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Bridging the design gap

Bridges : Design

Success in the design of innovative buildings does not always transfer to bridges. Dave Parker visited one company that thinks it can make the transition.

Bridge design is nothing new to Buro Happold, says bridge group leader Davood Liaghat.

'Many of our design groups have been involved in bridge projects over the years, but it wasn't until three or four years ago that the practice started to discuss setting up a formal dedicated bridge group.'

Convinced that the booming bridge sector offered realistic rewards, the practice began to recruit experienced bridge designers.

Liaghat himself came from Robert Benaim Associates two years ago and others made the same move: Flint & Neill provided at least one of the team. From the outset, says Liaghat, the group had a very clear brief.

'In the shorter term we would continue to enter design competitions, to get our identity established. There has been a boom in such competitions recently.

They are a good marketing exercise and help attract the right sort of engineer to join us. And they allow us to develop relationships with signature architects like Chris Wilkinson. But inevitably success rates are low, and they don't pay, ' he adds.

The longer term strategy is to 'find a niche' where high tech skills are still needed but the structures are more 'bread and butter', and hence more profitable. However, a potential niche does seem to have been located, one that needs both bridge and building technology expertise.

'Linkspans these days are more like suspended buildings, ' says senior engineer Karim Yngstrom. 'Usually they're fully enclosed, with full services, and have a complex interface with the buildings on either side.'

The group has already tackled two linkspan projects, at Warwick University and the Oxford Science Park, working directly for Buro Happold's structures group. Some 50% of the group's workload comes from 'internal clients'. But linkspan projects are starting to come in directly now, with two Scottish commissions already in the bag and a third Middle East project in the final stages of negotiation.

Such structures can be no less demanding a design challenge than any other bridge. The 100m long Middle East example will have three spans of more than 30m linking a giant shopping mall to a waterfront complex over a busy dual carriageway. A steel torsion box main deck will carry not just pedestrians but travelators as well. Naturally, it will be fully enclosed by curving glass and aluminium cladding, and will have the latest air conditioning.

These direct commissions for the bridges group are related to recent design competition successes, Liaghat believes. Chief among these was the group's triumph in the competition for the £2M, 170m span Ponte Della Musica, across the Tiber in Rome. Working with PowellWilliams Architects, Buro Happold beat off the challenges from 103 other entries - 'some of them from world famous bridge engineers and architects, ' Yngstrom notes. Other competitions overseas brought solid if less spectacular results, with several shortlistings.

'We did a lot fewer competitions last year than the year before, and in future we'll be much more selective in what we enter, ' says Liaghat. 'But as a marketing exercise it has paid off - we're getting more enquiries as a result, and more commissions.'

Inevitably, many of these are for 'landmark' footbridges, some of them remarkable in concept. Perhaps the most eyewatering was penned by a long term Buro Happold collaborator, German tensile structures pioneer Frei Otto. The eight span, 150m long Mechtenburg Bridge consists of a series of interlocking fans made up of 70mm diameter steel bars clamped together.

'We've come up with a multipositional clamp design which acts as a pin joint, ' Yngstrom reports. 'But it was a nightmare analysis - every component except the clamps was different.'

By comparison the £1M Cannon Wharf swing bridge across the River Wensum in Norfolk is almost conventional. A lightweight cable stay design, the 70m long pedestrian crossing rotates horizontally around the central axis of its twin towers.

Completion is scheduled for December this year.

More of a sign for the future is the Ratnapurna Bridge in Sri Lanka. A straightforward concrete arch crossing 50m ong with a precast prestressed deck, the structure was designed with local engineers under technology transfer initiatives and featured readily available local materials.

The practice is really hoping to get into design and build projects, and a major collaboration is in the final stages of negotiation. In the meantime, however, the group is engaged in preparing concept designs and budget costs for a particularly sensitive pedestrian crossing. It might be a cable stay design, a shallow concrete arch, or a steel girder structure, nothing too extreme. Its sensitivity, however, lies in its location - across a flood-prone stretch of the Avon just downstream from Buro Happold's riverside head office.

'I'm not looking forward to this one, ' admits Liaghat. 'It's too close to home, everyone will be looking over our shoulders'.

Another challenge is perhaps the wackiest of the recent crop of unorthodox pedestrian crossings. Designed to provide both a conversation piece and an opening crossing over the 10m wide canal which runs through the giant Paddington Basin redevelopment project in London, the bridge will feature a glass tube strengthened by a steel helix.

This helix will be rotated by an electric motor, probably via a small rubber wheel in contact, and will screw the bridge out over the canal.

The deck, however, will not rotate. It will cantilever the full 10m out from one end and be pulled forward by the rotating helix when the bridge moves.

Unusual challenges like these add spice to life and keep the problem solving muscles in fine trim. What Buro Happold needs now is to see its name attached to a few more conventional viaducts and road bridges.

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