Tensairity bridge in Maribor

The 'tensairity' pedestrian bridge in Maribor, Slovenia 2010 is a light-weight tensegrity and air-balloon bridge in preparation of the cities 2012 program as the European Capital of Culture. Competition entry by Aurel von Richthofen in collaboration with 'tensairity' consultant Prof. Dr. Rolf Luchsinger. 

Context:
The proposed new footway and bicycle bridge Lent–Taborsko nabrežje in light of Maribor's preparation to the European Capital of Culture in 2012 is a light-weight construction combining air beams and tension cables in a new structural concept called “Tensairity”. The design calls for a modern solu- tion that bridges Drava River at the location of the oldest bridge in Maribor, between the old port – Lent, and the Tabor bank (Taborsko nabrežje), and significantly shortens paths for pedestrians and bicyclists, thereby improving connections in the city centre at both banks of the river. Spanning the 130 meters wide riverbed of Drava River in three elegant ar- ticulated girders, supported by two columns anchored in the riverbed, with a height clearance of 3.60 meters the new footway bridge is designed to become the recognizable and innovative, transparent and neutral structure of the city.

Structural System:
The new structural concept Tensairity is a synergetic combination of a pneumatic structure and a cable-strut structure. The main function of the pneumatic structure is to stabilize the cable-strut structure. Tensairity struc- tures have a multitude of very interesting properties. The beam or shell like structures are very light, thus ideal for foot and bicycle bridges. Furthermore Tensairity structures are load adaptable. The load-deformation response of Tensairity girders can be controlled by the air pressure which allows the girders to adapt to changing load conditions. The controllable adaptiveness of Tensairity structures to changing load conditions is the most outstanding feature of Tensairity compared to con- ventional trusses or girders. The adaptability is an inherent feature of the structure itself and follows from its design and concept. As a consequence, Tensairity girders can be viewed as a machine, where energy is converted into work. The energy in form of the compressed air is used to lift a weight. The machine aspect of Tensairity structures can give a different look on civ- il engineering structures. In light weight structures, the live load normally dominates the dead load and thus mainly defines the design of the structure. Such a girder is therefore designed for a maximal live load which is normal- ly given by the building regulations. Most of the structures never experience the maximal load during their life time. The price for a nevertheless relative security is that most structures are under almost all conditions way to strong and way too heavy. In contrast, a Tensairity girder can be adapted to the cur- rent load situation simply by pressure variation. This enables an important safety concept for Tensairity. The idea is to design the Tensairity structure in such a way that the load-bearing capacity of the bending stiff elements in the girder is large enough to carry the dead load of the structure even with zero overpressure.

Tensairity is a machine and not a beam:
The fundamental Tensairity beam consists of a cylindrical airbeam, a compression strut tightly connected with the airbeam and two cables spiraled around the airbeam and at- tached at each end with the compression strut. The compression strut can have minimal dimensions leading to the light weight property of the new structural concept. Further- more, the pressure in the airbeam is solely determined by the load per area and indepen- dent of the span and slenderness of the beam. Therefore, the synergetic combination of an airbeam with cables and struts is ideally suited for wide span structures. In the case of the new footway and bicycle bridge Lent–Taborsko nabrežje, a sequence of interlocking gird- ers span the river.

Materialisation:
The airbeams are made of translucent EPFT material, standard in contemporary construction industry and recyclable. The girders rest on steel bracing and concrete foundations. The bridge-way is clad in local wood. They are equipped with spotlights with color changing capabili- ties. The light shines through the glassy end plates into the pneumatic structure and illuminates the Tensairity girders from inside. The color of each Tensairity beam can be dynamically changed and controlled by software and interesting light patterns over the whole bridge structure can be realized. Neutral and transparent during the day, the bridge can display dynamic lights during the night. The orchestration of the light will be the work a scenographer and may be integrated into the festive activities of the European Capital of Culture in 2012. Artist may pro- gram the bridge to resonate the ambient light of a second sun-down or use it to reflect the city's state of mind and emotion.