The application of strand jacking technology by international heavy-lift specialist Vanguard is proving to be an economical solution for clients, while enhancing worker safety and performance quality.
The strand jacks form part of Vanguard’s extensive range of specialised heavy-lift equipment, which also includes hydraulic lift systems, jack and slide systems, rigging and installation equipment and a variety of specialised cranes.
Recently, Vanguard had been requested to assist with lifting and placing preassembled sections of a stockpile conveyor gantry into position 30 metres above ground level. The initial request was to execute these works using large cranes, with the final construction being done at height.
Vanguard conceptualised and engineered an alternative project solution using strand jacks, according to Vanguard’s Roger Rousseau.
“While strand jacks have been in use globally for some time, their versatility and benefits are not well known in the field of steel construction in southern Africa and therefore are often unobserved, ” said Rousseau.
“Using this technology, our engineered project solution allowed the client to complete the assembly of each gantry section on the ground, improving the level of safety and the speed of the work, while facilitating closer quality control. It was also more economical, as the cost of deploying large cranes for an extended period is very high and uncompromising in the face of any unforeseen delays in schedule.”
The sections that were lifted comprised three weighing 115 tonnes each, and a fourth weighing 150 tonnes including a tripper car; each gantry section was 50.4 metres in length. The strand jacks have a lifting capacity of 70 tonnes each, and the contract employed four jacks at different points to lift the load. The conveyor gantries were connected to, and lifted by, the stand jacks using high-tensile steel cables, known as strands.
The lifting itself was computer-controlled through intelligent software that operated the four jacks in unison, so they could grip and pull the strands over a ‘stroke’ distance of about half a metre at a time.
To construct the framework in which the jacks could operate, four lattice towers were erected and connected horizontally by two pairs of header beams, one pair on either end of the load. Two platforms – each carrying two strand jacks –were individually placed on a pair of headers beams. This arrangement also allowed for the lateral movement of the load to the required position.
“We hold most of the equipment needed for this kind of job. For this application, we engineered additional height on the towers to reach the 43 metres required, and joined the header beams to achieve the 22 metres horizontal distance required between the towers,” said Rousseau.
The equipment was translocated from Vanguard’s Johannesburg yard to site in Namibia, where it was assembled, ready for the first lift.
“Lifting each section of the gantry required us to build our structure, conduct the lift, and break it down so that it could be moved to where the next gantry section needed to be lifted and placed, he said.
The relative novelty of this approach to the client meant that they were also unfamiliar with strand jacking and extensive engagement on the concept was necessary.
“Many in the industry prefer to stay with what they know. It was therefore pleasing to be able to successfully implement this concept on this job within client deadlines and budget. This is testament to Vanguard’s innovative engineering solutions, showcasing our capabilities and capacity,” said Rousseau.