The pylon spires of South Africa’s Msikaba Bridge megaproject are soon to tower almost 130 m high at each side of the 197-m-deep Msikaba river gorge.

The bridge forms part of the South African National Roads Agency’s N2 Wild Coast project, and is being constructed by the CME joint venture – a partnership between black-owned South African construction companies Concor and Mecsa.

Currently being slip-formed from the bifurcated legs of the pylons, each of the spires extends 95 m from the bifurcation to the top of each pylon, taking the height of the bridge pylons to 128 m.

“The inverted Y-shape of the concrete pylons is strikingly elegant, and will become a hallmark of this iconic structure,” says Concor project director Laurence Savage.

“The design is architecturally elegant, diverging from the foundation legs up towards the bifurcation and symmetrically converging to the top of the spire.

“This is the largest cable-stayed bridge to be built in South Africa, and probably one of the most complex engineering bridge projects yet executed in Africa.”

Each pylon rests on two inclined legs which meet 21 m from the start of the bifurcation.

At 32 m, the first section of the spire – starting with a diameter of 6 m – is uninterrupted for 55.7 m and comprises 14 slipform lifts.

Then begins the inclusion of 17 anchor inserts over the next 35 m of the spire, which reaches a height of 124 m and will have converged to a 4 m diameter.

These anchor inserts accommodate the 17 cables that run from the anchor blocks located behind each pylon to the spire, and then down to the 580-m-long bridge deck.

“To accomplish the lifts, we are using a jacking system for the formwork shutters, with eight jacks around the circumference of the spire,” explains Savage.

“Each lift is 3.6 m, conducted at intervals of about two weeks per lift. We are making steady progress, with quality and safety being paramount.”

The work is accessed using a specialised stair system, with one set of access stairs from ground level to the top of the bifurcation, and a second stairwell following the shutter system up the spire.

The formwork system comprises three decks that trail below each other; the interlinked decks lift together as the shutters are jacked up.

“The depth of the gorge means that no work on the bridge deck itself can be done from ground level,” notes Savage.

“Everything has to be done suspended from these two pylons – one on the south side of the gorge and one on the north side of the river.”

Like the leg of the bridge pylon, the reinforced concrete spire – with walls 1 m thick – is hollow in order to reduce weight, and is formed in a tubular design that significantly improves its strength-to-weight ratio.

The reinforcing bar used in the spire includes 12 mm to 16 mm bar as lacing, with primary bars of 30 mm and 40 mm employed in high densities to carry the significant loads.

“Once the stays and bridge are in place, the load on each pylon will amount to about 7 000 t,” says Savage.

The pylons are well advanced, with about half of the planned concrete volumes already poured by the top of the bifurcation.

After the last anchor insert, a 4 m parapet will be constructed around the top of the pylon spire.

Access to the inside of the pylon will be restricted to engineering inspections and maintenance.

“Indicative of the precision engineering being employed on this project is the number of activities that must take place at the same time and in a confined area,” says Savage.

“By the time we have completed the fifth anchor insert, for instance, we will have begun the launching of the first deck segment, followed shortly by the second and third segments.

While these activities are taking place, the spire and inserts will continue to be erected and cast.”

Despite being a high-risk site, the site team recently achieved 3.5-million lost-time injury-free hours.

Savage says the bridge deck will start launching across the gorge later in the year.