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Articulated dilemma

The new bridges at Bellmouth Passage in London's Docklands needed to move to allow boats to pass through, so two design teams considered the merits of lifting versus swing bridges

Most construction steelwork is designed to stay more or less where it is - even cable structure. But as bridges have recently started to take a serious place in the architectural canon, the issue of articulated architecture is happily raised once again, once only respectable in the realms of Archigram.

The two bridge designs for Bellmouth Passage in London's Docklands, one by Birds Portchmouth Russum (BPR) and the other by Patel Taylor, demonstrate these ideas. Each was engineered by separate design teams at Techniker. Ironically, the winning Patel Taylor design was changed radically by the client and now resembles, in mechanical engineering terms at least, the BPR design, which later received first prize in the AJ/Bovis Lendlease award for the best architectural work exhibited at the 2002 Royal Academy Summer Exhibition.

Bellmouth Passage is a stretch of water running north to south between two of the former docks adjacent to Canary Wharf. It is crowded by tall office buildings. Two footbridges were needed across the 20m-wide passage to provide access from the office buildings to the Jubilee Underground station. British Waterways insisted they needed clearance of around 6m above water level to allow boats to pass through. So the two bridges were going to have to move in some way, probably laterally or vertically.

The Patel Taylor engineering team at Techniker looked hard at the alternatives of a swing bridge and a lifting bridge. A lifting bridge, whether it deployed a conventional truss, Vierendeel truss or beam walkway structure, was found to be more structurally efficient than a swing bridge, which involved a cantilever and a pivot that had to cope with heavy eccentric forces. Similarly, it was obvious from the analysis that a lifting bridge supported at each end was better than a cantilever swing bridge in terms of bending, deflection and reactions at the foundations.

One further advantage with a lifting bridge was that it could be used open or closed if stairs were installed to an overhead walkway. With this basic information to hand, structural brainstorming could begin.

One question was whether extremely thin pre-stressed ferro-cement columns could be used with a squashed oval cross section and a ratchet system of raising the walkway. And what was the best means of raising a lifting walkway? A rack and pinion was an obvious solution, but there was also the possibility of pairs of chains at each end, as at the Queensborough bridge at the Isle of Sheppey. The idea of using a very heavyduty screwjack introduced baroque design possibilities - think of Bernini's weird Baldacchino at St Peter's in Rome. And was it really necessary to have symmetrical supports on either side?

Gradually the engineers started veering towards the idea of hydraulic power, which was, after all, abundant in the watery environment. Ideally, were it to be counterbalanced, a lifting bridge could expend little or no energy. This is a principle long deployed in the theatre, where vast and heavy sets are counterbalanced so they can be 'flown' down from the fly tower - the movement started by an initial pull by only one burly fly-hand. But hydraulics are good for the initial push and make for a very smooth motion, and with all that water around, the counterbalancing could take the form of pumped water. And so the design evolved into its winning form, with an elegantly lightweight steel structure forming a high frame on either side of the water with a light walkway, and the mechanics of the hydraulic system hidden underwater.

Some team members, however, were still looking at first principles. What about a bridge that somehow drew back across the water and folded into an inverted U loop like a stiff watch-strap? Or what about a simple bascule-type lifting bridge with a pivot at one end or an open coil that retracted like the bellows of an old camera? Or a structure that submerged into the water? Or one similar to an attenuated, eccentrically pivoted bicycle pedal without the axle, whose walkway ended up in the air when it opened, its counterbalance hidden under the water?

There were other possibilities with swing bridges. A horizontally swinging bridge could, as it opened, unobtrusively fold its walkway down flat against the side of the wharf and below its edge. Should a swing bridge have one section or two? If there were two, how could they interlock safely in the middle? Could it be a telescoping bridge? Or could it be made in sections that folded back on top of themselves in an 'S' shape, rather like a poised snake on the edge of the wharf?

BPR's final design ended up as an elegantly free-form pair of asymmetrical swinging platforms, one with a cafe at its end and both arms swinging on giant slewing bearings. The slewing bearing, despite the heavy asymmetrical loadings, is actually well understood. It is found in every tower crane - where big concrete slabs are used as a balancing weight or 'kentledge' as engineers call it - giving the moving arm its ability to move freely and with little lateral input once the initial inertia is overcome.

It was proposed to drop the giant slewing bearing into a circular sheet-piled pit fitted with rollers, with the elements of the arm assembled on the wharf side and attached to the head of the bearing, and kentledge added on the other side of the bearing.

Although the Royal Academy liked BPR's design and gave the architects a big cash prize, the client did not, and Patel Taylor's lifting bridge design was earmarked to go ahead. However, tenants of the buildings adjoining the site of the lifting bridges soon complained that its structure would obstruct views from their windows and block light.

So Techniker and Patel Taylor started a new series of structural investigations. In a strange twist, the solution agreed upon was a horizontally swinging bridge, but with glass walls, a roof, a single arm, and a dead-Miesian orthogonal structure.

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