Keywords: mobile scaffolding, staged cross section, seismic zone, span to span.

This paper describes the engineering construction of the viaduct over the Fluvià River, on the LAV Madrid-Barcelona-French border, located in the municipality of Bascara, Girona, in the traditional itinerary to France. Bridge Technologies S.L. has developed the construction engineering of this structure. The main changes to the structure in comparison to the previous design are:

1. Changing the static scheme for the supporting horizontal loads: From a seismic-isolated scheme, with seismic devices, to a classical scheme, which are at a fixed point on one abutment.
2. Changing in the construction method: A mobile scaffolding for the deck construction was designed, originally, for road decks with spans of up to 50m, and had however to be used for a railway deck, heavier, and with spans up to70m. This forced the use of a staged cross-section with a first freestanding phase, capable of supporting the second phase, which completes the cross section of 260kN/m.

Both changes required a complete re-design of all elements of the substructure and superstructure.

The Fluvià Viaduct has a total length of 835m (45m + 10x60m + 2x70m + 50m) and is the longest viaduct on the line between Barcelona and the French border. It has an average height above the valley of 18m and crosses skewed the river Fluvià, upstream of a small reservoir which increases its width to around 100m, in an area of outstanding natural beauty.

This paper describes the static scheme, the construction process and seismic design of the viaduct over the river Fluvià in the Madrid-Barcelona-French border. Since this is a highly staged structure, not only in terms of the static scheme, but also its cross section is evolutionary, it has been necessary to develop a detailed analysis of the redistribution of the sectional forces of self-weight and prestressing, and the redistribution of normal stresses at each section.

It is important to highlight the following result of this analysis:

1. The second phase of the cross section, which is prestressed with only 40% of the total prestressing force, is found to have a higher than normal stresses arising from the sectional redistribution (pre deformation of the first phase is greater than the second, so that it creeps more, but connection between phases prevents free shortening of the first one)
2. A detailed study of the stress state and the sectional forces of the critical sections for quasi-permanent combinations, frequent and characteristic was carried out, allowing calculation of the stress ranges under service loads for structural testing of fatigue.
3. It was possible to accurately assess the evolution of the sectional forces of self- weight and prestressing, allowing anadjustment of the maximum ULS verification and dimensioning of the prestressing.

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