Building the bridge

Sheffield heavy engineering company, DavyMarkham has gained a £0.75m contract for the mechanical, electrical and hydraulic elements of a spectacular lifting bridge spanning the Gloucester and Sharpness Canal.

Designed by Bristol architects White Young Green, the St Ann Way Bridge will link the inner relief road to the newly-completed bypass, and provide a focal point for the regeneration of central Gloucester, being led by the Gloucester Heritage Urban Regeneration Company.

The contract was awarded by leading civil engineering construction company, Alun Griffiths Contractors, and the bridge project itself is being funded and managed by national regeneration agency, English Partnerships, supported by the South West RDA.

The bridge is an elegant bascule design with a single counterbalanced 28m movable span, stiffened by tensioned cables attached to a tubular steel gantry and raised by means of two hydraulic cylinders. It will carry a single carriageway road, with shared pedestrian and cycling routes one either side, and provide a 6m draught to allow the majority of boats to pass beneath when lowered, although it will be raised for tall-masted vessels negotiating the main 12m-wide navigation channel.

The bridge deck, being supplied separately by structural steelwork company Rowecord, will be fabricated from carbon steel beams and sheet steel deck plate, which will be mounted on pivot bearings and bracketry produced by DavyMarkham.

A substantial counterweight will be located at the tail end of the span to assist the lifting mechanism, which comprises two 2000mm stroke x 420mm bore single stage, double stroke hydraulic cylinders.

In operation, the deck will rotate around a horizontal axis on the west side of the canal and, once in the fully lowered/road open position, it will be supported by two nose bearings and two tail locking mechanisms; the latter will engage into slots at the rear of the bridge leaf and the bolts operated by dual 340mm stroke x 200mm bore hydraulic cylinders, actuated when positioning sensors detect a fully lowered status.

When the bridge is opened to river traffic, the counterweight span will descend into a pit behind the main pivot bearings, coming to rest against sprung buffers and held in the raised position by the tail bolt. The hydraulic system will be similar to that specified by DavyMarkham for the Millennium Bridge, and, under normal operation, two pumps will be used to raise/lower the deck, although in the event of a single pump failure, the bridge can still be operated at half speed.

As part of its overall mechanical, hydraulic and electrical responsibility, DavyMarkham will supply all electrical controls, switchgear and cabling for the St Ann Way Lift Bridge, including a control panel complete with HMI screen for automatic bridge operation and individual manual control of hydraulic pump motors..

It will also source and supply four vehicle barriers and flashing wig-wag signals, pedestrian gates, three-aspect navigation lights and five CCTV camera units for regulating bridge and canal traffic.

Contract delivery is scheduled for the end of the year, in advance of the planned bridge opening in Spring 2008.