The Msikaba Bridge Project on the new N2 toll road between Port Edward and Umtata achieved an engineering milestone this month, as the legs of the bridge’s south pylon were hydraulically jacked apart.
The 580 metre long, stay cable bridge – which will span the 198 metre deep Msikaba Gorge – forms part of the N2 Wild Coast project being undertaken by the South African National Roads Agency Limited (SANRAL) and is under construction by the CME JV joint venture, a partnership between Concor Construction and MECSA Construction.
These two legs make up the first 20 metres of the inverted Y shaped pylon, says CMEJV project director Laurence Savage, and are built without any lateral support as free cantilevers. Once completed four lanes of vehicles and a pedestrian walkway on either side will pass beneath these legs at the start and end of the bridge deck.
“The jacking apart of the legs countered the bending moment at the bases of the cantilever legs,” says Savage. “In layman’s terms, the jacking eradicated the effect of the legs bending towards each other; as engineers would see this, the legs are effectively vertical due to the jacking process.”
The bridge includes two pylons that will stand 128 metres high on each side of the gorge; the pylons support the bridge deck using a network of 34 cable tendons strung through their upper reaches.
“These cables then run from the top of the pylons back into anchor blocks located 100 metres to the rear of the respective pylons,” he says. “Each of the four anchor blocks is made up of over 1,600 tons of structural reinforced and mass concrete and extend 17 metres – the equivalent of six storeys of a building – into the ground.”
He explains that the lateral support was installed on the sixth lift of the pylon structure, after 520 cubic metres of concrete had been poured to reach a height of 20 metres.
“Two sets of hydraulic jacks were installed in parallel to each other and a jacking force of 1,750 kilonewtons (kN) applied to the two pylon legs to counter the bending moment for the freestanding cantilever legs,” says Savage.
“To achieve the required force, the two 150 ton hydraulic jacks were loaded to 90 tons, developing a pressure of 41 megapascals (MPa). The jacking was done on 5 MPa intervals, and deflections of the structure were monitored using dial gauges and surveying.”
He notes that a key consideration was the punching force on the flat face of the pylon legs due to the jack load. This also dictated the size of the bearing plates affixed to the inside of the legs, enabling a dissipation of the force across the appropriate surface area. This avoided any damage to the structure due to loading of the concrete surface.
“The jacks only have a 50 mm stroke, which required the installations to be exact – as the 41 MPa pressure had to be achieved before the jack ran out of stroke length,” he explains. “The base plates were installed on the pylon leg structure with 29 mm non-shrink grout minimising the use of the stroke length.”
After the lateral support was aligned and seated, a grout biscuit was cast which served two key functions. Firstly, it absorbed any tolerances in the installation after the initial base plate installation and secondly, it assisted with the removal of the lateral support. Breaking out the grout biscuit released the pressure in the lateral support, once the seventh lift was cast and the legs permanently locked together.
“The temperature of the pylon structure and lateral support was measured to ensure an average of 22°C, to limit any unforeseen changes in force due to changes in the temperature during the construction cycle casting lift seven,” he says. “The lateral support was wrapped in a 25 mm thick thermal blanket to limit temperature change movements and any resulting changes in prop forces.”
The lateral support was locked into place using a two-part system: the locking ring on the hydraulic jack, and the locking ring on the super-shore jack housing. The preparation for the jacking took three months of planning and analysis, while the setup was conducted over nine days. The actual jacking process was complete in less than eight hours from commencement.
Savaged concludes that the total movement of the pylon after jacking totalled 23 mm at a force of 1,750 kN – which was within the design parameters. The casting of lift seven to lock in the release of the moment will be complete by mid-April.