Bridges - The phrase ‘beautifully simple’ might have been invented for Poole Harbour’s new bridge Andrew Mylius discovers .
The iconography of Poole Harbour’s planned second bridge could hardly be more fitting.
Locals claim the harbour is the world’s second largest natural haven. Thousands of yachts tug at their moorings and on sunny weekends the horizon is serrated by billowing canvas.
‘When we saw the ‘twin sails’ design among all the competition entries in 2001 it leapt out as the right thing for this location - it was stunning, ’ enthuses Borough of Poole regeneration infrastructure manager John Rice.
Sailors anchored in Holes Bay, on the north of the harbour, should see construction of the nautically inspired bridge start early next year. Expressions of interest in building it are being invited through the Ofcial Journal of the European Union, with tender documents likely to be published in September.
It will be a twin-leaf basculestyle structure with a difference.
Instead of conventional rectangular leaves, the bridge’s opening span will be composed of two opposed, long triangles.
These will lie side by side when the bridge is closed. When the bridge is opened, though, they will rise skyward, like a pair of masts carrying full sail.
A second harbour crossing has been on the launch ramp since Rice joined the council 24 years ago. ‘We have an existing lifting bridge across the neck of Holes Bay that links the town centre with Hamworthy, where the Port of Poole is. It’s a rolling bascule design, which works well. It was state of the art when it was built in 1926, ’ Rice says affectionately. ‘But it’s carrying port as well as local trafc. The carriageway is only 5.5m wide and its capacity has become limited.’ In the mid-1990s the Conservative government sanctioned construction of a long, high-level bridge right across Holes Bay. That project was struck from the Labour government’s roads programme in 1997. Poole Borough Council went to the drawing board to look for ways of getting a bridge by different means.
‘There used to be a power station on the west side of Holes Bay that was demolished, leaving a huge empty site. We were looking for investment to redevelop that site and kick-start regeneration of Hamworthy, which is the second most deprived ward in Poole, ’ says Rice. ‘We wanted a bridge that would connect with the town centre but also support the port.
And we wanted it to be eyecatching.’ Gifford director Peter Curran who, with architect Wilkinson Eyre, has developed the twin sails proposal from concept to detailed design, says: ‘This bridge will have as its backdrop a very at but beautiful, tranquil landscape. Holes Bay is anked by sites of special scientic interest. We wanted the bridge to sit sympathetically within this setting when it is closed, but to erupt into drama whenever it opens - like the existing bridge, it’ll open 15 times a day. The whole thing will emerge out of the landscape.’ Curran admits that some structural efficiency has been traded for the sake of theatre but argues: ‘Is Gateshead’s winking eye [over the River Tyne] an efcient structure? It’s debatable structurally. But is it efficient in terms of bringing investment to the area? Hugely so.’ He and Rice hope to see some of the Gateshead effect once the bridge is completed in 2010 - developer JJ Gallagher has already applied to the council for permission to build 1,200 homes, shops and offices on the former power station site.
And Curran emphasises that the loss of structural efficiency to aesthetics is minimal. ‘The motivation for having triangular leaves was actually to keep things very simple, to ensure reliability.
‘Bascule bridges are inherently robust. Their only troublesome parts tend to be the mechanical interlock that links the two leaves together when the bridge is closed.’ On the existing bridge there is a male-female interlock system consisting of steel pins that are retracted into the end of one leaf to disengage it from its opposite number.
‘We set out to eliminate mechanical interlocks by spanning from abutment to abutment. Each leaf lands on something solid, making them both simply supported. That works perfectly under dead load, ’ says Curran. ‘Under live loading there may, in fact, be some differential deection between the two leaves and to deal with that we will need some kind of interlock. But it will be passive. We’re looking at putting tags on the inside edges of each leaf that will simply rest on the adjoining deck. There will be nothing active, nothing mechanical about them.’ The navigable channel into Holes Bay is 19m wide. Each leaf is nearly double that, at 35m long. The deck has a total width of 18m.
Leaves will be lifted by pairs of hydraulic rams - ‘one ram would do, but having two will provide redundancy, allowing maintenance of one ram at a time without affecting the bridge’s operating schedule’, says Curran.
No counterweights will be used to assist: ‘We looked at that carefully, ’ Curran says.
‘Counterweights didn’t make sense in the context of whole life cost. They would require larger supports to take the added weight and that didn’t stack up against the extra power needed to raise the bridge without them.’ Most of the mass of the saillike leaves is towards the base, Curran explains. ‘The centroid of a triangle is a third of the way up, compared with half way up for a rectangle. Yes, there’s a larger load to lift, and wind loading is higher than it would be for a single large rectangular leaf, but if that’d been proposed it wouldn’t have won the competition. And loading is far lower than for a pair of smaller rectangular leaves.’ Wind loading will be reduced by constructing the bridge’s two 2.5m wide footpaths, running along the outside edges of the bridge, from steel mesh.
‘One of the great strengths is that although this bridge looks amazing the working technology in it is very simple - it’s tried and tested, off-the-shelf stuff, ’ Rice says. ‘It’s designed to be deliverable and reliable. Like all local transport authorities we’re strapped for cash - we have a £14M budget [set at 2001 prices] that’s etched in stone.
And we’re going to be lifting it 5,000 times a year. It needs to give hassle-free performance.’ The lifting leaves will rotate around a simple trunnion bearing, with the rams pushing against lugs on their softs from pits in the main piers. ‘It’ll probably be a well-lubricated steel-on-steel bearing - nothing fancy, ’ Rice says.
Connections between the back span deck and piers will be monolithic; the deck’s hefty transverse steel beams will be cast into the heads of the concrete piers. Approach spans are simple composite structures of steel beams decked with concrete formwork and an insitu cast topping. ‘Apart from shaping the transverse beams to maintain the shape of the main span, it’s the cheapest you could go, ’ says Curran.
Piers will probably be founded on 36m long cast-insitu concrete piles. Caissons are technically viable but look more expensive than piled footings, says Curran.
Ground is mainly alluvium, consisting of beach deposits overlying Bracklesham Beds - a mix of clays and sands and siltstones. Piling is likely to require temporary casing and may call for permanent casing.