The Mill-on-Exe footbridge (Figure 1) was opened in 2003. The span is supported by two pairs of stay cables, whose tensions are balanced by a massive counterweight, and is equipped with six tuned mass dampers (TMDs), which are designed to suppress pedestrian-induced vibrations.
A team from the Vibration Engineering Section (Figure 2) supported two University of Exeter undergraduate engineering student projects – one about performance of the tuned mass dampers and one about flow of energy from the pedestrian generating dynamic response to damping sinks, including TMDs.
Measurements used a set of 21 Honeywell servo-accelerometers (Figure 3) and a set of 11 APDM Opal inertial measurement units (IMUs). Opals are intended for sports biomechanics applications, but they are well suited for research into human-induced structural vibrations e.g. of footbridges, and this is the first time they have been used to study the flow of energy.
Energy was supplied by undergraduate students, either jumping or running (Figure 4).
The testing provided a set of experimental modal properties, which validated the modeling by the consultant (Flint & Neill) and confirmed the effectiveness of the TMDs.