In October 2009, responding to an increase in bridge suicides, the City of Bern council passed an urgent motion requiring the installation of safety nets on five of the city’s tall bridges. Following the implementation of provisional measures featuring vertical safety nets that met neither aesthetic nor listed building criteria, the Engineering, Transport and Green Spaces Department commissioned a project team to devise a definitive solution. Schnetzer Puskas Ingenieure and Rolf Mühlethaler Architekt worked together to design a set of nets that would meet all the stated requirements.
The task involved installing suicide prevention measures in the form of safety nets on both Kirchenfeld Bridge and Kornhaus Bridge. These horizontal nets are intended to prevent suicides jumping from the bridges and protect those below. Though similar in construction, the two net designs differ in detail and are tailored to the span widths and specificities of each existing historic structure. What is particularly ingenious about the design is that all its components – including the net itself – contribute to load-bearing. The system comprises a mesh net at least three meters in width supported by a main bearer cable stretching between the two bridge abutments. The width, mesh gauge and wire diameter of the nets were determined on the basis of mechanical tests and studies carried out on a life-size model. Net width depends on how far beneath the roadway a net is installed. On Kirchenfeld Bridge the net is 3.8m wide and lies 3.1m below the carriageway slab. The net on Kornhaus Bridge measures 3.1m in width and is fitted 1.75m below the bridge. The bearer cable is supported by cantilevered steel girders fixed to the bridge pillars. Suspension cables connect the net to the bearer cable and to the superstructure of the bridge.
The result is an efficient cable construction that, together with the nets, functions as an integrated static system. It helps prevent suicide attempts and may save lives, whilst also meeting the design, urban planning and listed building criteria set by the authorities.