Msikaba bridge reaches new heights

The Msikaba River Bridge forms part of the South African National Roads Agency’s N2 Wild Coast Toll Road project.

Once complete, the 580 m long mega cable-stayed bridge will cross the Msikaba River Gorge at a height of 192 m, making it one of the highest bridges in the country.

Over the past year, the pylons have reached heights close to 80 m and the bridge’s four anchor blocks are scheduled for completion by the end of 2023. Preparations for the complex deck construction have commenced with erection of the deck scheduled to start in early 2024. In addition, all five of the tributary structures have reached completion. These include three reinforced concrete drainage culverts (ranging between 67 m and 100 m in length), one reinforced concrete pedestrian culvert and one 32 m long, three-span integral reinforced concrete bridge.

One of the key aspects of the bridge is its two identical, inverted Y-shaped, reinforced concrete pylons. Each of the 127 m tall pylons comprises two inclined legs, straddling the roadway below, and a tall cylindrical spire. The spires have a diameter of 6 m at the bifurcation, where the two legs meet, tapering to 4.5 m at the top.

Anchored into each pylon are 17 pairs of main stay cables (connected to the deck) and 17 pairs of back stay cables (anchored 130 m behind the structure). The cables are anchored by means of 17 internal structural steel anchorage liners starting at a height of 88 m.

As the pylons are almost completed up to a height just below the anchorage zones, the next phase of the pylon construction involves the construction of these anchorage zones of the pylons, including the installation of the steel anchorage liners.

These liners form a critical part of the structural system as they transfer the loads, generated by the superstructure, through the stay cables and down into the pylon shafts. The mechanism of load transfer between the steel anchorage liners and the concrete pylon is through shear stud connectors. Each 2 m tall liner has approximately 400 shear studs and weighs approximately 10 tons. The largest liner at the top of the pylon transfers a total of 1400 tons from the pair of main span and back span cables to the pylon. The combined load from all of the liners results in a load on the pylon of more than 20 000 tons at the top of the pylon inclined legs.

The liners are locally manufactured off-site in a facility in Witbank and will be transported to site once complete. The liners will be installed into position using a tower crane. With a construction tolerance of 10 mm, it is important that the liners fit perfectly upon installation. Adjusting the heavy liners at these heights will be a challenge. In addition, the alignment of the stay cable anchor positions is critical to the success of the project. Given the length of the cables (between 185 m and 308 m), the slightest offset on the anchorage position in the pylon will result in large offsets at the opposing anchorage positions located in the deck and anchor blocks. Therefore, before the liners are transported to site, they are pre-assembled at the fabrication yard and surveyed to ensure fit-up and alignment.

This mega-bridge was designed by HVA, a joint venture of SMEC, CH2M and Axis and is being constructed by the Concor Mota-Engil joint venture. The construction supervision of the project is being undertaken by SMEC South Africa. The first liner is scheduled to be installed in the pylon by the end of 2023.

Client

South African National Roads Agency (SANRAL)

Location

Eastern Cape Province, South Africa