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Road to nowhere

Bridges - A highly unorthodox pedestrian crossing outside Amsterdam rethinks the suspension bridge. Andrew Mylius reports.

The Dutch have a knack for pulling off impressive feats of construction as though they were everyday events. Last month, a tongue-twisting joint venture of contractors Van Hattum en Blankevoort, Heerema Zwijndrecht and Van Splunder Funderingstechniek floated a 186m long, 570t bridge deck up the Amsterdam Rhine Canal, just east of Amsterdam, during a 12 hour possession.

Two deft tandem lifts with a pair of barge cranes saw it and the bridge's pair of 'toothpick' pylons and single suspension cable slung into position, ready for the canal to be reopened to shipping at the stroke of midnight.

The crossing's highly unusual self-anchored design poses an erection sequencing problem - the cables have to be secured to the deck, which is in turn reliant on the cables for support.

This has been overcome with the aid of temporary pylons and cable stays, from which the deck will sag under beam action until the hangers are all in place this month.

Client the City of Amsterdam laid on a bit of a party - Arup director Angus Low, who has designed the bridge with Dutch firm Grontmij, went along to slurp Heineken and enjoy the spectacle with locals. But this only served to underscore the builders' laid back savoir faire.

'Can you imagine a British client or contractor inviting people along to watch an operation like this-' asks Low.

The E8.7M ($10.9M) Ijburg Bridge is a signature structure for an as yet unbuilt Amsterdam satellite town, shortly to be conjured up on land reclaimed from the Ij Sea. 'The bridge is needed in advance because, before it takes the citizens from Ijburg to Amsterdam, workers have to get from Amsterdam to build Ijburg, ' Low explains.

A road crossing already exists, but in a country where cycling and walking are primary modes of transport, separate provision for cyclists and pedestrians was a given. Excited by Gateshead's blinking eye and Swansea's River Taw pedestrian bridges in the UK, City of Amsterdam commissioned their architect Wilkinson Eyre to come up with a scheme for Ijburg.

Where Gateshead got twin inclined bowstring arches and Swansea a raked cable stay structure, Ijburg's landmark is an unconventional self-anchored, single-cable suspension bridge with subtle geometry.

As with its iconic UK bridges, Wilkinson Eyre delivered a trademark curved deck. 'It has a constant radius and is symmetrical about its mid-point, ' notes Low. 'The curve is partly to achieve a skew [dictated by land availability at either end] and partly to achieve the 'desire line' - it eliminates tight curves on to the ramps at either end, and the client was after a fluent shape.' Low is something of an exponent of curved bridges: 'I'm always telling people that a bit of curvature is worth paying a little extra for [because they look better]. 'It's actually quite difficult to build a perfectly straight line.' Overall price for Ijburg works out at slightly less than ú1,800/m 2, 'which is the standard price for a footbridge'.

Ijburg's irregular deck section grew out of an early design check by the client. The barges that chug up and down the canal rely on radar to spot and avoid other shipping. City of Amsterdam's modelling revealed that the deck design produced huge radar reflections that would have prevented barges seeing past it.

'What they wanted was a 'stealth bridge', ' Low quips.

'We had to reduce its profile.' In elevation, the deck is a slender 700mm deep at either end, deepening to only 1.98m at mid-span to resist aerodynamic forces. The steel box deck was also faceted like the surface of a Stealth Bomber to provide a less uniformly reflective surface.

Ijburg's radar dodging skin serves a dual function, helping the minimal structure avoid wind induced oscillation. The deck is 5m wide, 'which over a 170m span is quite narrow', Low notes.

'The frequency at which vortex excitation occurs depends on the depth and section of the structure. If you have a constant section, the whole deck is excited at the same time. But with varying depth, only parts of the bridge will be excited at different wind speeds.' To improve lateral stiffness, the pedestrian path peels off from the cycle lane at the bridge's quarter points, creating a stretched Xshaped plan. The ends of the X are bolted down to piled, reinforced concrete columns supporting the in situ concrete approach spans.

Resistance to torsional forces transmitted to the columns by lateral flexing of the deck under wind load is being provided by yoking the in situ concrete-bored piles together with a subterranean reinforced concrete box.

It was the challenge of casting and tying down anchor blocks in the peaty, marshy ground that dictated use of the selfanchored suspension system. The single central catenary cable splits as it passes over the 37m tall, inclined steel pylon at either end. These backspan stays are secured to the ends of the deck, with tension forces converted into compression and bending forces in the deck.

'A cable stay solution would have worked equally well, ' observes Low, 'but the suspension bridge gives more fluency. It was chosen for aesthetic reasons.' Unusually for a single cable suspension bridge, the deck is supported from single hangers rather than pairs. These attach to the deck edges, requiring a fairly high degree of torsional stiffness in the deck itself. In plan, the hangers pull the 140mm diameter, locked coil suspension cable into a very slight S.

Arup was still smarting from its experience with London's wobbling Millennium Bridge when design of Ijburg Bridge got under way. Low explored the potential for synchronous lateral excitation, caused by the fall of massed marching feet, early on.

'One of the things we had to explain was that the Millennium Bridge wasn't wobbling because of its structural type, but because of its frequency, ' recalls Low.

'Even if we'd understood all the issues in advance, we would probably still have designed it the same way, albeit with dampers, from the word go.' Curing the infamous wobble involved a 'belt and braces' combination of viscous and tuned mass dampers. Ijburg Bridge is equipped with three tuned mass dampers from the outset, with a central 9t unit to control full span excitation fl anked by two 3t units at the quarter points to take care of the half-span mode.

'So long as it's used as a cycle bridge there'll never be any problem at all, ' Low predicts.

Low admits that Arup 'went over the top' in London, designing the damping system for a crowd of two people per square metre. The anticipated maximum loading for Ijburg is a quarter of that.

For fabrication of the bridge, the contracting JV has exploited the easy transport afforded by Holland's canal system and farmed out parts of the structure to subcontractors around the country on a lowest-cost basis.

These were shipped to Rotterdam for assembly and were barged north for the lift.

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