Pedestrian bridges are often designed to be lightweight and architecturally elegant, but those same characteristics can make them susceptible to vibration. When excessive motion affects user comfort, tuned mass dampers (TMDs) can provide an effective solution.
A recent bridge project presented precisely this challenge. Analysis performed during the design phase identified the key dynamic characteristics of the bridge, including its natural frequencies, mode shapes, and modal masses. These predictions revealed two dominant vibration modes: a vertical bending mode and a torsional mode with closely spaced natural frequencies.
Figure 1 Placement arrangement of the TMDs at the midspan
Using these predicted dynamic properties, DEICON designed and fabricated two tuned mass dampers with a combined moving mass of 500 kg. The dampers were installed at the bridge midspan adjacent to the two main girders, as shown in Figure 1. Because the target bending and torsional modes occurred at nearby frequencies, the tuned dampers were tuned to an intermediate frequency between the two, allowing a single implementation to provide damping to both vibration modes simultaneously.
Figure 2 shows one of the TMDs being lowered into its housing cavity. Integrating the devices within the bridge structure allowed the vibration-control system to remain concealed while maintaining direct influence over the dominant modal responses.
Figure 2 One of the TMDs being installed
Following installation, field measurements were conducted to evaluate performance. Acceleration data were collected at midspan close to the edge, with the TMDs locked (inactive) and then repeated with the TMDs unlocked and operational. The results, presented in Figure 3. The measurements identified dominant modes near 1.1 Hz and 2.4 Hz, corresponding to the bridge’s primary bending and twisting modes. Despite being tuned between these frequencies, the TMDs successfully added damping to both modes.
Figure 3 Power spectra and time traces of measured acceleration at midspan of the bridge with and without the TMDs operational
The dampers were tuned to a frequency lying nearly midway between the two dominant modal frequencies. This strategic tuning enabled a balanced distribution of damping, improving the dynamic behavior of both vibration modes rather than focusing on only one.
Figure 4 PSD and time trace of walking induced acceleration at midspan
To quantify the improvement, the measured responses were processed using narrow-band filtering and damping estimation techniques. For the 1.1 Hz mode, the bridge’s damping ratio increased from approximately 1.2% to 3.7% when the TMDs were activated. In other words, the effective damping of the mode increased by nearly a factor of three. The 2.4 Hz mode exhibited similar benefits. Analysis of the filtered time histories and exponential decay curves showed that the damping ratio increased from approximately 1.8% to 3.4% with the TMDs operational. This represented nearly a doubling of the modal damping. Additional testing was performed using pedestrian loading. Three individuals walked across the bridge at a moderate pace while accelerations were recorded at midspan. The results, shown in Figure 4, demonstrated that although multiple modes were excited by walking activity, the maximum measured acceleration remained at approximately 0.25% g. This level was well below the pedestrian comfort threshold recommended by AISC Design Guide 11.
This project highlights an important aspect of tuned mass damper design: successful vibration mitigation depends not only on the size of the damper, but also on a detailed understanding of structural dynamics. By leveraging analytical predictions, strategic placement, and careful tuning, two relatively small TMDs delivered substantial improvements in bridge performance. The result was a structure with significantly enhanced damping, reduced vibration amplitudes, and improved comfort for pedestrians.