Figure 1 The Medical Center
The vibration of an elevated floor system with long-span trusses supporting sensitive medical imaging equipment was mitigated through a combination of tuned damping, increased stiffness in the floor framing, and load distribution between adjacent bays.
Given the stringent vibration criteria for medical imaging floor systems, neither stiffening—even by a large margin—nor tuned damping of reasonable magnitude alone could achieve compliance in certain floors. Meeting these requirements necessitated a combined approach.
Figure 2 The mode shape
Numerical analysis of four bays in the floor system revealed a pronounced vibration mode involving two bays, as shown in Figure 2, with a natural frequency of approximately 4.8 Hz. This low frequency coincides with the second harmonic of footfall excitation from fast walking, making the system highly susceptible to walking-induced vibrations. Mitigating this mode required substantial stiffening, the addition of tuned mass dampers, and placing a bridging truss through the existing joists to distribute vibrations between bays.
Stiffening raised the floor system’s natural frequencies, shifting the frequency of the target mode beyond the lower harmonics walking-induced perturbation, while tuned damping effectively suppressed the first mode and reduced peak floor velocities.
Vibration measurements of the stiffened floor system revealed a new first natural frequency of approximately 7.9 Hz for the two bays. To add damping to this mode, four tuned mass dampers—two per bay—were designed, fabricated, and installed at the bay centers, each tuned to the 7.9 Hz frequency.
Two TMDs installed underneath one of the bays
Figure 3 illustrates two tuned mass dampers mounted on a sub-structure that bridges the two wide-flange beams beneath one of the floor bays. Figure 4 presents the measured power spectral densities of the floor system, in response to a heel drop perturbation, before and after the dampers were activated. The substantial reduction in vibration power at the target frequency clearly demonstrates that the tuned mass dampers effectively suppressed the first vibration mode they were designed to control.
Figure 4 Power spectral density plots of acceleration measured at the center of one of the bays, without (blue) and with (red) the TMDs operational
To ensure effective coupling between the TMDs and the floor, the bridge structure to which each TMD was fastened was welded to the wide flange beams and also Hilti-bolted to the concrete deck.