Isolation tables and platforms are very effective for controlling incoming support motions at the base of the equipment, but they are not a universal solution. Generally, they can only be used on the equipment for which they are designed for and can be susceptible to resonances from motions of the supporting floor. They are best suited to slab-on-grade supports or sufficiently stiff structural floors that have vibration frequencies well separated from the isolation frequency. They should not be used as a catch-all for vibration control, and mitigation measures that target the source of vibration (i.e. source isolation, location of sensitive spaces away from corridors, service spaces, and environmental sources) or the vibration path (i.e. the structural system) should be part of the solution.
Supplemental dampers—TMDs and AMDs
Supplemental dampers are devices that oppose the motion of a floor that has been excited by an externally applied force (e.g. footfalls and rhythmic activity, ground-borne vibrations). If designed and implemented properly, they achieve three goals: maintain structural motion levels below targeted criteria, optimize remedial structural measures and the number of low vibration zones, and reduce the cost of adaptive reuse due to fewer and/or smaller structural elements.
Tuned mass dampers (TMDs) are an example of passive vibration control using devices that do not require any power and are generally maintenance free for the life of the building. As the name implies, TMDs are composed of a mass supported by a spring and viscous damper that can be tuned to the specific vibration frequency of the floor. Traditionally, TMDs have been used to control perceptible and excessive motions from wind loading and crowd movement, but recent research by the authors has highlighted the viability of TMDs for use in low-vibration environments, such as laboratories.
This viability was demonstrated by successful implementation of a TMD solution during the design of a new multi-story laboratory building. The performance objective was vibration Class A in all labs. A total of 166 TMDs were installed throughout the building, with 100 percent of the labs achieving Class A, and approximately half the labs achieving the more stringent Class B. Equally important to the performance was the cost savings on steel framing: approximately $5 million USD.
Active mass dampers (AMDs) are a recently developed active vibration control device that addresses some of the shortcomings with TMDs. TMDs control a single mode of vibration of the floor, must be spatially located at the point of maximum deflection, are typically heavy (upward of 1,814 kg [4,000 lb]), and require structural connections to the floor. Whereas AMDs can control motions over a wide frequency range (thus addressing multiple modes of floor vibration), they can be located away from peak deflection points, and are very light (66 kg [147 lb]).
