A blog from the UK about bridges and bridge design
08 June 2012
Glass Bridge, Coventry
I gather some of the locals describe this as the "bridge to nowhere". It spans across Lady Herbert's Garden, landing in front of the Coventry Transport Museum. It carries pedestrians above a surviving remnant of Coventry's mediaeval city walls. However, it is undeniably fairly pointless, as you can quite easily walk around the wall at ground level, barely lengthening your journey.
The glass bridge is something of a Millennial folly, part of an ensemble of artistic projects intended to liven up the plaza area in front of the Transport Museum. The most prominent of these is the Whittle arch, a four-legged monument to jet engine pioneer Frank Whittle. This can be seen in the photo on the right.
The bridge, designed by Ramboll and fabricated by Rowecord, is 130m long in total, curving gently from the east before descending in a dramatic 15m diameter spiral ramp at its west end. Some 40m of the bridge is unsupported at this end, subject to substantial bending and torsional effects. These are resisted by a 762mm diameter hollow steel tube, which forms the primary structural member for the entire bridge. Vibrations are prevented by the presence of three tuned mass dampers inside the tube, although some movement is still quite perceptible.
The spiral ramp appears to be driven by the space available to achieve suitably shallow ramp gradients, rather than just the desire to make a bold structural statement. Aside from the main ramped route, there is a small staircase tucked away behind this part of the bridge, small, unwelcoming, and not one of the bridge's more successful elements.
Other than the spiral, the other unique feature of this bridge is its parapet, comprising extensive glass fins. Quite how these survive unbroken in a somewhat run-down city centre like Coventry's is a mystery to me. They are intended to reflect and scatter light when the bridge is illuminated at night - they don't form a functional part of the parapet restraint system. I think they are quite over-done, obtrusive both in size and in number.
Although it's all essentially unnecessary, the bridge does provide some nice views down into the gardens. It's hard not to think that a more minimalist balustrade would have been better in this respect as well, less distracting, and focussing attention on the views rather than shouting "look at me" quite so loudly.
For the most part, the bridge sits on slender columns with only a single small bearing at the top of each, something made possible by the significant plan curvature, which allows it to remain stable without the need for paired bearings. These can't be completely avoided however, as it otherwise lacks torsional stability, especially as it straightens out at one end.
The deck system comprises a series of anti-slip "planks" separated by grilles at frequent intervals. This serves a dual purpose: lighting is hidden below, and at night the deck lights up as a series of stripes, requiring no further illumination. It also allows the deck to drain freely, rather than having to capture surface water run-off at one end, which would be a challenge for such a long bridge. The downside is that the number of individual light fittings required is substantial, and also that a very visible pattern staining is produced on the main tube by the free-draining water.
Perhaps the oddest feature of the bridge is a weird cantilever wall which delineates the transition between the relatively straight section passing over the gardens, and the spiral curve over the Transport Museum piazza. I'm completely at a loss as to its purpose, but it not only looks odd from below, it completely blocks views off the bridge at this point. Can anyone explain it?
Overall, I admire the bridge's structural impudence, but find the parapets far too "busy", and wonder quite how much it must cost to maintain.