10 February 2015

London Bridges: 39. Merchant Square Footbridge


The bridge at Merchant Square, Paddington, is the latest addition to a peculiar assemblage of spans built across a little-used canal basin in an attempt to flag up a developer's site as somehow far more interesting than it really is.

It's a replacement for the defunct Helix Bridge, another odd structure which rotated corkscrew-style while retracting (or advancing) across a narrow canal throat, but which had long since ceased to work. The ability to work was not a practical handicap for the Helix, as it spanned across the entrance to the extreme end of the canal basin, a body of water which has for some time been more of an ornamental pond than a useful harbour for canal boats. However, its function was less as a useful moveable bridge and more as a piece of kinetic sculpture, and the new Merchant Square Footbridge adheres to the same brief.

Given that the bridge's raison d'être is to open and close in a spectacular, unusual yet elegant manner, it is perhaps unfair to have visited it at a time when it remained resolutely closed (it opens every Friday at noon). However, this is how it will be experienced for the vast majority of its lifetime, so it is at least not unrepresentative. Indeed, the canal basin is, at present, physically barred to canal traffic by the floating equivalent of bollards, so there isn't even the chance of an errant barge passing up to it and demanding that the bridge be lifted to permit passage.

From the video showing the bridge being raised and lowered, this is clearly an imaginative and highly unorthodox design. The bridge deck comprises five separate fingers connected only at a common axle, which are arranged so that each rises by a different degree, creating, with an admirable sense of the poetic, the impression of a Japanese hand fan. The structural engineering would appear reasonably straightforward, but I imagine the mechanical engineering to have been a real challenge.

In its quiescent state the bridge retains a degree of visual interest, principally due to the manner in which the five bascule counterweights have been raised above the ground. This is a significant gesture, with the designer clearly recognising that the bridge must remain interesting even when unaroused. The counterweights are arrayed in manner which I find reminiscent of the Sydney Opera House, a group of heavyweight steel shells or petals which hint at the bridge's occasional party trick. Each counterweight is labelled clearly with its weight in tonnes, a charming idea.

The bridge is generally well detailed. The forward bearings are compact and fitted snugly, and the deck and parapets have been assembled with some precision. There are interesting details, such as on the stainless steel mounting plates for the parapets. These are cut square to the bridge span, as is the obvious thing to do, except where they pass over the canal edge, where they have been cut parallel to the canal and hence at an angle to the (skew) span.

The alternating inclination of the parapet bars provides the necessary strength and stiffness against pedestrian loads without requiring the usual larger posts. This is a device I have seen and admired on previous (unbuilt) Knight Architects bridge proposals, but in practice it is not entirely successful, principally because the bars are much thicker than seems visually appropriate. I imagine a more transparent quality was sought, but not achieved. Other than the chunkiness, the parapets are well-detailed, with an attractive wooden lean rail at the top, incorporating hidden lighting on its underside.

The feature I found strangest about this bridge is quite how close it sits to the water's surface. This is, I suspect, a forced consequence of the overall design concept, which requires the bridge deck to support pedestrian crowd loading in its closed position as a simply supporting beam, and its self-weight as a cantilever when it opens. With the deck divided into five fingers, each has to be stiff enough to satisfy both functions, and I guess this has governed the depth of the structural members required. The upper level of the deck is governed by the canal-side levels, and the depth in which to accommodate the overall construction is such that proximity of the soffit to water is unavoidable. The relationship feels visually uncomfortable, to me at least.

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