Part of an existing building was converted to a medical center with a number of operating rooms. Vibration Measurement of the bare floor system (without any fit-out) pointed to the first natural frequencies of around 6 Hz for various bays.
Floor Vibration Limits of Operating Rooms: ISO standard suggests that vibrations in operating rooms not exceed the baseline response values. This corresponds to limiting the RMS vibration velocity to 4,000 micro-inch per second (0.1 mm/sec) at frequencies from 8 to 80 Hz, with somewhat greater velocities acceptable for frequencies below 8 Hz.
At 8 Hz, RMS velocity of 4000 micro in/sec (0.1 mm/sec) correspond to RMS acceleration of 500 micro-g .
Considering that the addition of fit-outs and equipment would lower the natural frequencies of the floor system making it more susceptible to walking induced vibration and the strict floor vibration requirement of operating rooms, it was decided to a) stiffen the existing floor framing and b) add damping to the floor using tuned mass dampers (TMDs).
Eighteen tuned mass dampers were designed, manufactured and installed underneath the operating rooms’ as well as the neighboring bays. Figure 1 shows four of these TMDs, bolted to the concrete deck, underneath two adjacent bays of the floor system. The stiffening scheme comprised of welding WT sections to the bottom flanges of existing girders is also highlighted in Figure 1.
Figure 2 presents the measured vibration responses of one of surgery rooms floor to a heel drop perturbation without (blue trace) and with (red trace) the TMDs operational.
As noted on Figure 2, stiffening of the floor framing increased the natural frequency of the floor system (from 6 Hz) to 7.6 Hz. The TMDs were tuned to the elevated natural frequencies.
Clear from Figure 2, the large reduction in the vibration power at the target frequency points to the effectiveness of tuned mass dampers, dampening their target mode of the floor system.