Change in Mass and Damping on Vertically Vibrating Footbridges Due to Pedestrians - Topics in Dynamics of Bridges - page 38

Fig. 4.3 Actuator at footbridge mid-span with hinged load cell
Fig. 4.4 Normalized PSD of bending and torsional signals measured during the footbridge modal analysis
Figure 4.4 shows the normalized power spectral density (PSD) of a set of bending and torsional accelerations measured
during the modal excitation. From this, the footbridge frequencies can be identified, for both bending and torsional modes. A
finite element analysis of the footbridge identified the same modes, with small deviations in frequency. The first bending
eigenmode was found to be near perfectly sinusoidal in shape.
The modal frequency and damping were both found to be amplitude dependent. Figure 4.5 shows the dependency of
frequency on vibration amplitude, whilst Fig. 4.6 shows the amplitude dependency of the damping of the empty bridge. The
plotted points are a result of five independent vibration tests.
Frequency response curves were also established for the empty footbridge, once the actuator of Fig. 4.3 was installed, but
no discernable change in the structures modal properties could be measured.