In recent years, the design and construction of structures has advanced in leaps and bounds, thanks to the development of better performing materials, the use of complex software and simulators, and new manufacturing processes. One example is the TRUMPF pedestrian footbridge, designed by structural benchmark Mike Schlaich. It is a 28 m long by 10 m wide bridge made of a double-curved stainless steel sheet only 20 mm thick, which links two production areas of the company of the same name in Ditzingen (Germany).
Schlaich always uses as little material as possible in his designs, as is the case with the TRUMPF footbridge. The design of this structure is based on the funicular polygon and its opposite, the anti-funicular polygon. The law governing this technique, used since the beginning of the 20th century, states that any symmetrical structural design working in pure tension will work in pure compression if it is inverted with respect to a horizontal straight line (and vice versa). This is clearly understood if we take as an example two structures we are used to: a suspension bridge and a stone arch. Both structures, defined by a similar parabola, work respectively in pure tension and pure compression, one of them convexly (the suspension bridge) and the other concavely (the stone arch).
Schlaich based the design of the TRUMPF footbridge on this technique, using an orange mesh, which works exclusively in pure tension. If we turn the orange mesh upside down, we get a structure like that of the footbridge, which now works exclusively in pure compression. In fact, the holes in the steel sheet are not a coincidence or just a matter of aesthetics, but they indicate the flow of forces to the supports of the bridge. This type of design, beyond its undeniable plasticity, achieves great efficiency in terms of material requirements. It also makes it clear how forces travel through the structure from the loads to the foundations.
By Alberto López, Senior Structural Engineer at Amusement Logic’s Architecture Dept.
