineering & Steel Industries. Compared with steel bridges commonly used in developing countries, the life cycle costs of complex partial box girders can be reduced by about 20%. This technology is economically feasible. It increases structural efficiency by constructing a span using the three cross sections: I-shaped cross section, box cross section, and double composite cross section. It offers optimal cross-sectional arrangement for distributing tension and compression, using alternating sections. Tensile reinforcement plates are applied to the I-type girder or narrow-width girder in the positive bending moment section. The high-strength concrete composite cross-sectional box is applied to the lower flange in the negative moment section. The complex partial box girder is a steel-based bridge, and 100% of the process is performed at the manufacturing plant. Only the assembly and installation need to be done at the site. The bridge is structurally simple. It has fewer plate members and uses less steel materials because of the optimal cross-sectional arrangement. Since bridge manufacturing is carried out safely at the factory, the probability of accidents is low. It is aesthetically pleasing. The bridge can be painted in a variety of colors. How was this technology tested? What were the results? This technology was optimized for the ratio of the steel box part and the I-type girder part and the amount of concrete. Many structural analyses were conducted to verify the quality of this technology, and structural behavior evaluation was conducted by manufacturing an actual-sized test model. As a result of the test, it was confirmed that no excessive displacement or strain occurred, and sufficient safety was maintained. This technology has been used in seven construction projects and 11 design projects in the Republic of Korea. These include Gimcheon-si South–North Link Road, Gongam Bridge (Gongam Intersection in Yangsan City), and Sadong Port Maintenance Works Connection Road. Gimcheon-si South-North Link Road and Sadong Port Maintenance Works Connection Road. Photos courtesy of Daeyoung Engineering & Steel Industries. An international patent application was filed in Indonesia and Cambodia. Daeyoung Engineering & Steel Industries is working on applying this bridge construction method (design and manufacturing) in Southeast Asia, initially in Indonesia where it will be tested based on local design standards. Resources Daeyoung Engineering & Steel Industries. Complex Partial Box Girder. Ask the Experts Sung-Jin Lee Post-Doctoral Associate, Department of Structural Engineering Research, Korea Institute of Civil Engineering and Building Technology Sung-Jin Lee works the Department of Structural Engineering Research of the Korea Institute of Civil Engineering and Building Technology (KICT). He holds a PhD in Engineering from the Korea Maritime and Ocean University. Korea Institute of Civil Engineering and Building Technology (KICT) The Korea Institute of Civil Engineering and Building Technology contributes to the development of the Korean construction industry, improves quality of life standards, furthers national economic growth, and improves social welfare. 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