The 'quietest' lab: Achieving vibration control and balanced design at NIST
Engineers, lab planners and scientists developed unique lab layouts with groundbreaking temperature and vibration controls
By Dan Hahn, HDR Architecture
In the world of nanoscience, as the size of the material for evaluation diminishes, the challenge to control ambient conditions increases. Therefore, you can imagine the tightly controlled environmental conditions required at the metrology laboratory at the National Institute of Standards and Technology (NIST), where the seismic fountain clock measures time accurate to one second over the life of the entire universe.
To meet these requirements, engineers, lab planners and scientists developed lab layouts with temperature and vibration control not found anywhere else.
The vibration criteria for NIST is stated as four vibration amplitude functions given in terms of frequency. Numbers are represented in a vibration curve, in descending order from an extremely quiet standard to that of typical construction.
Criterion Type A1: Velocity amplitude of 3 micrometers/sec or 125 microinches/sec at frequencies below 4 Hz; velocity amplitude of .75 micrometers/sec or 30 microinches/sec at frequencies between 4 Hz and 100 Hz. These criteria could only be achieved with engineered vibration isolation slabs.
Criterion Type A: Velocity amplitude of .025 micrometer or 1 microinch at frequencies between 1 and 20 Hz; velocity amplitude of 125 microinches/sec at frequencies between 20 Hz and 100 Hz. Site studies showed these limits were achievable at grade with slab-on-grade design and special vibration detailing.
Criterion Type B: Velocity amplitude of 6 micrometers/sec or 250 microinches/sec at frequencies between 1 Hz and 100 Hz.
Criterion Type C1: Velocity amplitude of 12.5 micrometers/sec or 500 microinches/sec at frequencies between 4 and 100 Hz. Criteria B and C1 were achievable in conventional building frame design with a consideration given to frame stiffness and mass.
At NIST, as with any lab, different portions of the building were programmed with different uses, each required different levels of vibration control. General-purpose labs and a cleanroom had more conventional requirements, while areas of the metrology lab required the A1 criterion mentioned above.
Ambient conditions produce vibration-these run the gamut from drinking fountains and vending machines to air compressors and air handling units. Paradoxically, while the stringent environmental demands of a state-of-the-art lab require proximity of mechanical systems to produce close-tolerance environmental control, these systems are vibration inducing. To meet the NIST metrology lab’s requirement of maintaining temperature to +/- .01˚C, the vast quantities of air-handling equipment needed special attention paid to control of vibration through design. More importantly, to achieve these incredibly precise vibration controls, designers placed the NIST metrology wings underground with no superstructure directly above it, only two to three feet of soil and open area. Because wind does no blow directly upon the shell of the structure, the dynamic forces that could excite the building shell are reduced or even eliminated.
To further reduce vibration, and to allow for the exceptional quantity of ductwork required, the metrology floor was moved to a greater depth, reducing the amplitude of surface waves that naturally decrease with depth. Collaborating with scientists, and based on the prototype studies, engineers also placed giant masses of concrete in pits on air springs under the lab floors. It was found that data measured on the prototype T-shaped concrete slab showed that A1 criterion can be met on the mass in the pit when isolated with air springs down to 1.5 Hz, the lower frequency limit of the measurements.
A walk-on floor isolated from the pit and the mass where experiments take place allows for the experiment to be isolated from the vibrations introduced by the researcher himself (see Fig. 1). Study of the prototype led to a separate support system to isolate each component of the lab floor, pit and mass.
Figure 1. Design of the isolation slab included a walk-on floor, allowing the experiment to be isolated from researcher-induced vibrations.
Certainly, the most stringent of vibration controls is achieved only through layer upon layer of isolation techniques. For example, the mechanical air handling equipment is isolated internally within the unit using mechanical isolation devices. Isolators are also located within ductwork and supports. Then, equipment is isolated externally. The lab floor is further isolated from surrounding structural elements by vibration isolation joints.
Additional vibration control was studied for potential future use. Active control systems that would read vibration input and adjust spring response to eliminate the vibration input before it travels to the supported mass was evaluated. Variation in the design of the mass was also evaluated for potential improved response.
These controls are moving researchers and engineers into the realm of the experimental, forging virgin ground that can only be determined by meticulous testing and trial and error.
A final word
Lab designers need to fit the vibration control needed to the type of experiments being conducted. It can be a costly mistake to “over design” a lab. The best and most cost-efficient lab is one that is designed “dead on”; one in which the planners fully identify and understand the needs-and constraints-of the researcher.
When construction is completed on the NIST labs, they will contain a range of about a dozen concrete slab masses of various weights and heights, some 100,000 pounds and 3 meters tall, and others 10,000 pounds and 1 meter tall. All meet A1 criterion but vary to accommodate the size of an experiment. Other spaces in the lab will simply provide the conventional slab-on-grade construction, fitting the design to the use. All the vibration features of the NIST AML add to the flexibility, reliability, and redundancy of the whole facility.
In the end, understanding vibration from the source to the experiment led to the right choices. The best-designed lab is simply one that performs as intended. And, in this case, the quietest lab is the one that makes the most impact. III
Dan Hahn, senior vice president with HDR in Omaha, Neb., is the lead structural engineer on the NIST project.