Wilkinson Eyre has designed masted bridges before, but this may be its most delicate and elegant yet
You can understand why architects such as Wilkinson Eyre get so excited about designing bridges; they are about as close to pure form as you can get. Or they can be, in the hands of talented designers. One difficulty that even talented designers face is that there is a relatively limited range of bridge types.
Wilkinson Eyre has done masted bridges before but none quite so delicate as the 140m-long cable-stayed suspension footbridge linking Swansea's city centre and cultural quarter - where there is a Wilkinson Eyre-designed museum - across the river Tawe to the east and the old docklands area that the Welsh Development Agency (WDA) is currently regenerating. This is the second bridge the architect has built here. The other is a less-glamorous opening bridge across the tidal barrier at the mouth of the marina.
Glamorous engineering design was exactly what the WDA wanted, an emblem for the regeneration of Swansea's port area.
So not long afterwards, Wilkinson Eyre and engineer Flint and Neill started work on the design - not long after also bringing in Newport steel fabricator Rowecord Engineering, Martin Knight, who led the architectural design team, says: 'The real enjoyment is in the initial meetings with the various engineers. We have a formal idea and they have an intuitive grasp of whether you can do it or not. Then once you have settled on something which you feel is the right solution, they apply the maths to it and, very occasionally, you realise it can't be done and you have to find another way. But more often than not, if it's a good concept aesthetically, you can make it work.
'We wanted a curve in the deck's plan to make it more dynamic and interesting. The single mast is the simplest form, and the stayed mast with a curved deck often means you get a curved form in the cables themselves - you are creating volume with cables.'
The cables were 35 to 40m long, in the form of 70mm locked coils, which meant they were tens of tonnes each in weight. The geometry of the curve and the angle of the mast allowed the deck to be supported on only the inside edge. The deck is a closed steel box, so it can resist the torsional stresses created by this asymmetrical support. It came in nine 20 tonne sections, which were welded together in situ.
The configuration of the 42m-long mast is complicated. It starts at the base with a cross section the shape of a filleted square.
By mid-height the section becomes kiteshaped, and at the tip it changes to a triangular section. All this is done using cleverly cut flat steel plate. Rowecord Engineering, brought in early in the design, has, Knight says: 'a glorious mix of very sophisticated CADCAM cutting applications - and hard men bashing metal.' Time was short and the team was worried about the feasibility of producing complex, three-dimensional shapes for the mast. But with Rowecord's people participating, the all-flat plate design emerged. Knight says: 'We didn't have to change the original concept all that much. It was a matter of making it easier for Rowecord. The mast now has a bullnose at the front, and they contributed an arris detail to the back edge, which looks amazing. We didn't design it because we didn't think they could do it in time.'
The detail involves a bead of weld running from mid-point to the top of the mast, which produces a sharp joint and simplifies welding. Knight says: 'They were fantastic, proactive and enthusiastic.' The whole bridge was then modelled in three dimensions by Rowecord to check the geometry and tolerances before starting work on cutting and welding.
The base for the mast is a big concrete pilecap in the middle of the marina. The 78 tonne mast was made in Rowecord's works in Newport. Knight says: 'It was laid on a barge and floated up to Swansea. Using the biggest crane we could find, it was lifted into place and bolted on top of the pile cap. A set of trestles was laid out across the marina and the nine prefabricated walkway sections slid across into place, and then the cables were fixed from the mast to the inner edge of the deck and the trestles were removed' Flint and Neill deployed the widely used Oasys system from the Arup stable to check the basic engineering numbers and analyse the structural behaviour of the bridge. It is, says the engineering-savvy Knight, 'beyond the understanding of architects'.