BEHAVIOR OF FRP PEDESTRIAN BRIDGES UNDER HUMAN INDUCED EXCITATION

Abstract

The prediction of the dynamic response of pedestrian bridges under human-induced excitation is a challenge in the design of pedestrian bridges, caused by the wide range of variables and the complex interaction effects. The use of new, lightweight materials, like FRP, and the trend to design long span and slender constructions lead to structures more sensitive to dynamical impact, which caused some vibrational problems at newly built bridges in the recent past. This brought increased attention to this topic. The present thesis aims to analyze the dynamic properties of the new material fiber reinforced polymer (FRP), to estimate the changes they cause in the dynamic response of respective constructions and to validate the current guidelines. The first part of the research includes a literature review in terms of pedestrian loading, their interaction with the structure, the characteristics of FRP and the specification of the current guidelines. In order to analyze the dynamic properties and the effects on the dynamic response, the second part presents a parametric analysis of simplified bridge structures and their dynamic response to different loads induced by pedestrians. In order to classify the new composite material, the estimated mechanical properties and dynamic characteristics are compared to the traditional material steel. FRPs are significantly lighter and less stiff than steel. The first property leads to a higher fundamental frequency, the later one counteracts this effect. The actual fundamental frequency of the unloaded system, which is also the main component in the dynamical evaluation specified in the AASHTO guideline, depends on the ratio stiffness to weight. In contrast to steel, FRP is more sensitive to human-induced loads. The additional mass of the pedestrians changes the fundamental frequency of the system significantly, due to the high ratio of live load to construction weight. This circumstance is disregarded by the current guidelines, which might have led to the vibrational problems at newly built pedestrian bridges. Furthermore, the lateral-synchronization-phenomenon, which is also not mentioned in the guidelines, has a significant impact on lively footbridges. A general approach for the consideration of the additional impact is introduced.