The Anglo-German team of Knight Architects and lead consultant Knippers Helbig has won a major international competition organised by the City of Vienna for a new pedestrian bridge in the centre of Vienna.
The 380m-long bridge at Margaretengürtel links recreational parkland to the west with Bruno- Kreisky Park to the east to provide a pedestrian friendly route across a busy traffic corridor. The linear form of the bridge evolves from the landscape at each end, establishing itself as a new topographic element. The continuous landscape-like object merges the previously disconnected parks into a single entity and the sculptural presence of the timber structure brings a distinctive new identity to the location.
The bridge will be a major new element in an urban environment that includes the Margaretengürtel Station building by Otto Wagner (on the U4 U-Bahn Metro line) amongst its historic neighbours. The massive minimalism and formal simplicity of the new structure is considered in counterpoint to the rich architectural language of the neighbourhood.
The fluid plan alignment follows a gently curved path, which observes strict constraints on where supports can be placed. The curve accelerates at mid span to create a sinuous ‘S’ form at its centre, and it is at this point that the two dimensional path evolves into a three-dimensional place, spanning across the main highway and tramway corridor with tall, asymmetric upstands on opposite sides of the bridge and seating set into the curving, banked sides.
The solution deliberately explores a new direction in bridge structural form through the use of massive timber with minimalist detailing. The body is an efficient hybrid structure with a glulam core of spruce layers or ‘lamella’ of varying qualities, depending on the stress level in the particular location, and an outer layer covered by larch lamella which provides durability and good protection against moisture. The relatively large uni-axial bending radii of the glulam layers are easily achieved using standard production techniques.
In areas of load transfer and peak stresses the timber will be reinforced with threaded steel rods laminated between the lamella. The factory-assembled units will be transported to site in manageable segments of up to 2m x 2m x 20m where they will be bolted together and installed onto insitu concrete plinths, which provide protection against highway impact. The robust construction method combines with the concept of a semi-integral bridge to significantly reduce maintenance costs.
This innovative development of traditional timber building techniques demonstrates an approach to construction which is both spectacular and environmentally responsible, being both a carbon sink and an efficient use of resources. Using a locally common building material with a low processing cost, it is estimated the superstructure will contain approximately 795t of stored CO2 within the 960m3 of wood and will use a fraction of the energy in construction of a concrete or steel bridge of the same span.
The design includes for a new social building beneath the western approach which, in its initial form, provides south facing steps that will act as both access and public space and serve to further root the crossing in the landscape.
The Knippers Helbig / Knight Architects competition win follows their success in a 2009 competition for two 100m-long bridges at Opladen, near Leverkusen in Germany, which is progressing towards a construction start in late 2010.