Fatigue Analysis of a Lively Footbridge

Abstract: The commitment of building aesthetic bridges made of stiff materials results in theconstruction of slender structures highly susceptible to dynamic loading. In the caseof footbridges located over a highly trafficked highway, such dynamic loads maybe the aerodynamic pressure induced by the heavy trucks passing underneath thestructure and the pedestrian loading involving a pedestrian running and jumping.There have been reported cases in the literature that these types of dynamic eventscan cause discomfort to the users of the bridge, and thus, footbridges are usuallychecked in the serviceability limit state. However, footbridges are not checked forfatigue damage since the design codes do not demand fatigue analysis.In the present master thesis, a hypothesis is made: the dynamic loading actingon footbridges can cause fatigue damage. To test this hypothesis, a case study wasconsidered. The Smista bridge was chosen, which is a footbridge with noticeablevibrations even with the single action of a pedestrian running on the bridge oreven when a single truck passes underneath the bridge. The bridge was simulatednumerically using a 3D shell finite element model. Four load cases were studied:the aerodynamic pressure induced by the heavy trucks passing below the bridge,the pedestrian jumping in the middle of the bridge, the pedestrian running in thecenterline of the bridge and the pedestrian running near the edge of the bridge.The results showed that the dynamic events applied on the bridge did not producehigh enough response and stresses to account for fatigue damage accumulation.Therefore, it is concluded that fatigue design of such bridges, although very livelyand which can cause high discomfort to people, should not be added as a designcriteria in the design codes.