Tuned Mass Dampers and Frame Stiffening for a Medical Center Floor System

In recent years companies with medical imaging equipment have been leasing space in medical office buildings on the upper floors. This is problematic as medical imaging equipment can be very sensitive to vibration velocity. CT, CAT and PET scanners typically are sensitive to vibrations larger than 2,000 micro-in/s and MRI machines are affected by vibrations as little as 500 micro-in/s. Compare this to a typical office building which has a sensitivity of 16,000 micro-in/s (Murray 2016). Therefore, CT, CAT and PET scanners are 8 times more sensitive and an MRI machine is 32 times more sensitive than an office floor. Someone simply walking down the hall may induce vibrations in these machines that blur the images that they create. Since many office building floors may not even meet the office vibration criteria, meeting the criteria for such scanners is well beyond normal performance of an office floor system.

tuned mass dampers being installed It was desired to place an MRI machine on an elevated floor of an existing office building with 50-foot spans. The floor was constructed of steel open-web trusses with concrete fill over metal deck. As discussed previously MRI machines are highly sensitive to vibrations; this particular unit required a maximum velocity of 0.07 micro-meters per second (micro-m/s) or 25,000 micro-in/s. When the existing floor was modeled the velocity was predicted at 2.1 micro-m/s in the location of the MRI, thirty times what was allowed. To mitigate such a drastic difference, multiple strategies were implemented.

First the trusses were strengthened to three times (3x) their original stiffness. This stiffening only reduced the velocity to 0.62 micro-m/s, still far from what was required. Note that the velocity has decreased as well as the fundamental frequency increased to ~7.5 Hz. Stiffening the floor has a significant effect on vibration reduction, but even stiffening by a factor of 3 was not nearly enough for this situation. Next a bridging member (a truss similar to the original trusses in this case) was added to the model. The natural frequency remained at ~7.5 Hz but the resonant velocity further decreased from 0.62 micro-m/s (with stiffening only) to 0.22 micro-m/s (with stiffening plus bridging). The reason for the natural frequency remaining unchanged is that bridging increases not only the stiffness of the structure but it also increases the portion of the floor mass participating in vibration. As noted earlier the maximum velocity at the unit location is reduced but still not enough. Finally, TMDs were added to the model in addition to the stiffening and the bridging member. TMDs were included in the bay with the unit and the adjacent bay (perpendicular to the direction of the bridging truss). Placing TMDs in the adjacent bay was nearly as effective as placing it in the main bay as the two bays behaved as a continuous beam in the main modes. With 2 TMDs in each bay the velocity was reduced to 0.043 micro-m/s.

Following the installation of the TMDs, floor vibration at the target bays, without and with the tuned mass dampers operational, were measured. With two TMDs in place in each bay, the actual measured velocity in the floor system was approximately 0.07 micro-m/s which was the maximum allowed. More

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Tuned Mass Dampers for a Dance Floor

Dancing, as in other rhythmic activities, subjects the floor to repetitive loadings. 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). Depending on the type of dance, the dancers are either always in contact with the floor or the they are jumping in which the contact with the floor is not maintained. In the first type, the floor is subject to the dancing load at the fundamental (dancing) frequency and a very few of higher order harmonics (multiples of dancing frequency). The second type of dancing load is potentially more severe than the first type; this is mainly because in addition to the fundamental (dancing frequency) the floor is subject to many of its higher order harmonics. These high order harmonics can potentially resonate even the rather stiff floor systems with higher natural frequencies.

dance floor tuned mass dampers Following the measurement of vibration and finite element analyses of the floor system, five bays of a dance floor were identified as having objectionable floor vibration. As the outcome of the measurement and analyses, the natural frequencies and the shapes of the first vibration mode of these bays were identified. Subsequently, ten tuned mass dampers (TMDs) with the active mass sizes of 700 and 750 Kg were designed and built to mitigate the tonal vibration of these bays. Two TMDs were installed underneath each of the 5 target bays. Three sets of coils springs in conjunction with the above-mentioned active mass sizes were used for individually tuning the TMDs to the natural frequencies of the first vibration mode of the bays they were designed for.

Following the installation of the TMDs, floor vibration at the 5 target bays, without and with the tuned mass dampers operational, were measured. Comparison of the measured floor vibration without and with the TMDs clearly showed the effectiveness of the TMDs in adding a substantial amount of damping to the first mode of vibration of their corresponding bays. The tuned mass dampers successfully dampened their target modes, quieting the vibration of the floor system. More

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Active Control of Tonal Noise in Engine Air Induction Systems

Passive, tuned acoustic absorbers, such as Helmholtz resonators (HR) and quarter-wave tubes, are commonly used solutions for abating the low-frequency tonal noise in air induction systems. Since absorption at multiple frequencies is required, multiple absorbers tuned to different frequencies are commonly used. Typically, the large size and multiple numbers of these devices under the hood is a packaging challenge. Also, the lack of acoustic damping narrows their effective bandwidth and creates undesirable side lobes.

