Conceptual design and preliminary test of the novel structure of steel inner core-UHPC composite box girder

A novel steel inner core-ultra high performance concrete (UHPC) composite box girder structure is proposed, which is lightweight, economical and low-carbon emission. In this proposal, the steel inner core serves as both the load-bearing structure and the UHPC internal formwork. Eliminating the installation of longitudinal ribs in the steel inner core greatly reduces the amount of welding, minimizes sources of fatigue damage, and simplifies construction. The UHPC is externally wrapped around the steel inner core to ensure the structural durability. When the novel box girder is applied to the incremental launching continuous girder structure with a span of 70 m, the technical index per square meter of the main girder is: 163 kg of steel, 0.242 m3 of UHPC, and a selfweight of 820 kg. With the same span, the novel box girder structure exhibits significant advantages in comprehensive performance compared with other types of main girder structures. The flexural behavior of the novel box girder was investigated through a four-point bending experiment and the cracking characteristics of the test specimen on the bottom plate under different reinforcement configurations were analyzed. The test results showed that the amount of reinforcing bars in UHPC had a significant impact on the development of cracks. When arranged with a diameter of 16 mm and a spacing of 100 mm, the overall distribution of cracks was short and fine, and the crack resistance strength was 1.56-1.80 times that of the reinforcement configuration with a diameter of 16 mm and a spacing of 200 mm. The preliminary test and analysis results suggest that the proposed composite box girder can be taken as a new structural option for medium to large span bridges ranging from 40 m to 100 m.