Footbridge with inclined steel arch

A steel pedestrian bridge was designed and built in Southern Italy to provide a link between a parking area and public buildings across a small river that carries the winter rainy waters from the hills to the Adriatic sea. An important aspect of the design concept was related to the respect of environmental needs and preservation of landscape that were imposed by the local authorities and institutions. The footbridge is located in a coastal region close to the Adriatic sea and after heavy rains the height of the river reaches the embankments. Two additional flights were necessary for the access of wheelchairs and prams, which are connected to the end sides of bridge; in order to reduce the dimensions of these ramps the quote of the pedestrian deck should be as low as possible. According to these needs the deck should be sustained by structural elements placed at the extrados, so the deck was suspended to a steel arch. The arch was placed only in one side of the deck, it was inclined respect to the horizontal plane of the deck, to maintain a complete view of the landscape. According to this scheme, the deck, consisting of a stainless steel trapezoidal box girder had a cantilevered front section. Vertical permanent and service loads generate torsion due to the cantilever scheme; a force in the plain of the arch is present contemporarily with a horizontal component (in the pane of the deck) due to the inclination of the arch. The forces in the plain of the arch are beared by the arch itself; possible buckling failure was also checked. The deck is not straight, and the curvature in the horizontal plane was designed in order to bear the horizontal forces by assuming the deck as an arch structural scheme in its plane. It is clear that the form of the footbridge is strictly connected to the structural concept that see an inclined arch and the pedestrian deck suspended to the arch through steel bars. The most fascinating aspect of the design process was to achieve a slender inclined arch which can support a 10 m span box girder deck. The hollow-core arch was built by using a steel tube that was coated in hot zinc bath to improve corrosion resistance. In the paper it will be shown how the deck and the arch are interdependent structural systems. The first carries a part of the vertical loads as a stiff box girder, and is supported by the cabled-arch system. The arch is reliant on the deck to be its tie, to react the horizontal component of load generated by the inclination an also to provide a constraint against buckling through the suspending bars. The closed box section was chosen to carry these restraint forces. Torsion deriving from the eccentricities of vertical loads are carried by a hollow core beam, which is also subjected to tension forces. Box-shaped sections were also adopted in order to allow the introduction of electrical cables since a lighting system was designed for the use of the bridge after the sunset. Design concept will be described in the paper, with reference to the principal structural aspects and construction strategies adopted in the project.