Active noise control could address all of the above-mentioned issues. Most active noise control systems use feedforward adaptive algorithms as their controllers. These complex algorithms need fast, powerful digital signal processors to run. To ensure the convergence of the adaptation algorithm, the rate of adaptation should be made slow. This might lower the effectiveness of the controller during the transients, e.g., a fast run up of the engine in an induction or exhaust noise control application.

An alternative to the feedforward active noise cancellation is feedback-based active noise control. Feedback noise control strategies are more straightforward and computationally demanding than adaptive feedforward schemes and thus can be programmed in less expensive micro-controllers rather than digital signal processors. Moreover, contrary to feedforward scheme where the microphone and speaker are located upstream of the air filter and thus subject to the environmental elements, in proposed feedback scheme they are placed downstream of the engine air filter and are well protected.

DEICON has developed an active feedback noise control scheme for air induction systems and demonstrated its effectiveness in a laboratory set up. A number of 2nd order filters programmed in a microcontroller were used to control the engine noise at multiples tones. The effectiveness of the actively controlled system matched or exceeded that of the traditional induction system with multiple passive acoustic absorbers. More

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Active Control of Gear Mesh Induced Vibration

Gearboxes are typically sources of vibration, either because they are in the transmission path from the engine to the structure so they transmit vibration coming from the engine or they transmit their own vibration at the gear mesh frequency and its corresponding side-bands. The latter can be heard as a whining noise coming from the gears. The level of whining noise can be exacerbated if the gear mesh frequency matches one of the natural frequencies of any of the components or subsystems in the transmission path. In vehicular applications, rear axle gear whine noise is caused mainly by the gear mesh vibration in the powertrain. This vibration is in turn transmitted thru the rear axle gear housing, the corresponding sub-frame, as well as other support structures to the vehicle cabin as an unpleasant tonal noise dubbed gear whine.

Active vibration control, using proof mass actuation of the rear sub-frame was used to mitigate gear mesh vibration in an all-wheel drive test vehicle exhibiting whine noise at around 450 Hz. Proof mass actuators (PMAs) generate force by pushing against a suspended mass and thus do not need an anchor point.

The effectiveness of active control in absorbing the shaker induced vibration of rear sub-frame of a test car was successfully demonstrated by examining the extent of reduction in the vibration of the rear sub-frame as well as the sound pressure inside the vehicle. Two electromagnetic proof mass actuators, mounted on the rear sub-frame of the vehicle, were used as the active elements. An accelerometer placed next to each actuator was used as the feedback sensor. Moreover, rolling dynamometer tests showed the effectiveness of active control in substantial reduction in vibration of the rear sub-frame and pressure inside the cabin caused by the rear differential gear mesh. More

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Paper on Floor Vibration Control in the The Structural Engineer

Wide column spans along with the use of high strength material make modern floor systems flexible and oscillatory. Walking (as well as other human activities) can induce high levels of vibration in such floors. When the traditional floor vibration control solutions, such as adding architectural features, mass, and/or stiffness to the floor are either not practical or ineffective, reactive damping provided by tuned mass dampers (TMDs) are used for quieting vibrating floors. High level of effectiveness, negligible weight penalty and ease of installation make TMDs a cost-effective and non-intrusive vibration control solution for both new and existing floors. In addition contrary to the damping that can be provided by non-structural elements such as partitions, raised floors and paneling which is not readily quantifiable and may not be an option for a space in which a light fitout is required, tuned mass dampers provide predictable damping and can easily be retrofitted. The installation of TMDs on existing floors is the least disruptive (to the occupants) of any floor vibration control solution <\p>

The Structural Engineer Vol94 article on floor vibration control

The paper titled “Vibration abatement of rectangular, trapezoidal and irregular-shaped joist-framed floors, using tuned mass dampers” on mitigating floor vibration has recently been published in “The Structural Engineer”, the flagship publication of The Institution of Structural Engineers. The subject of the paper is about one of the DEICON’s projects on mitigating floor vibration using tuned mass dampers. The citation for the paper is:
Kashani, R., 2015 “Vibration Abatement of Rectangular, Trapezoidal, and Irregular-Shaped J oist Framed Floors, Using Tuned Mass Dampers” The Institution of the Structural Engineers The Structural Engineer Journal, Volume 94, Issue 1, (2016).

 
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