Keywords: railway viaduct, limited spans, slender pier structures, lateral deformations, in situ testing.

In designing railway viaducts with limited spans of 35m and continuous rails, severe deformations and acceleration restrictions are demanded in the Eurocodes. Both verifications, in the vertical and in the horizontal direction are mostly done separately and with simplified models. In the case of two railway viaducts recently built in Belgium, slender pier structures were required for architectural reasons. Using a simplified model of the structure could lead to stiff constructions and does not take into account vertical and lateral deformations of the piers which may influence the bridge dynamics. This paper examines the influence of these deformations on the bridge dynamics by using detailed finite element modelling of the whole structure. The results show that the combined deformations hardly influence the bridge dynamics. In the nearest future, in situ tests will be carried out in both cases.

The first viaduct consists of two one-track continuous railway viaducts in extending a historic viaduct on the existing railway line between Brussels and Ghent crossing the Pede valley. The existing structure, of 523 m length dating from the 1930s consists of sixteen three-hinged reinforced concrete arches of a 32m span. The final design in extending the bridge from two to four tracks consists of new steel fly-overs of box section supported on slender rectangular steel hollow ribs of a conical shape, which are fading gradually into the lower part of the existing piers. The superstructure is made continuous over four spans. The conceptual design of the piers has to deal with both vertical and horizontal forces. As a result of the eccentric railway load (on one track), the portal frame bends on the side where the charges are applied and rises on the unloaded side.

The second case consists of a double track railway viaduct of 600m with isostatic plate decks supported on slender concrete piers. The piers have a central rectangular shape which widens at the top in several individual concrete beams in a curved shape referring to an iris flower being the symbol of the Brussels Government. The upper part of the pier structure with a length of a 10m becomes part of the superstructure and takes the reaction forces of two individual isostatic plate decks of a span of 20m on both sides of the pier. With a height of 25m for the highest pier, the pier structure has a slender design. In addition, the tracks are curved in a horizontal plan introducing centrifugal forces. Eccentric loading on one side (loading of one track) may introduce both vertical and horizontal lateral deformations in combination with the centrifugal forces. After the construction of the viaduct, in situ testing using lorries has been carried out.

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