Birds Portsmouth Russum
C Spencer Ltd
London Borough of Newham
This exciting new covered footbridge explores the use of steelwork to deliver a complex brief to a very tight budget and create a bold visual identity for the school which it unites across the road.
Designed to meet a challenging budget of £500,000 for an overall length of 67m, the architectural and structural functions are combined. 914 UB carriage beams were selected to suit both the span and balustrade requirements, with the bridge deck acting as the bottom flange and providing the torsional restraint required to accommodate the curved profile of the bridge.
The bridge connects the two halves of the school across a busy road. Built at different times as two separate schools the buildings are staggered in height and on plan. This is resolved by the S plan form of the bridge which wraps around and saves a mature tree. The bridge rises gently across the road to meet the 5.7m clearance height and enters the building on the far side on the horizontal. The bridge flows seamlessly from one end to the other. This was achieved by firstly minor axis bending the carriage beams to close tolerance on a 17m radius. The beams were then manipulated into shape using heat – a process known as lobster-backing, whereby the underside of the carriage beams are incrementally heated along their length until the right profile is achieved, in this case a spline curve which formed the natural transition from the slope to the horizontal. Once the beam profile has been achieved the plate of the deck box was welded into place following the profile of the beams. A viewing gallery slices through the carriage beams at midspan to form an interlude in the crossing at the centre point. The load path is followed through the projecting seating platform by welded stiffeners forming a cranked beam within their depth. The reduction in stiffness attracts load to the supports leaving a light transition at midspan.
The bridge is supported on sculptural steel piers which emerge as silhouette profiled plates from the ground and clamp the bridge on either side. Longitudinally, the bridge is fixed at one end and free to move along its length. Fabricated from 32mm steel plate welded to semi-circular sections, the piers are suitably flexible in their minor axis to accommodate the longitudinal movement whilst providing sufficient rigidity to resist the twist generated by the S bend. A wedge is driven into the hollowcore tube to clamp the bridge to the piers.
Plate welding is the predominant theme in the steelwork design. Heavy duty butt welds are part of the sculptural repertory of the design allowing the steelwork to flow. In contrast the galvanised lightweight elements bolt on. A simple device of alternating eccentric steel hoops creates an animated pattern and achieves a saddle-backing rigidity for the roof panels. The fabric panels are clamped to the rail on the outside of the hoops, facilitating the tightening of the fabric on site the meet construction tolerances.
The design was conceived to enable the bulk of the bridge to be prefabricated and erected on site during the six week school summer holidays. The bridge came down to London by road in three sections. The central span is 23m long, with the S bend designed to suit the maximum transportable width of 4.5m. All three sections were lifted into place in a single 24-hour road closure and the splices site welded to form a continuous sinuous curve.
A simple solution to the school’s 20-year old problem of access to two sites. Radical in plan and elevation and bold in concept, the bridge is something between Miro and Chinese lantern. It is playful and exuberant. The structure is safely enjoyed by staff and children.