Civil Engineering Reference
In-Depth Information
Chapter 4
Change in Mass and Damping on Vertically Vibrating
Footbridges Due to Pedestrians
Christos T. Georgakis and Nina G. Jørgensen
Abstract
Pedestrian-induced footbridge vibrations are an issue that bridge designers often have to contend with. A plethora
of research in recent years has led to the development of load models and procedures that allow for the determination of
footbridge response. Nonetheless, measured footbridge responses often deviate from those predicted. One of the main
deficiencies of the existing models and guidelines is the exclusion of the effect of changes in the footbridge's dynamic
properties due to the presence of pedestrians. More specifically, any change in mass and/or damping that a pedestrian might
introduce to a bridge will affect the bridges overall dynamic response. This effect is an element of what is often referred to as
human-structure interaction. In this paper, the results of an experimental study to determine the change in mass and damping
of a vertically vibrating footbridge due to traversing pedestrians are presented.
Keywords
Vertical footbridge vibrations • Mass • Damping
4.1
Introduction
The majority of footbridges designed during the twentieth century were designed in a purely utilitarian manner, with the goal
being the creation of a path for pedestrians to traverse between two points, without the impediment of e.g. vehicle traffic or
bodies of water. Towards the end of the twentieth century this changed, as footbridge construction began to be viewed by
architects and city planners as a tool for urban regeneration and as a means to create landmark structures and large-scale
“sculptures”. This shift in design philosophy, together with a reassessment our energy-intensive transportation infrastructure,
has lead to a change in the perceived importance of footbridges. As a consequence, designers and engineers are paying greater
attention to the aesthetic details and the servicability of footbridges, making sure that they are both beautiful and “comfortable”.
Although footbridge comfort is often difficult to define, vibration comfort criteria within international codes and
guidelines do exist [
1
,
2
]. More often than not, the same codes and guidelines offer a means to assess the level of footbridge
vibrations. Nevertheless, even though these codes and guidelines have often existed and been used, in some cases, for
decades, it is not until recently that researchers and engineers have produced load models and procedures that allow for a
more reasoned determination of footbridge response [
3
]. Still, even with the recent advances in load models and response
prediction, measured footbridge responses often deviate from those predicted.
One of the main deficiencies of the new generation of models and guidelines is the exclusion of the effect of changes in
the footbridge's dynamic properties due to the presence of pedestrians. Any change in mass and/or damping that a pedestrian
might introduce to a footbridge will affect the footbridges overall dynamic response. This effect is an element of what is
often referred to as human-structure interaction.
To the authors' knowledge, there has only been one other reported systematic attempt to determine the change of a footbridge's
dynamic properties due to the presence of walking humans. Zivanovic et al. [
4
] showed an increase in damping of a 11.3 m long
concrete footbridge, as a function of the number of traversing pedestrians. Unfortunately, the study was unsuccessful in
generalizing the results in manner that would allow for their use on any footbridge or similar structure. Furthermore, no attempt
was made to quantify a potential change in equivalent structural mass, due to the presence of pedestrians.
