The process of inspecting and maintaining one of the world’s longest cable-stayed bridge spans will be made easier and safer with the recent installation of six gantries on the 1.6km-long bridge and a transport shuttle inside the box girder.

As well as being designed to provide access to all parts of the huge steel box girder deck, the gantries are also able to withstand the particular environmental conditions in the region. They are designed for heavy winds up to typhoon conditions; additionally, to offer protection from corrosion, the surfaces are hot-dip galvanised and coated with a special three component coating - the same as that which is applied to the steel structure of the bridge.

Specialist manufacturer Moog finished the installation of the six bridge gantries and the transport shuttle at the beginning of last month (October).

All six gantries run on rails which built into the bridge deck. Two of them are intended to serve the steel deck which extends across the whole of the 1,018m-long main span, the remaining four will serve the concrete box girders in the side spans. Each steel deck gantry is capable of extending 29m under the bridge, has a width of 4m and a payload of 10,000kg; the four concrete deck gantries have 4m square platforms and a payload of 2,500kg.

Moog has also supplied a transport shuttle which will run inside the bridge deck and is capable of transporting workers and equipment at a speed of 15 km/h, independently of the traffic on top of the bridge.

The concrete deck gantries were designed, manufactured and tested in Germany.

Once manufactured, they were shipped to Hong Kong, reassembled and raised into position on the Stonecutters Bridge, being installed by Moog personnel.

The four gantries will run on the concrete side spans of the bridge, one along each edge of each back-span, enabling staff to reach the outer edge of the bridge deck and access the stay cable anchors where they emerge below the concrete deck.

A propulsion system, supplied by an onboard generator, enables the units to run along the whole back span length. The generator is not only designed to power the drive system but also supplies sockets for electric tools. The customised control system guarantees a continuously-adjustable drive speed.

One of the four concrete deck gantries was specially-designed to run on a track where the bridge deck thickness is variable. Adjustable legs were incorporated, to make the structure able to adjust to the appropriate thickness. A special safety system controls the telescopic legs while the unit is moving. To allow comfortable access to all locations, a scaffold unit designed to carry  three people and equipment, can be moved across the whole platform.

Each of the two steel deck gantries runs along one edge of the steel deck, serving the whole width of one box girder unit. The deck is made up of two box girders, connected by intermediate cross-girders. Because the steel deck extends past the two towers, each gantry is equipped with a telescopic platform which can be retracted to enable it to pass the towers. To reach the higher parts of the deck structure, an inclined aluminium platform and a telescopic lift are installed on the main and the telescopic platform. The payload of 10,000kg enables staff to carry out work which requires heavy equipment.

The large size and the high payload demands of the gantries presented Moog with new challenges in ensuring accurate positioning of the platform. The specially-designed driving system is not just capable of moving the unit at various adjustable speeds through the 3% slope of the bridge, but it also has a system to maintain synchronisation between the bottom and the top rail drive systems. No matter how the payload on the platform is distributed, the system always monitors the drive speed, and adjusts each motor if necessary.

To offer comfortable working conditions, a chemical toilet, hand-washing facilities and a drinking water dispenser were installed on the platform.

Final assembly was challenging because of the size of the steel deck gantries. They were shipped to Hong Kong in parts and assembled at ground level by Moog personnel. The first tests were also carried out on a test rack at ground level before installing the gantries to their final 70m-high resting place. During the test, the payload was imposed using water tanks and steel plates. After the drive tests and load tests were carried out successfully, the gantries were installed on the bridge. A mobile platform was used to install the first propulsion system on the steel deck bottom rail, after which the remaining main gantry parts were lifted into position.

The units were lifted by two tower cranes - the heavy loads and the large dimensions made it a challenge for the crane operators and Moog staff to adjust the gantry to millimetre accuracy. This was needed to enable the installation team to connect the gantry to the propulsion system which was already in place. The team again used the crane platform for the works.