It is located some 35km west of the city of Mosjoen, close to highway 78 between Mosjoen and Sandnessjoen.

It will be the second large bridge project connecting Alstenoya to the mainland, coming more than 12 years after the Helgeland Bridge was opened. The region is no stranger to world-record scale bridges – the Helgeland Bridge's 425m long main span was the longest cable-stayed span in the world when it opened in 1992.

Sundoya Bridge is divided into three spans; it has a main span of 298m and two side spans of 120m. The main span will be the second longest span in the world for a continuous post-tensioned cast in place box section concrete bridge.

In terms of its design, consultant Dr Ing Aas-Jakobsen has followed a similar approach to that taken for the Raftsundet Bridge, opened in 1998, to which the Sundoya Bridge will almost be a twin. The two bridges have identical main spans, but Raftsundet has four spans as opposed to Sundoya's three. Contractor AS Anlegg, which is part of the joint venture building Sundoya, was also the contractor on the Raftsundet Bridge, and architect Boarch Arkitekter has also worked on the two schemes.

In January 2001 the joint venture company AF Sundoybrua won the contract from client Statens Vegvesen to build the Sundoya Bridge. This joint venture consisted of the contractors Reinertsen Anlegg and NCC Construction.

High performance concrete is central to the design of the bridge – both normal weight HPC and lightweight HPC. Normal weight concrete, at approximately 2500kg/m3, is used for the 120m side spans, while lightweight concrete, which weighs in at about 1970kg/m3, is used for construction of the 298m main span. This enables construction to proceed using the balanced cantilever method.

Local rock from Norway is used as the aggregate for the normal weight concrete, but the lightweight concrete required an imported solution. Normally the aggregate used for lightweight concrete in Europe is expanded clay or shale, but this material has high levels of absorption and for this reason, regulations prevent such concrete from being pumped.

In order to address this, the contractor adopted a similar solution to that used on Raftsundet Bridge – importing Stalite aggregate from South Carolina in the USA. Stalite is produced through thermal expansion of high quality slate, and results in a lightweight aggregate that gives concrete of very high strength at low unit weights. Its low absorption of approximately 6% and high particle strength are two of the factors that allow Stalite to achieve high strength concrete in excess of 82.7MPa, the manufacturer says. The bond and compatibility of the aggregate with cement paste reduce micro-cracking and enhance durability, and its low absorption makes it easy to mix and pump.

According to AF Sundoybrua quality manager Jan-Eirik Nilsskog, this material has given a very good result. It produces concrete that is easy to pour into the formwork and it gives a good surface finish, he says. It is being pumped some 120m along the bridge deck to the concreting position. Concrete is produced by a transportable mobile plant located only 1km from the bridge site. Constant monitoring of the concrete weight is necessary to ensure that the cantilevers are properly balanced. This is tested for each pour.

The project began in January 2001 at Aker Verdal with the production of caissons for the pier bases. In May 2001 the two caissons were towed 500km north to the bridge site.

The bridge is being poured in situ using special mobile construction equipment developed by NRS. The cycle for construction of each 5m wide bridge segment is a week, and two mobile units are being used on the Sundoya Bridge. These particular units were built for AS Anlegg to use on the Varodden Bridge in Kristiansand in Norway, and they have also been used by the same contractor on the Rafsundet Bridge. The design of the central part of the main span of the bridge is based on the use of lightweight concrete LC60 while other parts of the structure use the more standard type C65. Because of the aggressive marine environment, the quality of the concrete must be particularly good.

The structure is a single cell, prestressed rectangular box girder, largely built using the travelling formwork system from NRS. The box width is 7m and its depth varies from 3m at the centre of the span to 14.5m over the piers. Close to the abutments, concrete of quality C25 will be used inside the box girder as ballast. In addition, the designers have included the necessary elements inside the box girder in order to allow the possible addition of post-tensioning cables in the future. The long-term behaviour of such large spans is not fully known, so the possibility that the main span may sag over time has to be taken into account. The width of the road is a constant 7.5m from the barrier on one side to that on the other, and the total width of the bridge is some 10.3m. There is a 2m wide footway included in the width of the structure.

The pier shaft is formed with twin legs, which are hollow inside. The pier shafts incorporate permanent prestressing cables and they have a constant wall thickness and a width that varies parabolically over their height.

Temporary tie-down piers are used to construct the bridge - they are located 35m into each 120m-long side span from the main piers. Each consists of an I-shaped shaft, which is tied down to the ground using rock anchors and connected to the box girder by means of prestressing cables. The purpose of these structural elements is to support the cantilever and prevent rotation in strong winds. Once the bridge superstructure is complete and the main pier prestressing is fully tensioned, the temporary tie-down piers will be removed piece by piece.

The location of the bridge, only about 100km south of the Arctic Circle, has meant that special measures have to be introduced to allow construction work to continue all year round. Apart from the obvious need to provide site lighting for much of the wintertime, the challenge of concreting in temperatures which can be as low as –20°C has to be overcome. Hot concrete is produced for the bridge – sometimes up to 30°C – and the formwork has to be insulated to keep the concrete warm. Electric heating cables are also used on the end of the previous pour to warm up the concrete before casting.

Construction of the new bridge began in January 2000 and is expected to be complete in September this year. The construction of the cantilever started in summer last year and is due to be finished in April. When Bd&e went to press, the project was on schedule for opening to traffic in late autumn.

Printer friendly version

Email this article to a friend