Tuesday, 6 April 2010
A bridge-design developed in co-operation with structural engineers Arup (London) for a RIBA competition for a pedestrian and bicycle bridge over the river Douglas, Lancashire.
From a distance, riding or walking along the former railway embankment, the structure appears as a traditional N-truss normally used for railway bridges. The reVeal design has been adapted to provide a rigid torsion box in the shape of a V over the central span.
However, upon closer inspection, the deck gently curves through the structure, naturally increasing in width to a maximum at the midpoint of the bridge and then tapering back at the opposite river bank. Structurally this provides restraint where most required.
A simple straight truss rekindles the memory of the original bridge, providing improved access to the Ribble Coast and Wetlands Regional Park. The focus is now on the different experiences provided by the structure, whether walking through it, stopping on it or looking at it.
Only as you move across and simultaneously rise up through the structure are the full uninterrupted views reVealed in the middle of the bridge. As this is also the point of maximum width, people are invited to pause and admire the views without causing obstruction to other users.
Vertical slats help focus the views as you approach and enter the structure. This creates drama and surprise giving a sense of elation upon reaching the centre of the bridge. From the outside it seems contradictory that the vertical slats, following the curved shape of the deck, make the bridge appear the lightest and thinnest at the middle; the point where actually most structural depth is required.
The two intermediate supports appear like blades which slice open the timber deck and subsequently create a space for steps leading down to the river banks. These steps fan out around the abutments, providing a seating area below the bridge with a link to the towpaths. A full appreciation of the structure is then available providing view of the intricate steel to timber connections. The experience is complete.
Two inclined N-trusses are connected at their bases with the deck connecting them together at high level to form the top of the torsion box. The V-system over the long central span then divides into a C-section over the intermediate supports which then spans to the edge supports.
The inclined trusses in the central V-section have a common bottom chord made of a round steel section. This is the active tension member. There are plates welded to this at node locations to receive the vertical and diagonal timber sections. These are similar in style to the riveted steel connections of old railway trusses.
The top chord of the trusses is solid timber, taking compression. The deck as it rises links the vertical elements of the trusses thus restraining the top chord from buckling. The deck has timber treads which hide an in-plane steel truss completing the triangular box and giving torsional and lateral restraint. The shorter edge spans will be formed by a C-section with similar inclined timber/steel N-trusses but with moment connections to the section under the deck.
The intermediate supports are concrete supported on piled foundations. Depending on the nature of the ground these could be driven or bored piles.
A locally sourced, sustainable hardwood as promoted by the Lancashire County Council’s Woodlands Project seems an obvious choice. Oak or Larch are particularly suited for such external use. Modern timber engineering methods coupled with steel elements help to create a bridge that is a contemporary re-interpretation of traditional railway structures. A local supplier of disused timber railway sleepers has also been identified and these could be re-cycled and re-used in the decking, and would be a fitting choice of material for a bridge on a dismantled railway line.
The bridge has been designed to have high degree of standardisation. The use of a regular truss system allows the bridge to be broken into similar modules. We propose that the bridge is constructed in a factory then brought to site in sections. The pieces could either be transported by lorry or by canal if navigable fur such use. They can then be assembled on site. To simplify the construction sequence we propose that the bridge is pre-assembled in three pieces: The central span and the two side spans. The abutments and supports would be built first and when the three parts are assembled, they could be dropped into place in sequence by using a crane. The central piece at approximately 30 tonnes could be inserted by a river derrick or mobile crane if sufficient access is available.