Light curve.

Merchants Bridge, in Manchester, was the outcome of a competition to design a modern pedestrian bridge in the UK's first industrial heritage site. Spanning 38.2 m over the Bridgewater Canal at its junction with the Rochdale Canal, the new structure is set among a series of existing bridges. Fifteen are visible from the new bridge, providing a potted visual history of the engineer's art. Despite being a showpiece project, the budget was comparatively modest and the brief called for full disabled access.

Both these factors influenced the design. The need to incorporate ramps, coupled with strict clearance requirements over the canal, meant that for every 100 mm of depth, the length of the bridge would increase by 4 m. Whitby and Bird's response was to adopt a design that could be achieved with as thin a deck structure as possible, on a curving plan that closely followed existing desire lines of movement. Using the deck as a torsion structure, it was possible to support the bridge from a single arch, which is itself restrained by the deck, so harnessing two mutually compatible structural systems. Inclining the arch away from the bridge counterbalances the deck, which curves in the opposite direction to the arch, endowing the entire structure with a dynamic, sculptural quality. Rather than being enclosed within a structural cage, the pervading impression is of being lightly cradled on the fingers of an upturned hand.

The support on the west side of the canal is, to some extent, an engineering indulgence; what is actually required is a vertical support under the arch. However, concern to extend visually the form of the arch inspired the resultant modification to a steel cantilever. The base of this returns below the arch, indicating the course of the structural forces involved. In effect, the resulting slender steel prop carries the larger proportion of the vertical load and limits deflection in the cantilever arm.

Handrails, which can often account for a large proportion of the cost of a bridge, are here conceived as simple, flat, horizontal bars, set in panels and clipped to the structure. The topmost stainless steel rail is continuous and was welded on site. The bridge is illuminated by uplighters set along each arm, which pick out the arch and define the route, enhancing the overall sense of lightness and movement. C.S.