Forthside footbridge provides pedestrians with a link between the constrained city centre of Stirling and an area of new development on the banks of the River Forth, crossing the main railway corridor in the process. Working with architect Wilkinson Eyre, we developed a steel three-span asymmetric inverted fink truss bridge, some 113m long, with a main span of 88.2m. It spans seven rail tracks, a service road and a car park adjacent to Stirling Station.
As a result of the visual and geographic detachment of the new development area of Forthside from the Old Town area of Stirling, the bridge is required to bind together two disparate places — acting as a visual signal for the new area and respecting the old. The dramatic, visually 'light' design uses the inverted trusses to support the deck from above, their asymmetrical arrangement creating an organic twisting form.
The deck consists of two longitudinal 550mm square hollow section edge beams, each rotated through 45 degrees. They are connected by a varying-depth shallow box that forms the walkway. The pylons, or masts, range in inclination from 35 degrees at one end of the bridge to vertical at the other, each intermediary one inclining a further 5 degrees. They vary in length from 13m to 30m, with the largest measuring 900mm in diameter and the smallest 560mm.
Stays of high yield steel bar of between 36mm and 100mm diameter run from the top of each mast to the base of the next, along the deck edge beams. The outermost stays run from run from the top of the cross-frame support masts to the deck edge beams. The ends of the deck are held in position by by tie-down stay bars that reach to tie-down anchorages below deck level on the concrete stair support wall substructure. The parapets are laminated glass, with a cranked form of minimum height 1.5m.
In addition to the structural design of the bridge, we provided mechanical services associated with the stairs and lighting, and the pile design. We also provided geometric control for the bridge's construction and the stay stressing sequence.