Illuminated pedestrian footbridge in Catalonia moulded by complex site conditions
This footbridge over the new bypass of Sant Pere Sacarrera by Alfa Polaris is part of an Y-shaped itinerary that links the town centre with two footpaths that give access to a forest area and were interrupted by the new road. The client not only wanted to provide pedestrians with a pleasant user experience, but also carry out an attractive design that would improve the visual experience of the drivers, avoiding luxurious solutions.
The aim was to get a proposal with reduced whole life cost and environmental burdens. For this reason the design team proposed a Corten steel truss girder structure for the deck (very efficient from a structural point of view and whose rusted skin protects itself from further corrosion); the use of LED for the street lighting over the footbridge lamps (with long life span and energy efficiency); and polymeric timber for the decking over the structure (with almost no maintenance).
The design of the new footbridge was strongly conditioned by the asymmetric shape of the cross section of the road trench, marked by the different level of the two banks. A footbridge composed by two elements was proposed. The main one is a two span continuous deck formed by two Warren truss girders with variable depth, linked together at bottom flanges by transversal beams. The latter, in turn, give support to the variable wide polymeric timber decking through longitudinal joists.
The main span of this element crosses the road, while the second one remains integrated to the north abutment, where a zig-zag ramp raises its level and therefore helps reduce the longitudinal slope of this element. The second element is a square platform integrated to the south abutment which creates a sort of balcony piazza that provides users great views over the town and its surroundings.
The main girders have a slight curved shape in elevation and their depth varies, being maximum at the north end (where the deck width is minimum) and minimum at its south end (where the deck width is maximum).
Since the upper chord of the truss has no bracing, in order to prevent it from lateral buckling, the designer improved the torsional stiffness of the bottom chord by giving it a hollow box shape, reinforced the lateral stiffness of the web members and increased the width of the top chord, to which an inverted U shape was given. This allows enough room for the lights to be embedded in it.
High efficiency low maintenance LED technology lamps were installed on each of the bays of the truss. This solution not only provides an agreeable street lighting along the crossing, but, by means of the combination of light and shadows, in the night it highlights the beauty of the structure.