Tuned mass dampers (TMDs) were used to abate resonant vibration of the floor system induced by the rhythmic (dancing) perturbation at an upscale intimate concert venue/night club. The composite floor system is made up of 2.5” lightweight concrete on 3” deep, 20 gauge steel deck supported by wide flange steel beams and girder framing. The typical dimensions of the bays are 30’x32’. The first resonant frequencies of the five bays with the most severe vibration are in 5.5-7.5 Hz range, with the typical damping ratio of 2.7%.
When one of the harmonics of a rhythmic perturbation (such as stepping load in dancing) matches one of resonant frequencies of a floor system, that floor vibrates excessively (resonates).
With the floor vibration measurement results identifying the first natural frequencies of the 5 target bays on hand, 10 tuned mass dampers (2 per bay) were designed, manufactured and installed at the center of the bays. The rather large load density on the floor necessitated the use of rather large 750 Kg (1700 lb) TMDs.
Dancing load density (load/area) depends on the type of dance, the number and weight of people dancing as well as the size of dancing area. The frequency of dancing load depends on the tempo of the music (normally between 1.5-3.5 Hz and even as high as 4 Hz).
Figure 2 shows two tuned mass dampers bolted to a subframe structure bridging the two center wide flange beams, underneath each bay. Each pair of tuned mass dampers were tuned to the natural frequency of the bay they were designed for.
To ensure effective coupling between the TMDs and the floor, the bridge structure to which each pair of TMDs were fastened was welded to the wide flange beams and also Hilti-bolted to the concrete deck.
The blue trace in Figure 3 presents the vibration response of the floor to a heel drop perturbation without the TMDs operational (TMDs locked). The red traces in Figure 3 depicts the same measurements when the TMDs are operational (TMDs unlocked). As shown in Figure 3, the large reduction in the vibration power at the target frequency indicates that the tuned mass dampers have dampened the first vibration mode they were designed for and tuned to, effectively.