Job that came in from the cold

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Aesthetics determine a rail bridge solution

Complex falsework supports timber shuttering

Winters in Sweden are so cold that even hardy Swedish steel fixers sometimes feel the chill when working at 39m over water. 'The biggest problems with this job have been the weather and safety, ' says Torsten Bergstrand, main contractor Skanska's project manager on the Staksund railbridge.

Spanning 120m over the Staksund river, the new bridge will carry Banverket's (Swedish Rail Administration) new high speed commuter line between Orebro and Stockholm, some 60km away. The unusual design, by architect Soderberg & Palm and engineer ELU Konsult, consists of a post-tensioned box section insitu concrete deck suspended on steel hangers from a pair of cast insitu hollow concrete arches, which rise up to 39m high.

Winter assembly of the timber formwork for the concrete arches required the men to remain stationary in cold winds of up to 36m/s for many hours, and the risk of falls and exposure were Bergstrand's biggest headache. Scaffolders in particular were not happy and Skanska invested heavily in modern inertia reel harnesses. Fortunately everyone remained clipped on and there were no major accidents.

At the north end of the bridge trains will emerge from a tunnel and pass through an architect designed portal containing a glazed arch, which is also being built by Skanska on a separate contract. 'Local residents would only allow us to build a bridge that was aesthetically pleasing, ' says Bergstrand.

Measures have been taken to shield local residents from the sight and sound of the rail line by making use of the hilly wooded topography of the area. The river is navigable and popular for yachting as it flows into Lake Malaren and then on to the Baltic. An arch solution was preferred, partly as a result of the requirement to provide a 20m head clearance above the water to allow river traffic passage.

Construction was divided into two phases, casting the arch followed by the deck. The first phase began with the construction of mass concrete springers for the arches. 'The biggest construction problems we had were with the foundations, ' says Bergstrand. Skanska had to blast down further into the cofferdams than expected to find competent Stockholm granite, which caused a five month delay to the programme.

Design of the falsework for the huge arches was a major challenge for UK supplier RMD, which delivered 300t of its Rapidshor and Megashor falsework systems for Skanska to erect under supervision. The falsework was supported on four huge 1000kN jacking towers, two on the banks by the springings, and two placed midriver on driven steel piles. Skanska was required to provide unobstructed clearance to river traffic during assembly of the falsework.

The concrete arches taper from 4m side boxes at the springers to 2.5m at the apex, and were formed in a way that shows the love for, and abundance of, wood in Sweden. Carpenters put the 22,500m 2offormwork together solely from 50mm by 100mm softwood battens, with no plywood or Ikea components in sight. This gives an attractive patterned concrete surface but limits reuse of the forms. Roaring site bonfires are not allowed in environmentally sensitive Sweden, even in the depths of winter, so used wood is hauled off site for recycling.

Six pours from each bank meeting at midspan were needed to complete the main arches.

Space inside the formwork was tight and access very difficult, which made steelfixing potentially dangerous work. A culture evolved where the steelfixers 'lived' inside the forms, having steel, food and warm drinks passed to them rather than go through the long process of climbing out.

The arches were completed in January and the bulk of the falsework has now been struck down to the deck level. Formwork for the deck is of a similar construction to the arches, although more straightforward and less exposed. Once the deck has been cast the hangers will be fitted and the deck falsework taken down.

'Everything on this job has been interesting, ' says Bergstrand, 'but the concrete arches have made it very special'. The bridge is expected to be completed in the autumn.