Dynamic performance of the Mill on the Exe footbridge

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.

Mill on the Exe 1
Figure 1: Prof. Reynolds explains the operation of tuned mass dampers to MSc students Elmar Talibli and Ed Court

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.

Mill on the Exe 2
Figure 2: James Bassitt, Vincent Ao, Yan Xu and Ahmed Mohammed taking a break.

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.

 

Mill on the Exe 3
Figure 3: Thomas Lynchehan function tests a servo accelerometer

Energy was supplied by undergraduate students, either jumping or running (Figure 4).

Mill on the Exe 4
Figure 4: Josephine Benthall crossing the bridge

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.

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