Why read this
The first-ever RIBA design competition for a roadbridge produces an innovative winner
Traffic management on a very difficult site is the key to success
Precast concrete facilitates construction
Just south of the ancient market town of Sudbury in Suffolk the busy A131 crosses the River Stour.
Every week 10,000 HGVs rumble over the modest 34m span concrete structure, along with 90,000 other vehicles. Few drivers take particular notice of the experience. Even fewer realise that this is the only crossing of the Stour for miles. So when a 1993 bridge appraisal suggested there were serious deficiencies in the 90 year old structure, and that a 3t weight limit should be applied, Suffolk County Council knew it had a major problem on its hands.
'The final straw was the condition of the foundations, ' says SCC bridge improvements and maintenance manager Nigel Burrows. 'There were obvious inadequacies in the transverse beams and slabs, and the cantilever footways were also suspect. But when a live load test showed the 300mm square concrete piles were 'pumping', a simple strengthening exercise suddenly became far more complicated.'
A foretaste of the problems ahead was what happened as soon as one lane of the bridge was closed to allow trial holing to take place. Sudbury gridlocked. Any long term solution would have to have traffic management and congestion minimisation at its heart. As the local authority's engineering department began to consider its options, the sheer scale of the challenge began to emerge.
'Of all the bridge sites I've worked on, this is the worst, ' Burrows admits. 'It's in a conservation area for a start, with listed buildings dotted all around, and there's limited access at one corner only.
'Services in the bridge include gas and water mains - and eight BT ducts which we were quoted £500,000 to divert.'
Only two realistic options emerged. The existing bridge could be patched up in the hope it would last until the longawaited, frequently postponed Sudbury southern bypass actually opened. 'But this could be ten years off, or it might never happen at all, ' says Burrows.
Alternatively, the bridge could be replaced. With the engineers unable to guarantee that a repaired bridge would last more than five years, replacement was the only option .
Suffolk's own internal engineering consultancy took on the brief, working very closely with local planners and English Heritage. Four concepts were presented, ranging from a bowstring arch to a five span brick faced concrete arch crossing with mediaeval overtones.
This was the preferred option, which went on public show in September 1999. At first the response was reasonably good, Burrows reports, with 30% of those who attended the viewing in favour, 25% against, 'and the rest mixed'. But when the impact of the construction phase on congestion in the town became clearer, public sentiment changed dramatically.
'There would have to be a complete revision of the traffic flows in the town, with the introduction of a one way system, the realignment of corners to allow HGVs to get through, and major restrictions on parking', Burrows explains.
'Worst of all, there would be single track working across Ballingdon Bridge for up to two years.'
Opposition to the proposals was organised, vocal and effective. In December the county's planning committee voted to postpone a decision on the bridge, pending the outcome of a hastily-organised local referendum. 'This voted 93% in favour of strengthening instead of replacement, ' Burrows reports.
'But as there was only an 8% turn-out, both sides of the argument claimed victory.'
Finally, in January this year, there was general agreement between the county council and the opposition groups that the only way forward was a design competition under the aegis of the Royal Institute of British Architects.
'RIBA was very pleased', comments Burrows. 'It had done competitions for footbridges before, but never a roadbridge. As we had been given only six months to come back to the planning committee with new proposals, we decided on a restricted competition.'
This would have two phases; an open competition first, with five entries only going on to public exhibition. An advisory panel with local representation was set up. After several meetings, what Burrows describes as a 'detailed and quite complicated brief' was produced.
Over thirty architect/structural engineer teams entered the first phase. Five were shortlisted. Given that the teams had only 4-5 weeks to prepare their proposals for such a sensitive and difficult site, says Burrows, it was hardly surprising that none were '101% fire proof'.
'Those solutions that appeared to minimise disruption were less appealing aesthetically, and vice versa, ' he adds.
'We invited the shortlisted teams down to Sudbury to give detailed presentations to the advisory panel - on such a difficult site we felt those who had to choose had to do more than look at pretty pictures.'
Following the briefings the advisory panel made recommendations to the jury panel, which was made up of two Suffolk County Council members, the county's director of environment and transport and two non-voting RIBA representatives. In June the final decision was announced. A three-span precast concrete arch design by Brookes Stacy Randall and Ove Arup had won the day, largely, says Burrows, because of its innovative approach to traffic management (see box). The planning committee is still to rule, and the final go-ahead is expected in October. Cost is estimated at just under £2M.
Cutting two ways
Arup associate Stuart Smith says the Ballingdon Bridge project was 'arguably the most constrained we've ever done - certainly at this sort of value.' Brookes Stacey Randall partner Michael Stacey adds: 'We enter a lot of architectural competitions, but this is the biggest and most comprehensive brief we've ever received.' They believe their joint design, a 'three-dimensional' precast concrete arch structure, was chosen because it offered the best combination of engineering and aesthetics, and minimal disruption of the A131.
'Traffic had to continue to flow across the bridge, ' Smith points out. 'There were no practical diversions, no possibility of setting up a temporary crossing alongside.
'However, the original bridge is quite wide. Even at the southern end, where there's a house on each side, there is 11m available.
So we came up with a three lane solution, two for traffic, one for working in.'
At competition stage the plan was based around a purpose-made two lane temporary bridge with a pedestrian footway on cantilever outriggers. This would be installed after the eastern parapet had been removed to make space. Traffic would flow normally over the bridge while the western third of the original bridge was demolished, new foundations and piers installed and the first of the two precast concrete box arches erected.
Moving the temporary bridge over the new section would allow the operation to be repeated on the eastern third. Then comes the clever bit.
Smith explains: 'The temporary bridge will split into two sections longitudinally. One half will be returned to the bridge's eastern side and traffic will start flowing each side of the centre. We'll then demolish the rest of the old bridge and erect infill units.'
Since the competition the design team has revised its plans around adapted standard temporary bridge units, but the basic principles will remain.
Ongoing design development is mainly focussed on the big precast units and the best way of getting them into position. Single lane working will still be needed, especially in the early stages, but for most of the projected 58 week construction period the threat of gridlock has been lifted.