Construction of swing bridges is now rare in the UK. But a Birse-led design and build team has taken on the challenge in the east Yorkshire town of Goole. Damian Arnold reports
SWING bridges are only called into action a few times a year but if something goes wrong with the bearing on which the steel structure turns, long-term chaos could result.
At the £4.75 million Dutch River Bridge built by a Birse-led design and build team in Goole, east Yorkshire, lengthy closure is not an option, given the cross-town traffic that already backs up a long way on both sides of the temporary bridge now in place.
Mindful of the chaos caused at structures resting on a bearing that gets damaged ? the Glasgow wing tower being a prime example ? the solution for one of the biggest swing bridges to be built in the UK in recent years was to protect the key bearing so that it only takes the weight of the structure when it swivels.
'Because the central bearing is not in use for the majority of its life it will last longer, ' says Birse project manager Dave Lowther, who adds that the bearing itself is an off-the-shelf manufacture that will m inim ise delays if it does need to be replaced. 'It's not having to bear the weight of the structure and being bumped and ground by heavy buses or lorries.
If that bearing breaks or needs replacing, then you could do so without closing the bridge, ' he says.
The rest of the time the bridge rests on two central pintel bearings and two bearings taking the weight of the swing bridge's tail. When the bridge is hydraulically jacked up to rotate, these bearings are then retracted and the load rests on the single central bearing on which it moves.
A strategy to keep maintenance to a minimum was just one of the selling points of the design and build team, which includes Birse and structural designer Cass Hayward.
The agreed solution was a steel cable-stayed bridge under a design that kept faithfully to the illustrative drawings produced by the client. But key elements of the design have been evolved by the Birse and Cass Hayward team.
They were assisted by mechan ical and engineering consultant Bennett and bridge fabricator Butterly, both of which worked on the award-winning Falkirk Wheel boat lift.
A lot of thought had to go in to whole-life costing because most of the people on the job had not been involved on a swing bridge to this scale before ? so rare is it for one to be replaced. Consequently the engineering is pushing the envelope of what is possible.
The original span of the central swinging section, for example, was to have been 11.5 m, but this was lengthened to 15 m to accommodate the bigger vessels that are expected to navigate the river in future.
'The span of the swing bridge was about as long as it could be to remain a swing bridge, ' says Mr Lowther. 'If the span had been any longer it would have needed to be a bascule br idge.' Faced with complicated engineering work on a long span, Birse's construction strategy centred on building two steel jetties stretching out into the river that would support 120-tonne cranes. With the reach of each crane stretching to 24 m, the whole width of the river is spanned ? ruling out the need for complicated and costly marine construction.
'We won the job on the jetties. It's a method we use quite often. It means we can use a crane at all times and are not reliant on marine construction, ' says Birse site agent Richard Burton.
'The tide can be very fast and the water level can rise 5 or 6 m very quickly, ' he adds.
BIRSE Dutch River bridge project manager, 47-year-old Dave Lowther, based in Tadcaster in Yorkshire, has had the challenging task of co-ordinating all the parties on the design and build contract.
Lowther's project management sk ills have been recognised on his last three jobs. These include the refurbishment of Leeds Station, which won an ICE Yorkshire award and for which Lowther was shortlisted for the ICE's civil engineering manager of the year.
Project managing Dutch River Bridge has been one of the biggest challenges of his 15 years at Birse, where he has chief ly worked on water and rail-related jobs. In fact, he says, it is probably the biggest challenge he has faced since sinking one of the last concrete gravity dams in the UK, in north Wales, more than 15 years before.
'I've enjoyed the job because it has been unusual.
Swing bridges don't get replaced every day and it brought a new set of challenges. I have been quite lucky doing those types of jobs, ' he says.
The winning tender also had to convince the client that construction would be considerate as well as clever.
Residents, some with front doors inches away from the steel boundary wall of the site, were particularly worried about noise created by the piling operation.
Testing found that they were less annoyed by the noise of a hammer than by a vibration drill. To reduce the inconvenience to locals, strict working hours were enforced.
The very tight footprint of the site in which one of the UK's biggest swinging bridges is to be built was the main challenge cited by project manager Dave Lowther.
'We had to design the new bridge in exactly the same footprint as the old bridge, with all the same road alignments, ' he says. 'There was no room for manoeuvre and what we were providing is twice as wide as what we had before.' Planning was therefore key to the job, especially to ensure that materials were delivered at exactly the right time.
At least some of the materials could be sourced from the old bridge. The original wooden piles were taken out of the ground and reused as part of the bridge's fender ? which is capable of def lecting 2,000-3,000 tonnes of shipping, the typical weight of the big vessels that deliver grain to Goole port, at up to 4 knots ? at the water's edge.
Piles used on the job included second-hand gas pipes that had not been used. The 16 kg recycled plastic curbs, known as enviro curbs, can be manhandled. Conventional curb segments that tend to weigh more than 20 kg now have to be lifted by mechanical means under HSE guidelines.
There has been a swing bridge on the site since 1760. The first cable-stayed timber swing bridge lasted 120 years, as did the second. The new bridge has got 120 years to live up to but the low maintenance strategy has given it every chance.