Minimizing vibration in sensitive laboratory areas
There is a “whole lotta shakin' goin' on” in laboratories across America. Not from earthquakes or tsunamis but from a source much more common and insidious: people going about their daily routines.
Too often, a laboratory technician views a distorted specimen through a 400 × microscope. The optical precision balance waves slightly rather than giving a steady and accurate measurement. The culprit, of course, is a structural system that ineffectively deals with the vibrations caused in virtually every building. Even very slight movements, imperceptible to a person sitting still, can disrupt sensitive laboratory equipment.
Many medical laboratory facilities are designed as shell spaces to house future research programs that obtain grant funding well downstream of the building design process. To provide adequate functional flexibility, laboratory facilities are usually designed in modules. Improperly designed, these modules can exacerbate vibration problems. The good news is that proper architectural planning and structural design can provide floor framing that is capable of mitigating vibration.
Floor vibration criteria for equipment used in the production of integrated circuits.
If the structural engineer is proactive and invited to assist in the early stages of design, before the module is established, he or she can exert a powerful influence on the floor system's vibration characteristics. With thoughtful design, the engineer can create a floor system that properly damps vibrations for sensitive equipment in an affordable and functionally flexible structure. This article presents a straightforward overview of laboratory floor vibrations and suggests some strategies to improve the behavior of most laboratory building structures.
What's Causing the Shakes?
Many administrators and technicians assume that roadway traffic and mechanical systems are the primary sources of floor vibrations. Although these sources can certainly cause problems, the primary source of nuisance vibrations in sensitive laboratory equipment is generally as close as the opposite side of the work partition—colleagues simply walking down the corridor to and from their offices, going about their daily routine. Each step creates a forcing function, and the walking pace applies the force at a particular rhythm or frequency.
Each floor structure has a natural frequency that is a function of its span, stiffness, and mass. Typical floor systems fall in the frequency range of 3 to 8 hertz (Hz). If the forcing-function frequency caused by walking is close to the natural frequency of the floor framing, nuisance vibration will likely occur. While this vibration is typically not perceptible on a physical level, the ramifications to sensitive laboratory equipment can be significant.
How Shaky Is Too Shaky?
There are no code-specified criteria for the design of floor framing to control vibrations. As a result, structural engineers must use their experience and judgment in developing appropriate vibration performance criteria and damping measures. Laboratory planners and architects must assist by defining the future usage and types of equipment anticipated. One common strategy is to segregate laboratory floors into zones defined by the types of expected research and corresponding equipment. To facilitate this process, the Institute of Environmental Sciences and Technology developed the “BBN Criteria.” The table list groups of sensitive equipment with their corresponding vibration criteria. Vibrations are generally measured in microinches per second, commonly called “mips.” Figure 1 illustrates these groups as Velocity Criteria (VC) curves relative to the normal human perception threshold.
For example, a typical office building can generally accommodate vibrational velocities of 4,000 mips or higher without designers needing to pay attention to vibration control. Higher velocities, however, will generally cause significant and objectionable floor vibrations. The VC-A curve (see table) is most commonly applicable for general laboratory facilities. It prescribes a maximum allowable vibrational velocity of 2,000 mips, which is adequate for most typical bench microscopes up to 400 × magnification. With thoughtful structural design, VC-A is readily and economically achievable on elevated floors.
Comparative reinforced concrete beam depths for 85 and 100 paces per minute (ppm) at a 41′ 0″ beam span.
Designing to the requirements of curves VC-B (1,000 mips), VC-C (500 mips), and VC-D (250 mips) (see table) requires more extreme structural measures. These measures often include deeper beams, smaller bays, and additional floor mass, which increase structural costs. It generally makes more economic sense for equipment within the VC-B to VC-E zones to be housed on slabs, on grade.
When providing an analysis for vibrations induced by occupants, the structural engineer should typically consider at least three walking speeds. Speeds of 50, 75, and 100 paces per minute (ppm) are commonly used. Historically, 100 ppm is the most accurate and stringent measure, although the need to control construction costs often pressures owners and their design teams to reduce this criterion to 85 ppm. While lower walking speeds of 50 to 75 ppm are sometimes used, research suggests that floors designed on the basis of these lower speeds may not perform as well as expected and should not be used unless walking speeds can be controlled. Breaking long corridors into shorter segments can sometimes reduce the design walking speed.
