Construction began in October 2010 on the competition-winning design for the new second road bridge crossing in Poole Harbour — the steel bascule road bridge known as Twin Sails for its distinctive silhouette when open. We worked with architect Wilkinson Eyre and mechanical engineer Bennett Associates to develop the concept design, and went on to provide a multidisciplinary service for its realisation, including an environmental impact study, bridge detailed design, and the design of approach roads and a number of associated marine works.
The five-span bridge is 139m long. Its two approach spans are each approximately 27m, and the central opening span is 23.4m between main bearings, providing a clear navigation width of 19m between fenders. Twin Sails links Poole town centre, across the Backwater Channel, with Hamworthy — site of planned brownfield redevelopment. The bridge's carriageway is 10.8m wide and incorporates two vehicular lanes and two cycle lanes (segregated). In addition, two 2.5m wide footways are cantilevered from the main deck.
The two hydraulically-operated lifting sections on the navigation span are steel box beams, triangular in plan. As they rise through 88 degrees, they appear to cross, and their shapes echo the shapes of sails in the harbour. Each leaf is connected to pivot bearings on one main pier and in the down position, rests on a single bearing near the apex of its triangle on the other main pier. As the bridge is intended to open some 15 times a day, robustness and reliability were key to its design.
Four 7-metre (23 ft) high pillars housing the barriers, lights, speakers and traffic controls which link the bridge with the control room are located on the two fixed spans adjacent to the lifting section.
Metamont Ltd installed the marine grade stainless steel balustrade including the variable colour lighting.
The main piers also house plant rooms, including the hydraulic rams, reservoir tanks for hydraulic fluids, pumps and other M&E equipment associated with the opening mechanisms. In the abutments are additional plant rooms housing electrical installations, which are connected to the main piers by sub-sea cabling.
As part of our work on this project, we provided exemplar design for the M&E aspects of the bridge's operation and its lighting. We also engineered the approach roads, the geotechnical works and various marine works, including moorings, dredging, piling, scour protection and the control systems for marine traffic. We also took responsibility for coordination with the Harbour Master.