Viaduct over the Nalón River in Spain - A new experience with 'strict box' composite bridges

The motorway viaduct over the Nalón River is 1100 m long and 27 m wide. The longitudinal structural system consists of a single strict composite box that takes advantage of the different properties of structural steel and concrete, going beyond the double composite action systems already in use for a long time. The box girder is enlarged by means of composite trusses connected to the upper slab at both sides of the deck. Longitudinally, the structure can be divided into two sections:
- a 660 m long curving approach section over the river plain
- a 440 m long straight section crossing the riverbed. The central part of this section is a 124 m span rigid frame composed of a box girder with varying depth and two elegant piers, one on each side of the river.
The bridge was constructed by launching from both abutments. The launching of the main section, due to the varying depth of the box girder in the main span, required longitudinal auxiliary elements in order to launch it in the same way as the spans of constant depth. A temporary pier was built in the middle of the river in order to reduce the cantilever in the main span to a moderate length. The viaduct construction was completed in May 2007.