A tuned-mass damper (TMD) is a specially-designed structural/mechanical element that can be incorporated into a new structure during the design phase or added to an existing structure to reduce the vibration of the structure. TMDs are most effective where the structure's motion is caused by one or more resonant modes of the original structure. In essence, the TMD extracts vibration energy (i.e., adds damping) to the structural mode it is "tuned" to. The end result: the structure feels much more stiff than it actually is.
A TMD has three basic elements: mass, stiffness, and energy dissipation (damping), so there are three forms of tuning required in the design of a TMD. The stiffness and mass of the TMD are selected to provide a TMD resonance frequency very close to the structure's resonance frequency. The TMD damping level is selected to optimize the energy dissipation over the effective bandwidth of the TMD. The TMD mass is selected to provide the desired level of vibration mitigation. It is reasonable to expect the TMDs to provide about a 70% reduction in the vibration. If any one of the parameters (mass, stiffness, or damping) is changed, the other two must be adjusted to maintain the tuning (performance).
Designing a tuned-mass damper is a multi-step process. First, structural dynamics analyses of the structure are performed to determine the TMD characteristics (mass, stiffness, and damping) required to achieve the desired level of vibration mitigation. The next step is to convert these properties into a physical form appropriate to the site (e.g., size, shape, location). A prototype of the TMD is manufactured and then tested to verify or fine-tune the design. We have worked with local fabricators and machine shops (Johnston Products and Halsey Manufacturing) to covert our designs into physical parts. Finally, the TMDs are manufactured and delivered to the jobsite for installation. We perform a final check and verify performance after installation is complete. Please take a look at the PDF file for more details.
Tuned-mass dampers can be designed to satisfy virtually any criteria. In many cases, the architect will prefer to have the TMDs concealed within the architectural envelope of the structure. The image sequence below illustrates how TMDs can be attached to the primary structure, but hidden within the architectural finish of a flexible walkway. No one will know the TMDs are there. Without the TMDs, this walkway would be highly susceptible to walking-induced vibration that could be disorientating. The TMDs silently "absorb" the vibration making the walkway feel more stiff than it actually is...the TMDs make this walkway functional.
The animations below illustrate how a tuned-mass damper moves in relation to the primary structure. Both cases represent vibration caused by someone (not shown) walking on the walkway or floor. The motion, in this case, is magnified so that it can be seen, but the relative motion of the structure and the TMD is realistic.
In this case, a suspended walkway is susceptible to lateral side-to-side walking-induced vibration. The image on the left-hand side of the animation represents the walkway's motion without the aid of a tuned-mass damper. The image on the right-hand side includes a TMD located at midspan and concealed within the architectural envelope. The TMD mass is the square block that moves side-to-side.
Vertical vibration of floor systems, particularly those with steel beams and a composite deck, is very common. In this animation, the TMD is placed at midspan of the beam and is designed so as to not interfere with the suspended ceiling below. The downward deflection of the beams caused by each step is visible in both images; however, the post-step vibration is dramatically reduced by the tuned-mass damper.