Frequency effects of pedestrian loads on footbridges

Abstract

Current Standards suggest for a footbridge dynamic analysis periodic load models which do not consider the imperfection nature of human walking. They not only do not represent pedestrian dynamic loads precisely but also exaggerate a tuned mass damper efficiency. The purpose of this study was to expose this lack of realistic load formulation and the differences obtained between a footbridge response by means of the Standard models and a more realistic loading scenario which considered the existent frequency scatter in walking loads. Another aim was to show the tuned mass damper loss of reduction when a frequency scatter was introduced to pedestrian dynamic loads. The study started with a literature survey which allowed the author to understand and collect what researchers had developed so far regarding footbridge analysis. Next, a dynamic analysis was carried out by applying periodic and near-periodic walking loads to a finite element footbridge, and running a time integration dynamic analysis in order to obtain the structure response under each loading scenario. The results indicated that existing load models underestimate in some cases the levels of vibrations reached by a footbridge, compared to the footbridge response obtained by means of a more realistic loading scenario. Moreover, it was found that indeed a tuned mass damper amplitude reduction becomes overestimated when near-periodic walking loads are used. All the results obtained by the simulations were compared to existing formulation in order to prove its accuracy. The principal conclusion was that actual Standards do not represent human-induced loads in a precise way and, as a consequence, tuned mass dampers designed according to them are expected to provide an amplitude reduction which cannot be reached.