Butterfly Wings. An innovative bowstring concept to span the Ebro River in Logroño (Spain)

Las Norias footbridge design is the result of a competition (25 proposals) to solve the connection of two existing neighbourhoods of the city of Logroño separated by the Ebro River. The bridge is an innovative type of bowstring arch that, looking for the minimum environmental impact, clears the river with a single 102-m span. Its deck width varies between 5,0 m and 10,0 m, looking for a suitable adaptation to the landings, and its arches rise 11,0 m over the deck, which is suspended from them by spiral-strand cables. These cables are contained in cylindrical surfaces due to the lean of the arches and the curved edges of the deck. The deck, with an innovative design, is made up of a post-tensioned timber slab, lying over a double-Y-shaped steel spine with variable depth. The cross section of the arches is also variable, with maximum depth and minimum width at their springings and the inverse proportions at the centre, in order to avoid out-of-plane buckling.