Abstract:

A new footbridge has been built at Valladolid (Spain) to connect the old city with a new residential development located at the other side of the River Pisuerga. The footbridge has a main span of 110 m and consists of a tubular steel truss with a hexagonal cross-section stayed with a three-dimensional cable cage.

The staying system combines 6 longitudinal cables and 15 transverse cables with a hexagonal shape.

The longitudinal cables are anchored to the piers in the main span in order to increase its resistance to vertical and horizontal loads in all directions and to enhance stiffness. These cables are located externally to the tubular truss and the required eccentricity is achieved by means of struts placed at each corner of the hexagons like radii pointing out from the centre of the tube. Transverse cables are essential in order to keep the shape of the hexagonal cross-section under the effect of the longitudinal prestressing and high live loads unsymmetrical. A solution with all the prestressing cables and anchorages similar to those commonly used in prestressed bridges was preferred in order to simplify execution and detailing and, thereby, reduce costs.

The great deviation angles of the transverse cables at the intermediate corners of the hexagon required a detailed study of the more appropriate geometry and materials to be used in this element.

Meticulous design was also necessary in order to place the anchorages and the deviation units of these cables at the tip of the tubes used in the struts and to respect the lightness of the global design.

These struts are just 200x200 mm RHS.

Special attention has been paid to the interaction between structural and architectural issues since the footbridge is part of a museum designed by the renowned Spanish architect Rafael Moneo, who was also involved in the design of the footbridge. During the whole design process a close relation existed between the architects and the engineers in order to keep the initial ideas alive and to achieve a pleasant result in relation to the aspect of the global treatment of the zone (the museum, the footbridge and the treatment of the riversides).

A complex design procedure has been necessary due to the non linear behaviour of the structure (tension-only behaviour of the cables, boundary conditions during construction, great deflections) and the vibration control that is essential to this type of footbridges. Thorough dynamic analyses have then been carried out to guarantee that transverse oscillations that could bother pedestrians do not occur in spite of the flexibility and lightness of the structure. Geometrical aspects have also been quite complex during the design and the construction of the footbridge. All the cross-sections along the bridge are different and neither the cables nor the struts are located in a plane for each group. A detailed study was necessary in order to ensure perfect compatibility between the steel and the cable elements.

Even though the river is not very deep and a construction procedure with provisional supports on the river bed could have been envisaged, a construction procedure independent from the river seasonal variations was preferred. The welding of the different parts of the structure as well as the installation of the anchorage and deviators of the transverse cables is easier to be carried out on the riverside at a reduced height from the ground. Therefore, the main span including Piers 4 and 5, as well as the timber pavement was ompletely assembled on the riverside and was then moved to its final position in a simple manoeuvre.

Fig. 1 Footbridge elevation: Concrete section (stretch 1) and steel section (stretches 2, 3, and 4).

Fig. 2 Cross section at central mid-span (left); schematic view of two modules of the structure at the main span, with colours indicating the different groups of elements (right)

Fig. 3 Schematic view of the stages of the displacement manoeuvre of the footbridge from its construction position at the riverside (right picture) to the final position