Portland, Oregon NOISE-CON 2011

2011 July 25-27 Vibration Sensitive Facilities Near Rail Lines Considerations and Mitigation Ramon E. Nugent, P.E. a) Acentech Incorporated 250 N. Westlake Boulevard, Suite 150 Westlake Village, CA 91362 Jeffrey A. Zapfe, Ph.D. b) Acentech Incorporated 33 Moulton Street Cambridge, Massachusetts, 02138 Certain issues arise when vibration sensitive facilities and equipment are located near rail lines. Vibration is often one of the greatest challenges, particularly as equipment becomes more and more sensitive. This paper will address the following considerations, 1) site evaluation and prediction of the vibrations in the completed building, 2) mitigation options that can be incorporated during design, 3) high performance isolation system design, and 4) manufacturer’s criteria. Case studies will be used to illustrate key concepts. 1 INTRODUCTION Often equipment used in research and production of opto-electronic and nano-electronic systems, as well as instrumentation used in state-of-the-art research in the physical and biological sciences, cannot function effectively if it is subject to vibrations that exceed small threshold values. However, facilities are often located near transportations systems that produce high levels of ground-borne vibration. The facility developer thus is presented with the challenge of devising designs in which the vibration disturbances are within acceptable limits. Using three case studies, this paper will address the vibration considerations when vibration sensitive equipment or processes are located near rail lines. 2 VIBRATION CRITERIA

Unfortunately, early in the design process the building owner often does not know what specific equipment will be housed in the building. To help the design process, in the 1980’s Eric Ungar at Bolt Beranek and Newman developed a set of curves to help define classes of vibration sensitive equipment1. The generic vibration criteria (VC) curves are still widely used today to a) Email: rnugent@acentech.com b) Email: jzapfe@acentech.com

design vibration sensitive facilities. The VC curves are currently referenced in the noise and vibration assessment guidelines published by the Federal Transit Administration and the Federal Rail Administration2, 3. Figure 1 shows the common family of VC curves. The criteria are frequency dependent and extend from 4 cycles per second to 80 cycles per second.

The vibration criteria are given in terms of 1/3 octave frequency bands. The criterion level is defined by the flat section of the curve. All of the curves tip up at low frequency because, despite what most researchers believe, most instruments are relatively insensitive to low-frequency vibration. This occurs because most instruments are affected by relative motion between components (a mirror and a laser for example). At low frequencies, well away from internal resonances, most instruments behave like rigid bodies and there is little relative motion between components (all of the components move together).

VC-A is appropriate for general labs where people use bench-top microscopes and balances. We also use this criterion for vivariums, although vibration effects on animals are not well understood. For reference, the human perception threshold ranges from 4,000 to 8,000 micro-in/sec; so, in many cases, the concern is vibrations that cannot be felt by individuals. Most manufactures of sensitive equipment have very detailed and specific vibration criteria. Vibration criteria are not always expressed in terms of vibration velocity like the VC curves. Acceleration is common for magnetic resonance imagers (MRI’s), and displacement is common for electron microscopes. Often the instruments are more sensitive to vibration at one frequency than another and the criteria will reflect those differences. Another consideration is the averaging process; peak hold for example is a worst case measure of the vibrations over the sample period. Peak-hold is appropriate for instruments that can be affected by even short term disturbances. 3 CASE STUDIES The following case studies will illustrate:

1) site evaluation and prediction of the vibrations in the completed building, 2) mitigation options that can be incorporated during design, 3) high performance isolation system design, and 4) manufacturer’s criteria

3.1 Case Study 1 – Cambridge, MA MRI The photo in Figure 2 sums up the challenges of locating a vibration sensitive facility in a busy urban environment. This was a new building in Cambridge MA that was located near a busy street, a subway line and a spur rail line that carried one or two trains per day. Our client wanted to locate a research building here that would contain, among other things, a vibration sensitive MRI suite on the ground floor. Our client wanted to know what vibrations could be expected in the building when it was completed.

The design process for the new building began with a site measurement to quantify the levels of vibration at the site where the MRI unit would be installed. The vibrations were measured on the surface at the setback distance where the building was to be built, and at depth in boreholes. (If the building is going to have a basement it is desirable to also measure the vibrations in boreholes at the planned depth of the foundation. If possible, an even better measure is the vibration in the basement of an existing building, provided the foundation type and distance from the rail line are similar.)

