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Triple jump A new crossing in the Netherlands has been designed as an unusual combination of cable stay and bascule bridge, reports Helena Russell. Photographs by Paul Collard.

New urban centres are springing up around the Netherlands as part of the Dutch government's strategic development plans to provide more housing. A number of towns have been earmarked for expansion including the northern town of Zwolle where some 1,200 new houses are expected to be built by 2010.

But with no ideal site for the development, the chosen location required a new bridge across the Zwarte Water to connect the new centre, Stadshagen, with the existing town.

Although only 194m long, the bridge manages to include three types of structure in four distinct sections; an approach viaduct at each end, a cable stayed deck in between and a bascule which lifts to allow boats through.

One of the most important considerations for design of the bridge was its looks - in particular the way the light would fall on it. So for such a small and relatively low crossing, the concrete pylons are high and wide, and something of a feature. Each are 17m wide at the base, tapering to a knife blade-like point at their full 50m height above water level.

Preliminary design was by consultant Gemeentewerken Rotterdam, the design arm of Rotterdam City Council, and set out the general form of the bridge. Architectural input from Maarten Struijs then suggested the pylons should be as close as possible to Stadshagen on the west bank, in line with two blocks of flats.

Dutch consultant Grontmij bid for the design work, but called in Maunsell to help out with the unusual design, explained project manager Arie Monster. 'We had already done some work with Maunsell,' said Monster, 'and with their wide experience on cable stayed bridges it seemed obvious to team up with them.'

Maunsell's engineers were put in charge of the cable stayed bridge, pylons and cables, and east approach span design, with Grontmij designing the remainder, and being in charge of overall project management. But close co-operation was needed between the team members, particularly as the bascule bridge and the cable stayed structure work together as a single element.

One of the unusual features of the cable stayed bridge is that the pylons have cables only on one edge. The weight of the deck is counterbalanced by the height of the pylons and sheer mass of the concrete bascule chamber on which the pylons sit.

Piles had to be used under three of the five piers, and under the base of the bascule chamber. Installing them in the sandy ground was not easy, explains Grontmij project engineer Walter Zwart, as the contractor found big boulders in the sand layers. About 15% of the steel piles broke while they were being driven, Zwart recalls.

To prevent uplift, the piles had to be driven 22m into the sand, says Zwart, as on this section of the waterway the level can rise by up to 1.4m when strong west winds funnel water along the canals. There are plans to reduce this water rise by installing a huge rubber inflatable dam across the neck of the inlet some distance away, but until then the piles must cope with the uplift.

The navigation channel, which passes through the bascule bridge, carries ships of up to 3,000t, so a guide structure was essential to protect the piers and main pylon of the bridge. The structure, made of wood and welded steel pipe, is attached to steel tube piles driven into the river bed, and rises and falls with the water level. When hit it is designed to deflect by about 300mm.

Even at the time of design, the local authority held the possibility of future widening in mind. Designs include provision for an extra lane on the southern side.

But according to Maunsell associate David Dawson, the most difficult part of the design was how to build the bridge. Dutch Main contractor Dubbers-Malden began construction of the 17m long by 15m wide bascule chamber by pouring a 1m thick concrete slab underwater. A further 2.3m was then added to form the reinforced concrete base slab. The chamber itself is roughly 10.5m high, with side walls rising to form the bottom of the two pylons.

Inside the chamber, three 2.5m deep manholes allow access to the stressing anchors. Six vertical prestressing cables are used to control the tension on the rear face of the pylons.

'We could have fastened the stressing anchors in and then tensioned them from the top

of the pylons,' admits Zwart, 'but then we would still need to know that the anchors were correctly fastened in, so we chose to do it this way.' This also allows access should the bridge be widened.

The pylons were formed in 3m lifts. Over half of the 56m long cable stayed deck, to just past the third cable, was built on falsework. Three cables were then fixed from each pylon to the edge of the deck, and the falsework removed. The cable stayed deck was originally designed as a thick plate, but Maunsell redesigned it with main and transverse beams to reduce weight.

A 20m span orthotropic steel deck forms the cantilever span of the bascule bridge which is expected to be opened about 10 times a day. When fully open, a notch designed into the face of each pylon enables the deck to fit flush into the pylon, retaining the smooth shape of the structure.

While this is a pleasing detail, Zwart confides that it is unlikely the bridge will be opened to that extent very often, as few of the boats require so much headroom. 'Maybe only when the architect visits,' he laughs.

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