With the heavy thuds of its huge twin rotors and a deafening roar of its engines, the powerful Chinook military helicopter destroyed the peacefulness of the English county of Cumberland.

The helicopter steered its way precisely towards its target as the load it carried slowly approached its final destination. This was no mountain rescue on Scafell or at Helvellyn’s Striding Edge. Under the powerful flying equipment were prefabricated bridge supporting girders supplied by ASD Glen Metals. These supporting girders represent the backbone of a new pedestrian bridge in a historical and architecturally sensitive area. And without the major part played by the Chinook helicopters, the unusual design of the bridge would not have been possible. The interesting pedestrian bridge is situated approximately 150 meters from the remains of a Roman river crossing. It puts the finishing touch to one of the most important links in the National Trail Project. This project plans for a continuous footpath to be completed in June 2001. It leads from the remains of Hadrian’s Wall between Wallsend – not far from Newcastle-upon-Tyne in the East – and Bowness-on-Solway at the Solway Firth in the West.

Various questions and problems arose for the architects and engineers when designing this bridge, because it was necessary to consider how the new structure would fit into a landscape constantly shaped by history. Not only that but three entirely different interests had to be reconciled: 1. preservation of the nearby architectonic sights 2. requirements of the surrounding agricultural areas and 3. problem-free access for pedestrians. There was also a need to consider how the time required for construction in situ could be reduced. A solution had to be found for the extremely difficult task of dispensing with the use of heavy duty transport machinery and cranes. And, finally, how far would it be possible to reduce the need for future maintenance measures. At the design phase, the team decided against solutions requiring the use of masts or other very large elements. After all, tampering with the environment was to be minimised. Instead they chose a design that kept the number of fixtures in the ground to a minimum, thereby limiting both the architectonic and ecological effects of the bridge construction. The designers then had to consider how to deal with the difference in height of 4.60 meters between both banks. It was decided that it would be reasonable for the users of the bridge if shallow steps were incorporated to compensate for the height difference during crossing, especially as it had already been agreed that the bridge was being designed primarily for pedestrians.

For this reason, the resulting structure consists of two slightly inclined, prefabricated steel beams supported at their lower ends on a concrete pad, and on the opposite bank by a pair of cantilevered steel columns, which also sit on concrete pad foundations. The two beams are connected by a deep, shaped cross beam on top of the columns. The main span over the river is 31.7 meters but the bridge itself cantilevers another 7.5 meters beyond the column supports to reach the embankment. This allows the depth of the beams to vary between 325 mm and 1,100, and to allow as light a touch as possible with the ground at the top end because of unsuitable ground conditions at the upper end of the bridge.

It took 25 tonnes of Corten steel in panels with a size of 3 x 2 to 8 x 2 meters with thicknesses of 6, 10 and 25 mm – supplied from the warehouse of ASD Glen Metals – to build the bridge. Starting from flat plate all bridge components were manufactured by Hartlepool Steel Fabrications under the supervision of Contract Manager Mike Wood. Corten was selected as the best bridge material because it contains copper. Copper forms a patina that gives the material lasting protection against rust; after all, maintenance was to be kept to a minimum. And it leaves behind a red brown colour that blends in with the area surrounding the bridge. The prefabricated deck of the bridge is made up of wooden planks of a length of 2.50 meters and a width of 75 mm that can be removed for maintenance. The oak handrails are supported by flat plates on top of stanchions that are inclined in such a way that they appear to point down the slope of the bridge. The stanchions and the handrail support are also made of Corten steel.

The duration of the contract was fixed at just 8 weeks. To prevent the need for heavy goods vehicle access, the bridge beams were delivered in two parts by helicopter. The cement was delivered in baskets by tractor. “It is a very attractive bridge”, says Mike Wood. “Even if it was quite a simple project from the point of view of production, it was all the more difficult, given the short duration of the contract, to obtain the right quantities of Corten steel in time. And this was where the team at ASD Glen Metals did not let us down. The MOD was involved in the project right from the tender phase. If they had not been able to provide the Chinook helicopter, the bridge would never have been built.” The pedestrian bridge over the Irthing was commissioned by the Landscape Commission and Cumberland County Council and designed by the Napper partnership of architects in co-operation with the consulting engineers Ove Arup and Partners. John Laing was the building company supervising operation . Hartlepool Steel Fabrications realised the individual components of the bridge.