A Study of Vibrations of a Slender Footbridge Due to Human Movements

Slender footbridges can be susceptible to large vibrations due to human movements. The low natural frequencies and damping of these systems can result in excessive or annoying movements. This paper presents details of the vibrations analysis of a two-span steel footbridge, designed and built by a group of architecture students at Virginia Tech, Blacksburg, Virginia, USA.

The footbridge is comprised of three main segments that were fabricated in shop and shipped to the site for installation. The main supporting members were made of two 200 mm deep steel I-beams, which resulted in a span/depth ratio of about 66. As the structure was only designed for static loads, design modifications to the boundary conditions and member connections were made to reduce the level of vibrations due to the pedestrians’ movements on the footbridge. Following the completion of the construction, dynamic testing of the structure was conducted. The main objectives of these tests were: (1) to conduct a modal analysis of the footbridge, (2) to study the effects of human-structure interactions (HSI) on the dynamic properties of the footbridge, and (3) to evaluate the vibration of the footbridge due to different numbers of pedestrians walking or running over it. The presence of people resulted in a reduction in the natural frequencies and an increase in damping ratios. The low natural frequency of the footbridge made it susceptible to excessive vibrations when a group of people ran over it. Finally, two observers evaluated the intensity of vibrations as people crossed the bridge. The results were compared with the provisions of some guidelines on human vibration perceptibility.