Work starts on Msikaba Bridge deck

Work has started on the Msikaba Bridge deck, and it took three trucks to transport the first steel-bridge deck segment in three sections from Middelburg in Mpumalanga, and three to five hours to offload each segment at the bridge near Lusikisiki. 

The 580m Msikaba Bridge will cross the 195m deep Msikaba river gorge and once completed, will be the longest span cable-stayed suspension bridge in South Africa and the second longest in Africa after the 680m Maputo-Catembe Bridge in Mozambique.

The anticipated completion date is the end of 2024, according to the South African National Roads Agency SOC Limited (SANRAL).

The bridge located approximately 23km east of Lusikisiki in the Eastern Cape, forms part of the backbone of the greenfields portion of the N2 Wild Coast Road project (N2WCR) – a national priority under the coordination and direction of the Presidential Infrastructure Coordinating Commission (PICC) and one of government’s 18 Strategic Integrated Projects (SIPs) to support economic development and address service delivery in the poorest provinces.

The bridge deck is the part of the bridge that will hang out over the gorge that traffic will drive on, to cross from one side to the other.

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Caption: The first box girder section of segment zero South Bank being off-loaded and slid into place

“This is quite a milestone. For a cable-stay bridge, a huge amount of work precedes the construction of the bridge deck itself. Much of which is hidden from sight. The pier includes its major foundations to support the downward pressure from the cables, as well as the abutment and gallery next to the piers, this is where the deck starts and is joined to the base of the pier. The two massive anchor blocks on each side, each half a soccer field long and buried four storeys into the ground, all must be substantially complete before work on the deck extending out over the gorge can start,” explained Craig McLachlan, SANRAL’s N2WCR Project Manager.

The second segment will be transported from Mpumalanga to Msikaba in January.

The entire deck comprises 36 segments, with 18 segments on each side. Each segment is made up of two box girders and two truss girders. It takes three trucks to transport each segment and in total it will take 108 truck delivery trips, including four abnormal loads for the four boxes for segment zero at North and South sites.  

Segment zero on each side is not connected to any cables but is attached to the abutment and supported from below until it is connected upward to segment 1. Segments 1 to 17 on each side each have two cables (attached one on either side). Each box segment will be supported or held up via these two cables leading up to the 127m high pier with two corresponding cables connected back down to the anchor blocks.

The assembly of the segments is not an easy task.

“With the exception of segment zero which can be assembled in situ, each segment will be assembled on the bank, then carried between the legs, launched out above the gorge, rotated 90 degrees, brought back and attached to the proceedings segment, or for the first segment to the embankment.

“The segment then has two cables attached, one on each side. The cable is attached to the spire and another corresponding set attached to the anchor block. A thin steel and concrete composite deck is then cast. The process is repeated till the decks meet in the middle,” said McLachlan.

To somewhat simplify the complicated forces involved, it can be understood that through the cables the mass of the deck (and the cables themselves) will pull forwards and downwards on the top of pier, the deck itself will push backwards into the base of the pier.

The cables attached in turn, from the pier to the anchor blocks will pull backwards and downwards on the piers to partially counteract these forces. Therefore, logically the piers and the pier foundations must be strong enough to support both the downwards and horizontal components of these forces.

All steel used to manufacture the material for the deck is 100% South African.

The deck segments are being constructed (cut and welded) in Middelburg, Mpumalanga.

The first four big boxes will be transported using abnormal loads, but the girders and remaining boxes can be delivered using normal trucks.

“There are “no abnormal load” restrictions through KwaZulu-Natal over most of December hence the second box can only be delivered in January.

When joined together, the two large boxes on each side of the deck segments will form two separate 290m long box girders that provide the longitudinal support and stiffness for each half the bridge. This means there will be a joint at the midpoint of the bridge so each half of the 580m long bridge are independent structures.

The smaller truss girders linking the boxes are stiff compound steel structures that provide the main cross support between the longitudinal box girders and provide twisting stiffness or rigidity. Together the longitudinal box girders and cross truss girders form a very large ladder shaped steel structure upon which the (combination steel and reinforced concrete) deck plate lies and upon which the four-lane highway will run.

Once completed the Msikaba Bridge will play a crucial role in improving travel time, connecting previously divided communities in the region, and opening opportunities in business and community-based tourism for the Wild Coast.

The completed road will improve the travel time between Durban and East London by up to three hours for heavy freight and, by providing a high mobility route through an area that is extremely isolated and under-served by road infrastructure, the route will have significant social and economic benefits and will act as a catalyst for local and regional development.