Secondary vibration sources such as mechanical rooms and truck docks should be located as far away from sensitive equipment zones as possible. Rotating or oscillating equipment should be located adjacent to columns, away from beam midspans to best control vibration from this source. Heavy oscillating equipment such as laundry machines should generally be located on slabs, on grade.
If stringent vibration criteria (less than VC-A of 2,000 mips) are anticipated, the owner should hire a vibration consultant early in the project design. An expert in the evaluation of laboratory equipment, the vibration consultant will meet with the user groups to identify specific equipment types, needs, and locations. Conferring with the equipment manufacturers, the vibration consultant will specify the VC and maximum vibrational velocity applicable to each equipment type.
Concurrently, the structural engineer should be exploring several feasible structural floor systems for both vibration performance and economy. The vibration consultant can help evaluate each system to determine the lowest (minimum) vibrational velocity (e.g., 2,000 mips) that the proposed floor system can accomodate. This equates to the most sensitive piece of equipment that can be placed on this framing. Looking at more than one system ensures that there will be optimal choices to best meet vibration requirements and provides several price options. Ultimately, this approach will generally yield a cost-effective structural system that best serves the laboratory's use.
The structural engineer should also team with a vibration consultant during the reprogramming of existing laboratory facilities. The consultant will conduct field tests to record actual vibrations from the building while in use (e.g., from occupants walking, mechanical equipment, laundry equipment, cage-washing equipment, etc.). The structural engineer can then use this input where required to evaluate floor-stiffening solutions and support specific vibration-sensitive equipment. Similar to the process followed with new buildings, the vibration consultant will evaluate the floor system and assign the proper minimum vibrational velocity (e.g., 2,000 mips). In many cases, several iterations are required between the structural engineer and the vibration consultant to ensure that the existing floor framing is damped appropriately.
Our firm has developed a computer floor vibration model to determine minimum vibrational velocity (in mips) for checking preliminary structural schemes. While this should not be used as a substitute for a vibration expert, it does allow for rapid and more accurate structural pricing in the preliminary design stages.
Starting With the Grid
To accommodate modular lab units, laboratory facilities are typically designed with column grids spaced in a multiple of two or three modules. This commonly results in column spacings of 22′ 0″ or 33′ 0″ in one direction, with perpendicular grid spacings in the range of 30′ 0″ to 35′ 0″.
Laboratory structures can be appropriately supported by either structural steel frames or reinforced concrete beam and slab systems. Both systems require careful vibration analysis. Concrete framing systems, however, must be framed very differently from traditional office building layouts to effectively and economically control vibration. Detailed vibration studies by our firm have shown the following:
For column spacings in the range indicated above, reinforced concrete structural systems with total structural depths of about 25" are well suited to accommodate VC-A vibrational velocity (i.e., 2,000 mips) at 85 ppm. These systems can be readily formed with standard formwork, resulting in maximum economy.
If the column grid is larger or the walking criterion exceeds 85 ppm, dramatically deeper concrete floor systems are required for comparable vibration performance. These deeper systems usually require significantly more expensive custom formwork solutions. Figure 2 shows the vibrational velocity for various concrete beam depths accommodating a span of 41' for both 85 and 100 ppm. A 40"-deep beam is required to meet the VC-A vibrational velocity for the 100 ppm criteria. Conversely, a 30"-deep beam can meet the same vibrational velocity for 85 ppm.
A successful laboratory design begins with an experienced design team that understands the end user's needs. Vibration sources have been widely misunderstood and, therefore, are often not properly mitigated in the design process. With proper understanding and planning, vibration in laboratory facilities can be reduced significantly and, in many cases, virtually eliminated. HD
Muhammad A. Cheema, PE, is a structural engineer and Principal with Walter P. Moore Engineers + Consultants. Houston office and can be reached at firstname.lastname@example.org.