I recently posted the winner in the Foryd Harbour Bridge competition, a design-and-build contest for a £4m opening footbridge at Rhyl in North wales. It was an unusual twin-leaf bascule bridge, with a central mast resembling that of a ship, to be designed by Gifford and built by Dawnus.
I've been sent information and images for three of the four other entries, and rather than wait for the promoter, Denbighshire, to make them all public, I thought I'd just get right on and show them here. As ever, click on any image for a larger version (links are given only where I've found relevant information on a firm's website).
First, a few thoughts on the engineering aspects. The swing bridge (from Knight Architects & Parsons Brinckerhoff) is at a disadvantage on the reliability front, as swing bridges require interlock or jacking mechanisms at the ends in order to properly secure the bridge in its closed position. This isn't necessary for a bascule bridge, which can secure itself in place by virtue of its dead weight. As a result, a bascule solution has one less mechanism to cause problems.
The Knight / PB design also shares a disadvantage with the winning Gifford design, of having a central pier, and therefore no fixed link from shore to the operating mechanism. This means that electrical and hydraulic supplies have to be buried below the river bed, adding to the environmental impact of construction, and also that access for maintenance is rendered more difficult if the bridge mechanism were to fail while in the open position.
In favour of the swing bridge, and the Mott MacDonald bascule variant, is that the centre of gravity and centre of rotation of the bridge can be located at the same point. This balances the bridge such that the mechnical power required to operate it is much lower - the weight of the bridge is balanced in all positions. Essentially, the mechanical plant only needs to overcome wind loads, and again the swing bridge is best for this, with perhaps the lowest operating cost of any of these designs. The Flint and Neill / Moxon bascule cannot possibly have the two centres coexistent, so is in an unbalanced position throughout most of its opening cycle i.e. the machinery has to push the bridge's weight around as well as overcome wind loads.
From a construction point of view, I'd imagine the PB / Knight design causes most disruption to the estuary simply due to the sheer size of what must be assembled and the possible need for temporary propping in an unbalanced construction state. However, the differences will be small.
The Flint / Moxon design will clearly have structural issues to be addressed by virtue of being supported on one edge only - it must be very carefully balanced, and even then design against twisting due to wind when open might be a challenge.
However, the most unusual of these designs is the Mott MacDonald bridge. I understand that the "ring" feature rotates about its virtual centre on roller bogies , which would mean it is similar in design to the Bellmouth bascule bridge, and the Falkirk Wheel. There's a clear visual resemblance to the latter, although I think the Wheel's fins are architectural, rather than forming a counterweight as on the Motts design. I do wonder how the Motts bridge behaves laterally - the ring doesn't look wide enough to be stable under transverse wind loads.
Visually, my favourites are the Flint / Moxon and Motts designs. The PB / Knight bridge is just too "samey", with echoes of bridges like the South Quay Footbridge and Swansea Sail Bridge. It is very nicely detailed, especially in avoiding the expense of a torsionally stiff box girder.
I very much admire the simplicity of the Flint bascule, and while the Motts bridge has plenty of "logo potential" the ring and its fin look a little small and out-of-scale to the whole bridge on some of the images. Nonetheless, I prefer all three of these to the actual winner.
Alun Griffiths / Parsons Brinckerhoff / Knight Architects / Davy Markham
Kier Construction / Mott MacDonald / Atkins Bennett
Morrison Construction / Flint and Neill / Moxon Architects / Atkins Bennett