Figure 3 is a photo of our case study from Cambridge. This is picture shows the principal vibration source, a CSX diesel locomotive pulling freight cars. In this area of the rail line the train speed was about 15 mph and the track condition was marginal, about what you might expect for a lightly used spur line. The pipe in the foreground next to the orange cone is a borehole. The blue box is our radar gun for measuring train speed.

We set up a line of surface vibration sensors at 13 ft, 52 ft and 118 ft from the tracks as depicted in Figure 4. At the time of the measurements the foundation design had not been decided so we also wanted to know what the vibration levels were at depth - in case they decided to incorporate a basement into the design. Three boreholes were drilled next to the surface sensors, the first two were about 20 ft deep, and the third borehole went all the way down to bedrock at about 118 ft. The first 20 ft or so was glacial till. This was followed by a clay layer that went all the way down to the bedrock. As it turned out, the depth of the bedrock varied from 65 to 135 ft deep.

Because the operations were on a rail spur, the train schedule at the site was very unpredictable. We set up a digital tape recorder to record train events over a period of 12 days to be sure that we captured multiple events. Over that time we measured the vibrations from about 36 trains or about 3 per day. Figure 5 presents the vibration spectra that were measured on the surface 52 ft from the tracks. As indicated by the range levels represented by the black circles, there was considerable variation on the train vibration levels, especially at low frequencies. The researchers indicated that even short duration events could disturb their experiments so we used an upper-bound spectrum to characterize the vibrations at the site. The maximum levels were on the order of 10,000 micro-inches per second, which is high for a general lab space (2000 micro-in/sec is typical for a general lab).

The curves in Figure 6 show the upper-bound spectra measured on the surface and in the boreholes at 52 ft and 115 ft from the tracks. The surface spectra (solid lines) have the behavior we would expect, with very little attenuation with distance at low frequency. At the 52 ft sensor position, the vibration levels in the 23ft deep borehole were not dramatically different from the surface. The vibrations on bedrock at 118 ft deep, on the other hand, were considerably lower (about 20 dB). This occurred at low frequencies, which is good because low frequency vibrations are the most difficult to isolate.

The next step was to predict the vibrations in the new building. Factors considered were soil/foundation coupling loss and structural amplification/attenuation effects. Generally vibrations decrease as you go up in a building, although this is not always the case – particularly if you excite the vertical accordion mode of the building. Once you have predicted the vibrations inside, you can compare them to the instrument criteria and decide if mitigation is needed.

The vibration measurements provided valuable information. These were some of the things that we took away from our test: First, because the levels at 23 ft deep were not that much lower than the surface, there were no compelling vibration reason to choose a basement over a mat foundation. While the bedrock performance was impressive, there was a real concern how you take advantage of this. If the building is essentially supported on stilts, will soil contact from 118 ft on up degrade the advantage of the bedrock support?

These levels were a concern to our client because they exceeded the criterion for the MRI system that they were planning to procure. The question was whether they could deal with the disruption 2-3 times per day.

After review of our report our client opted to proceed with the following design. They used a mat foundation supported on piles to bedrock and pile caps. They also designed a bathtub in the MRI suite that could be used to house an isolation system, if it was needed. The plan was to re-measure in the bathtub once it was completed and to either design the isolation system if it

was needed, or fill it with concrete if it wasn’t. They ended up filling it with concrete because the MRI system that was ultimately selected had an isolation system that was sufficient to protect the MRI.

Figure 7 is a photo of the building during construction. It illustrates some other design issues, namely airborne noise due to the train - in particular the horn blasts as the train approaches the grade crossing in the foreground. The MRI suite is in the right side of the building and the subway runs under the street you see in the foreground.

One question of interest was how did the vibrations in the completed building compare to the ones we made on the green field site? Figure 8 shows what we found in the MRI room. This chart shows the vibration spectra that were measured in the completed bathtub. The MRI was located about 85 ft from the tracks, and was essentially at grade. We didn’t measure at exactly 85 ft before construction, so we extrapolated the pre-construction spectra from 52 ft and 115 ft.

