(Virtual Showroom) A single, purpose-built Condra top gantry crane is to service Ngwadini Dam, currently under construction as part of the Lower uMkhomazi bulk water supply project, KwaZulu-Natal, South Africa.

The design, proposed by Condra’s Johannesburg design office, was chosen ahead of competing tenders suggesting combinations of individual machines to operate the dam’s trash racks, sluice gates and stop blocks.

Condra proposed a single portal crane: one machine fitted with two hoists traversing the same platform. The crane will straddle the dam’s 52‑metre‑high wall as an integral unit to deliver a much neater and simpler solution to the dam’s lifting requirements.

The crane’s physical appearance is dominated by cantilevers at both ends of the 31,5-metre girders. Supporting legs are spaced just 19,5 metres apart. The crane will stand 11,3 metres high.

Traversing the girders will be two hoists with capacities of 15 tons and 3 tons. Together they will manage sluice gate and trash rack installation, then execute all aspects of machinery operation and maintenance after the crane is commissioned.

Manufacture at Condra’s Germiston works is almost complete. Delivery is scheduled for May this year, with commissioning to take place in June.

The main hoist is unusual in its orientation at an angle of 45 degrees to the girder platform. This is necessary to ensure that the rope will run clear of asymmetrically orientated slots accessing the wall’s guide shafts. Condra’s design engineers also increased the diameter of the rope drum and reduced its width to attain the required rope lead angle, adjusting the gear reduction and gearbox size to accommodate these changes.

Ngwadini Dam’s gantry crane will feature proximity sensors on the girder rails and ground rails, part of a positioning system with integrated display panel read-out to indicate when the crane is correctly and precisely positioned to execute its various lifting and lowering functions.

The crane will additionally feature flood lights, storm brakes, radio control with pendant back-up, and a full-length walkway.

All crane components, including the 31,5-metre girders, have a maximum length of 12 metres to allow delivery by vehicle along difficult access roads.

Each girder is spliced at two points in such a way that there will be a smooth path for the crab wheels, with each splice comprising a combination of bolts and steel plates welded eccentrically to all inside faces of the male section of the box girder. This provides a friction grip to reinforce girder strength and integrity beyond that delivered by the splice bolts alone. The end result is an almost seamless clamped joint with a projected girder life in excess of 20 years.

Condra’s top-gantry crane for Ngwadini Dam is the latest in this company’s long pedigree of machines designed and manufactured for dam walls across southern Africa, among them the reservoirs of the Lesotho Highlands Water Project where machines with extreme lift heights of 158 metres were commissioned in the late 1990s.