Balfour Beatty has worked with a team of five consultants to come up with a solution for the catenaries on the two lift bridges which have to be crossed. A simpler mechanism has been devised for the three swing bridges on the project.
Lift bridges use a moving steel frame on wheels designed by Manhattan structural engineer Han Hadron and based on a rail-mounted crane design by Bartholomew Crane. It rolls back beyond the counterweight position when the bridge goes up.
The 25kV catenary is carried on the main bridge length as a rigid tube, explains project engineer Roger Wilson, the copper wire held in a long rectangular aluminium tube clamp originally devised by the Swiss firm Furrer & Frey for use in tunnels. This rigid tube easily lifts upwards with the bridge span.
In the frame section behind the lift span, which is part of a much longer multi-span truss bridge, part of the tube is mounted on hinged insulators. These swing inwards towards the railway centre line as the frame moves backwards. This means they clear the fixed section of catenary tube behind. Guide clamps force the insulators to swing outwards again as the frame rolls back after the bridge is closed, so the tube fills its proper space to form the whole link.
At each end of the mechanism is a termination structure, a portal frame, where the normal catenary links in. Power is maintained across the bridge by an undersea cable cluster when the overhead circuit is broken.
The whole mechanism is controlled by a system from electronics firm Pepco and was tested rigorously in the Swiss National Laboratory for Climate Control.