Self-stressed Bridge in composite materials

Description of the research

The arch consists of an elastically bent pultruded pipe whose the geometry is that of an elastica, a cable anchored at the ends of the bow to maintain its shape, and two zigzag stay strings. To build the footbridge, the top of two such arches are linked together in the middle and a bridgedeck is installed on crossbars between the arches. Inexpensive pultruded elements also allow for a minimization of the number of assemblies between the elements: Cables and stays are continuous strings if possible. The proposed footbridge also enables high degree of industrial prefabrication, and thus leads to an improved quality, from the pultrusion process to the assembly of the arches and of the bridge. The lightness of the structure offers new possibilities for the installation of this ready-to-use bridge. The shape of the initial selfstressed structure must be computed before installing the cables and stays. The stress-equalized geometry (optimized geometry) is found numerically with an algorithm based on the method of force densities (MFD). This method yielded satisfactory results: The maximum force in the stays under the initial state of stress decreased by about a third compared when optimizing the geometry. On the behaviour of the footbridge under load however, the choice of the initial geometry has no significant effect.

Other improvements (e.g., control) have to be tested. The static and dynamic studies show that the dimensions and sections needed for the footbridge elements are both reasonable and feasible. The proposed super lightweight footbridge also shows a satisfactory dynamic behaviour with respect to its interaction with pedestrians, which often is not the case with classical steel lightweight footbridges. The improvements made possible when optimizing the shape of the structure under a selfstressed load state open new perspectives for further optimisation. A subsequent study aims to develop an ‘‘interactive” shape optimization technique, i.e., a real-time control of the shape. The purpose of the shape control is to ensure an equal distribution of static or quasi-static live loads in the FRP cables and stays, and thus to maximize the security margin for this type of composite structure. The stresses in all the stays can be equalized by interactively modifying the geometry of the bridge.