Vibration is going to happen. Period. Vibration cannot be eliminated, but it can be reduced. In the vast majority of cases, vibration levels are so low that no special effort is warranted. But let's face it, if you're reading this, you're probably concerned about vibration for one reason or another and you're looking for a solution or options to avoid a problem that requires a solution. The first step in successfully addressing vibration-related issues is to pick the right team to work with. After that, it's all downhill. Here are several strategies we'll consider for controlling vibration:
Vibration specifications are intended to help you avoid vibration problems in the first place. But consider this: equipment manufacturers write their specifications with their best interests in mind. Imagine that. They tend to impose strict limits on your structure to avoid any hint of an issue with their equipment. As a result, manufacturers’ specifications are frequently incomplete, overly conservative, and require intelligent interpretation to apply them properly for structural design or evaluation.
You need an experienced structural dynamicist to sort through these and other potential pitfalls on your behalf. A careful review of the specifications by a knowledgeable expert can significantly reduce your project cost. Poor specifications are common. Engineers who recognize that aren't.
Many potential vibration pitfalls can be avoided through a combination of effective space planning and structural design. In some cases, it is sufficient and appropriate to design the floor system to have a minimum resonance frequency. But here's the rub: analysis software available to engineers is only as accurate as the assumptions made by the engineers using the software. These assumptions require experience obtained from measuring the vibration response of existing structures.
We routinely acquire vibration data from existing structures, which we then use to refine our assumptions for future structural dynamics analyses of similar structures. Many medical and research facilities are designed today using tried, but not true, assumptions.
Examples of specially-designed vibration mitigation systems include fluid (or gas) viscous dampers and tuned-mass dampers that offer cost-effective solutions in certain cases. Both of these systems reduce the amplitude of the vibration by extracting energy from the motion. The fluid viscous dampers produce a resistive force based on the relative velocity between two points on a structure (e.g., lateral velocity between two floors of a building). These devices operate over a wide frequency range, but may require a relatively large stroke (relative displacement) to be effective.
The tuned-mass damper is effective when the relative displacement is small; however, beneficial damping is only available over a limited frequency range centered on the low-damped resonant mode for which the TMD is tuned. A more thorough discussion of tuned-mass damper design and operation is provided here.
The best approach for controlling vibration is to prevent the vibration in the first place. Stationary sources of vibration, such as air handling units and other fixed-location mechanical systems are usually mounted on springs (vibration isolators) to isolate the source from its support. The spring stiffness is selected based on the equipment weight to provide an isolation system resonance frequency lower than the dominant disturbance frequencies produced by the equipment. Vibration isolation is extremely effective and should be considered whenever practicable.