Different Types of Shock and Vibration Mitigation Systems
A tuned-mass damper (TMD) is a specially-designed structural/mechanical element that can be incorporated into a new structure during the construction 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 structure. The TMD (or multiple TMDs) extracts vibration energy from the structural mode it is "tuned" to. The end result: the structure feels much more stiff than it actually is. Tuned mass dampers often provide a much more economical means for reducing vibration than, say, increasing the stiffness of the structure.
Designing tuned-mass dampers is a multi-step process:
A similar process is followed when designing TMDs for much larger bridges and buildings. The physics is identical in all cases.
Tuned mass dampers will reduce the vibration relative to the pre-installation levels. An example of the original vibration level and the vibration level measured following installation of the TMDs is shown in the figure. Two commonly-used reference levels are shown in the plots. The "Perception" level corresponds to the ISO-defined root-mean-square (RMS) vibration level where most people become aware of vibration. The "Office" level is commonly used as the maximum-desired vibration level for office environments and is defined to be 4 times higher than the Perception level. The Perception level is often used as the maximum-permissible vibration level for hospital operating and patient rooms.
In the example shown here, the original vibration levels significantly exceeded the desired maximum vibration level for an office environment. We performed a site vibration survey to measure the vibration and to determine the resonance frequencies of the floor system and designed four tuned mass dampers to address the 5.8-Hz floor vibration mode and to be placed on the floor slab (under a raised floor system). The vibration levels were measured once again following installation of the TMDs. The vibration levels near the TMD design frequency are 70% lower than the original levels - a very successful result.
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.