Stability of network arch bridges under traffic loading

Network arch bridges are tied arch bridges with inclined hangers that cross each other at least twice. Specific favourable load carrying properties can be achieved with this type of bridge due to this particular hanger arrangement. Hangers are kept in tension by permanent loading. Traffic loading causes reductions in some hangers while increasing the tension in others. Network arches therefore act very much like an interacting system of superimposed trusses in which axial forces predominate in the chords with only little bending effects, allowing for slender cross-sections. The numerous inclined hangers provide stabilisation against global buckling in the arch plane in this structural system. Specific patterns of traffic loading can be found in which the tension force in individual hangers is significantly reduced. If the intensity of such traffic loading patterns is high enough, several adjacent hangers may become slack. These slack hangers cease to provide stabilisation for the arch and can trigger a particular in-plane buckling mode for the global structural system. This effect was investigated during the independent check of the network arch bridge crossing the Brandangersund in Norway. The present paper reports on the strategies of checking this effect developed during this project.