Footbridge 2017, Conference 6–8.9.2017 TU-BERLIN
Since I’ve been dealing with tubes with flattenend ends over the past 30 years, our occupation here is constructing a bridge out of small tubes, which all together constitute a large tube in form of a pedestrian bridge. Themes like sustainable design, resource-saving materials, minimizing costs by planning and creating something new out of old but best practiced materials are the main thoughts for this structural concept. A modern construction that fits into its environment, that is actively used while it has a welcoming design are important issues for planning a bridge.
The proposed footbridge is basically designed by a string of triangles. The advantage of the design is that there is only a little construction height below the footpath. Therefore, the bridge does not have to be built at a high ground level. Instead there is a respectable height over the footpath that helps to minimize the internal forces inside the construction.
The design of a tube with numerous small tubes produces many joints. To minimize costs, a system with simple nodes must be found, while the nodes have to fit with the tubes. The design basically consists of two types of tubes, one as a net following the surface of the cylinder, the other to reinforce tangents of the profile. To prove the idea, we checked an existing tubular steel system.
Tubes in two lengths and two forms of the so-called „System 180“ are screwed together and form a large tube in their composition. System 180 consists of tubes made out of stainless steel that ends are flattened. The centres of the flat ends are drilled. The diameter of the hole corresponds to a quarter of the flattened end of the tube. By this composition, the German law of steel construction becomes fulfilled (the double of the length of the diameter to the edge in direction of power). The special features of System 180 are the four projections extending from the flattened end of each tube, which fit into four cavities on the other side of the flattened end of the tube it becomes connected with.
The combination of projections and cavities increase the strength of the joint by 50% compared to connections held by bolts only. By calculating a construction made from System 180, the static specification of the tube can be transferred to the joint. Thus the calculation is quite simple – the important proof focuses on buckling, other proofs like tension or forces within the knot have not to be considered. The area where the tube goes into the disc made by the flattened end is weak concerning bending in one direction. Accordingly by designing a structure, it is important to support the weak part, which is normally solved by constructing triangles.
Since the load-bearing construction overlaps the running path, a rain protection is conceivable without additive surface mounting needed. Snow and ice are largely excluded. Handrails and for example, visitor guidance systems and signposting can be attached to several joints easily without creating additional bends in the construction. To form the length of the bridge, the flattened ends of the main tubes must be bent at slightly different angles.
Every part of the construction is contributing to strength, like a net in the shape of a tube with a diameter of approximately 3.50 m. The construction is light and the costs are reasonable. All parts are made of brushed stainless steel without any coatings. For forming the tubes and for mounting the construction no welding is necessary. This proposition of how to construct a footbridge out of tubes is ecologically sensitive. The material used integrates itself gently into the environment through its reflective surface while having a long duration. The bearings can be minimized, since no horizontal forces act in the ground. It is a truism that bridges serve to connect. But, in fact, it is quite rarely mentioned, that disconnection precedes separation. In its appearance, the construction is light, the connection of several tubes and in its result the combination of triangles gives it a dynamism and modernity.