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Turning the tide

Ground engineering Birkenhead roro

The most spectacular sections of the Twelve Quays project are the $5M pontoon and linkspan or ship-to-shore bridge designed by Transmarine and manufactured by Harland & Wolff Heavy Industries in Belfast.

The 1,700t, 70m by 48.2m steel pontoon has two hydraulically operated ramps which allow for different vehicle deck heights, which vary with the state of the tide and how heavily laden the ship is. The Mersey presented particular challenges, with a wide tidal range of 10.9m between highest and lowest astronomical tides.

'The Mersey also presents special problems because it is fast flowing and has relatively high waves, ' says Transmarine managing director David Byrne, adding: 'The linkspan is, we believe, the largest ever installed in the UK and one of the largest in the world.'

Designed to take full HB loading and to carry 44t trucks, the linkspan can cater for four lanes of traffic over its 16.3m width. The motion of the pontoon from both the river and loading from ships and vehicles places extraordinary requirements for movement range on the bridge.

'While the shore end hinges are fixed, the pontoon end hinges can differ in level by 1m, ' says Byrne.

'It's a truss bridge, but the transverse structure had to be designed to take the torsion without introducing 'hot spots' or high stress areas, ' he adds.

Hot spots would experience accelerated metal fatigue and have a shorter life.

The alignment of the linkspan at 60infinity to the centreline of the pontoon was dictated by the approaches to the docks, and the proximity of the Queensway Mersey tunnel.

Piling was limited to only 28m from the tunnel, again influencing the geometry of the berth layout.

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