By Julia Graham
Much of today’s high-tech lab equipment is extremely sensitive. This currently includes electron microscopes, magnetic resonance imaging, silicon wafer production, and opto-electronics, but the list continues to grow. As a result, eliminating vibrations has become increasingly important as the scale of investigation and manipulation by these devices has grown finer—think nanotechnology, where operations can only be viewed through a high-powered microscope and troublesome vibrations may be humanly imperceptible. For example, when a probe is being inserted into the nucleus of a single, living cell, it doesn’t take much of a disturbance to throw things off. A small vibration, imperceptible to you or me, can drive this type of equipment (and its operators) to distraction.
Trying to quantify this type of concern can be daunting for even an experienced engineer. Let’s face it—even when talking about structural frequency or stiffness, few people ordinarily think in terms of micro-inches per second or kips per in-sec. But at RWDI, this is our native tongue. As experts in designing structures and systems to control vibration and its effect on equipment as well as human comfort, this is the way we think.
It’s actually very straightforward
Vibration typically results from external and/or internal sources. External sources can be things like road and rail traffic, construction activity, and heavy industrial operations nearby, whereas internal sources may include walking, building services, and other lab equipment.
When a new installation is being considered, careful site selection offers a good way to head off vibration problems from external sources before they arise. Staying far away from road and rail traffic, construction, and heavy industry often is the best method for eliminating external vibration—and probably the least costly alternative. But when it is not possible to optimally site a new facility, or when newer, more vibration-sensitive equipment is going to be placed in an existing facility with external vibration sources nearby, various remedial measures are available. Here are a few examples.
- Vibration from roads is best controlled by keeping the roads in good repair and avoiding features that will cause impact loads, such as speed bumps and curbs, near vibration-sensitive spaces.
- Vibration from rail traffic can be controlled by using resilient materials below the rail ties or ballast and by keeping both tracks and trains in good repair.
- Barrier walls can be used to mitigate the effects of vibration from both rail and road traffic.
- The effects of vibration from construction activity can be controlled through long-term monitoring and alarm programs.
- Vibration from heavy industry can be controlled by placing vibration-causing equipment on properly designed isolated slabs, resilient material, or isolation springs.
These methods, however, can be costly and may not always be able to reduce the vibration to acceptable levels. Overall, striving to optimize the site selection is the best method for controlling these external sources of vibration. To assist with this optimization, a vibration survey is recommended as part of the site selection process.
The internal vibration sources can generate higher levels than external sources and can be addressed by appropriate structural design (e.g., floor structures with adequate stiffness and mass), proper location of sensitive uses within the lab, and isolation of building services and vibrating lab equipment.
Our work on the Louis A. Simpson and Kimberly K. Querrey Biomedical Research Center at Northwestern University is a prime example of how we help designers accommodate internal vibration sources, in this case footfalls.
Accounting for vibration in design
In order to determine the required performance for specific equipment, vibration criteria are typically used. Some, but not all, manufacturers provide specific sensitivity limits for their equipment. However, in some cases, the specific equipment to be used in a space has not yet been selected prior to required progress of the building design. In these cases, the use of generic vibration criteria can be helpful.
Concerns about vibration sometimes arise when an existing piece of equipment is slated for relocation, for example when moving to a new facility. We can avoid problems in the new location by taking measurements in the existing location (assuming the equipment is operational and functioning effectively) to use as criteria for the new location. This can be a preferred strategy, especially when manufacturer-specified criteria are unavailable or are suspected to be overly conservative.
In general, vibration-sensitive equipment is best located on a slab-on-grade foundation to limit transmission and amplification of vibration from building services and footfalls. Controlling vibration on locations above grade tends to be more difficult, although this is more the case in steel structures than in concrete. Certain types of specialized equipment (e.g., NMRs) should be located away from the exterior façade of a building to avoid the negative effects of high background noise and potential wind induced vibrations on lightweight building elements such as windows. The distance from mechanical and electrical equipment should also be considered when placing sensitive laboratory equipment.
Equipment that must be supported on the structure requires special consideration. Vibration generated by footfalls decreases with increased floor stiffness and higher natural frequency. Long span floor systems should be avoided, especially in lightweight steel/concrete composite construction. To optimize the floor system, the span lengths should be considered early in design in the context of the type of sensitive equipment that is being planned for use in the space. Spans longer than 35 feet are usually impractical for vibration sensitive applications. To limit vibration, some laboratories use separate structures to support floors and equipment mounts.
All building services require a high degree of vibration isolation. Major pieces of equipment (e.g., chillers, diesel generators, etc.) should be located in a separate utility building, or at grade level when possible. Stiff, massive support structures are preferred to enhance vibration isolation of mechanical equipment.
To learn more about our structural vibration control capabilities, please visit rwdi.com/expertise/all-services/structural-vibration.