The project is a road construction project with a total length of 4.68 km connecting Yeongjong International City and Cheongna International City in Incheon, Korea and was implemented as two Lots to enhance the economic cooperation and connectivity between the two economic free zones(Yeongjong and Cheongna) responding to the increasing demand for transportation. CP-1, which includes 2 kilometers of marine bridges out of a total length of 2.65 kilometers, was tendered with Hanwha E&C on a technical alternative bidding basis for detailed design. After winning the tender design, BANDI carried out the detailed design for a technical alternative proposal and we were in charge of the main bridge design. 

The length of the sea-crossing bridge in the CP-1 area is about 75% of the total length. Therefore, the main bridge through the sea required a landmark character, and it was essential to ensure the safety of the bridge foundation of the offshore bridge located in a large-scale fault fracture zone. In addition, a bridge crossing plan was required to allow pedestrian and bicycle traffic as well as six-lane vehicle traffic.

As a result of detailed investigation such as soil investigation and eletric-resistivity survey, it was confirmed that there is a fault fracture zone with a maximum width of 235 meters at the end of CP-1, which is classified as the deepest area of the offshore bridges. This area is a curved section of the roadway and is connected to the beginning of CP-2, so the plane line cannot be changed. Therefore, a curved cable-stayed bridge with a main span of 280 meters was required to install the bridge foundations completely avoiding these fault fracture zones. In conclusion, a curved cable-stayed bridge with a main span of 280m that can satisfy both road function and structural safety was proposed, which will be the longest curved two-pylon cable-stayed bridge in Korea.

In particular, the planning of the girders was crucial because we wanted to increase the sense of openness for the driver by applying a single stay cable installed in the center of the girder. Therefore, the bridge girder of the main span bridge applied streamlined steel box girders with high torsional rigidity, which reduced the weight of the bridge superstructure and minimized the load transferred to the foundation. In addition, the pier head part of the girder adopted concrete box girders rather than steel girders, applying a composite hybrid construction method that connects steel girders and concrete box girders. The concrete girder at the pier head was connected to the concrete pylon with a rigid connection to withstand large torsional moments due to the curved plane. The connection between the concrete box girders and steel box girders was planned using PT-BARs and prestressing tendons for stress distribution and efficient reinforcement.

The main span of the cable-stayed bridge was to be constructed using the balanced cantilever method using a 200-ton derrick crane, and the side spans that could not be constructed using the balanced cantilever method were to be constructed using a large block method using a floating crane.

A cable-stayed bridge with a curved plane cannot avoid the unbalanced forces caused by the cables installed on the pylon being installed asymmetrically on the left and right sides of the pylon. Therefore, we planned the pylon to be sloped transversely to reduce the unbalanced forces caused by this asymmetrical installation, and the pylon cables were also arranged eccentrically to reduce the transverse moment acting on the pylon as much as possible. 

On the other hand, the cable is a PSS (Parallel Strand System or Multi Strand) cable, which allows small-scale individual tensioning of the cable and individual strand replacement.