Blinking marvellous

Now they have a seventh. Installed last November, but not due to open until September, the newcomer is a delicate ribbon of white steel that loops 50 m up and 105 m over the River Tyne in one great parabolic bound. Unmistakably a turn-of-the-millennium lightweight engineering structure, it is a deliciously minimalist echo of the grand lattice steel arch of the 1928 Tyne Bridge – the precursor to the Sydney Harbour Bridge – and a new starting point to the rapid-fire sequence of bridges.

Seen purely as a civic monument, this graceful white bridge is an instant hit – Tyneside's answer to the St Louis arch. Moving in closer, its practical purpose becomes evident. By 18 thread-like diagonal steel cables, the near-vertical arch supports a near-horizontal arc of steel, which swoops out over the water to support a dual deck for pedestrians and cyclists. The designers, Wilkinson Eyre Architects and civil engineer Gifford and Partners, have one more trick up their sleeve that propels the Gateshead Millennium Bridge into a higher league than London's Millennium Bridge – it swings open to allow ships to pass beneath. And the way it opens is the cleverest and most innovative bit of all. The entire bridge pivots as one piece about its two bearings, one on either bank. As the overhead arch swings downwards, the deck moves up, and when they are level they offer headroom of 25 m above water level to allow large vessels to pass underneath. It has been dubbed "the winking eye", as it opens and shuts like an eyelid rolling over the eyeball.

Keith Brownlie, Wilkinson Eyre's director in charge of bridge design, modestly describes it as a variation on the bascule bridge principle, in which the bridge opens by swinging upwards. By a great leap of imagination generated by the engineer and architect working in unison, the innovative Gateshead Bridge came into being with bearings housed on opposite riverbanks, so that the bridge remains straddling the river even in its open position. It is, in fact, a suspension bridge and bascule bridge rolled into one (see "How the eye winks").

Brownlie explains the rationale of the competition-winning design: "The design is a simple response to the brief, which called for a low-level bridge, but one with empathy for the existing high-level bridges. That's why we have a low deck and a high arch. But the brief also demanded clearance for shipping, and out of that practical requirement we extracted the drama of the opening of the bridge." Apart from a £9.2m grant from the Millennium Commission, the £22m bridge has been funded entirely by Gateshead council as the first instalment of a £250m clutch of cultural goodies that it is developing on its riverbank. These include Dominic Williams' competition-winning conversion of the Baltic flour mill into a lavish contemporary art gallery and Foster and Partners' grand multiple concert hall. The new footbridge will link the vibrant nightlife of Newcastle Quayside to Gateshead's new cultural centre, and has been planned as a component of the two cities' joint bid for European Capital of Culture in 2008.

Both the arch and the pedestrian deck are constructed as hollow box-frames of steel plate. However, the cycle track is a deck of narrow aluminium ribs, which cantilevers out from the box-framed deck and will be separated from the pedestrian route by a fence in perforated stainless steel. The ribbed cycle track is intended to cast a patterned shadow on the water surface below and form a translucent halo to the arch when raised to the open position. An elaborate computer-controlled lighting system will extend the spectacle into the evening.

Gateshead's Millennium Bridge has basked in ample publicity, nearly all of it positive, courtesy of two iconic CAD images drawn up by Wilkinson Eyre as part of the competition entry. One of these was even used for a Royal Mail first-class stamp in June, five months before the arches even appeared on the ground – surely a first for an architectural or engineering project.

One of the bridge's several delights is how closely the finished product matches the original design. "We worked extremely hard at the front end to retain the simplicity of the original design in the face of the constraints of cost and buildability," says Brownlie. "The only real differences are that the steel plate has been upgraded and extra internal stiffening has been added to the box sections, but both of these are invisible from the outside." Does this mean that the Gateshead bridge has emerged free of fiasco, such as has bedevilled London's bridge? As the bridge is not due to open to the public until late summer, and its rotating action will not be tested until next month, it is too early to say. On the other hand, the immense pair of steel arches weighing 850 tonnes dropped into position like a dream in November, and Gifford has given written assurance that this cable-stayed bridge will not wobble alarmingly.

Even so, one problem has already cropped up to dismay architect, client and onlookers. At the river harbourmaster's insistence, 16 steel bollards have been sunk prominently into the river-bed to guide ships beneath the central apex of the bridge. Even Gateshead council leader George Gill has publicly decried the bollards, arguing that "they detract from the elegant design of the bridge" and that large ships rarely pass that far up the river anyway.

But if the bollards detract from the design, their effect seems to be negligible in the eyes of enthusiastic Tynesiders. Last November, Newcastle's Evening Chronicle published a glowing "millennium bridge supplement", which claimed: "The overriding opinion of locals is that it is money well spent." The extent of media coverage within and beyond the city confirms that this is one of the few millennium projects that has caught the public imagination in a big and positive way. And well it might: as a piece of city-centre urban renaissance – practical, monumental and iconic in its simple graceful form – they don't come any more inspiring.