The results are very interesting in that the vibrations in the completed building were not that different from the pre-construction measurements made on the surface. In this case, they were about ½ as severe. In fact they were comparable to what was measured on the clay layer before construction. Another interesting thing is that, driving the piles to bedrock did not have a dramatic effect on the vibration in the building. We attribute this to the soil contact all along the pile and under the mat foundation.

3.1.1 Case Study 2 – Long Island, NY This is another before and after example. This is a sensitive facility on Long Island in New York. Here the building was about 585 ft from a commuter rail line, the Long Island Railroad as shown in Figure 9. The sensitive areas were located on a grade-supported slab at ground level. Again vibration measurements were made before and after the building was constructed. Figure 10 summarizes the measured vibration spectra at this location. In both cases the train-induced vibrations were about 10 times higher than the ambient vibrations (no trains). But, the vibrations in the completed building were only about ½ as severe as the pre-construction levels. So the presence of the building didn’t affect the vibrations significantly. Another thing that was nice was the ambient vibrations in the completed building were actually a little lower than they were before, which means we did a good job of controlling vibrations from the building mechanical systems. 3.1.2 Case Study 3 – NMR Mitigation In the third and last case study we examine what happens if mitigation is required. This case involves a nuclear magnetic resonance (NMR) facility where the NMRs were to be located in the basement of a research building about 350 ft from a freight rail line in North Carolina. Figure 11 shows the relative position of the facility to the rail system. Vibrations from freight trains exceeded the instrument’s criterion and we were asked to help design a mitigation system for two of the NMRs.

In designing supplemental vibration mitigation one must always be careful because most pieces of sensitive equipment have their own internal isolation systems. Stability issues can arise if one soft spring is simply stacked on top of another soft spring. Stability effects can be mitigated by using a large intermediate mass between the two isolation systems; this is commonly referred to as an inertia block. The inertia block serves to dynamically separate the two isolation systems and you can effectively treat them as performing independently of each

other. As a rule of thumb, the inertia block should be 10 times the mass of the isolated payload (the isolated portion of the sensitive equipment).

Inertia blocks are often recessed into bathtub spaces in the floor and are difficult to see once they are installed. We had a recent project where this wasn’t the case. This project required a very large block (80,000 lb), and it had a large open area under the floor where the block could be easily seen. Figures 12 and 13 show photos of this inertia block system from below and above the floor, respectively. The block is “T-shaped” to keep the vertical center of gravity at the top of the isolators – this helps reduce vertical-horizontal coupling (horizontal motion produces rocking). The eight isolators are air-springs with a natural frequency of about 1.5 Hz.

Figure 15 shows how the systems in North Carolina performed. The red line shows the vibration on the floor next to the block which clearly exceeded the NMR’s criterion. The black line shows the vibration on the block which was well within the criterion limits. There is a little amplification at the resonance of the isolator, which is normal. This is usually not a problem because there isn’t a lot of energy at low frequency.

4 CONCLUSIONS Vibration sensitive facilities can be built near operating rail lines if proper design considerations are implemented early in the design. These considerations include field vibration measurements to determine levels of vibration at the surface and below grade as appropriate, determination of the sensitivity of the equipment, allowance for implementation of mitigation designs, and post-construction vibration measurements to verify that the criteria have been met. 5 REFERENCES 1. E. E. Ungar, D.H. Sturz, and C. H. Amick, “Vibration Control Design of High Technology

Facilities”, Sound and Vibration, July 1990. 2. U.S. Department of Transportation, Federal Transit Administration, “Transit Noise and

Vibration Impact Assessment,” FTA-VA-90-1003-06, May, 2006. 3. U.S. Department of Transportation, Federal Railroad Administration, “High-Speed Ground

Transportation Noise and Vibration Impact Assessment,” October 2005.

Fig. 1 - Generic Vibration Criteria

Fig. 2 - Case 1 Problem Overview

VC-D (250)

VC-C (500)

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Electron Microscopes (> 30,000x), NMR, Mass Spectrometers, Cell Implant Equipment, Microelectronics 1/2 micron line width

Unisolated Laser and Optical Research Systems, Microelectronics 1/4 micron line width

Fig. 3 - Case 1 Measurement Site

Fig. 4 - Case 1 Measurement Layout

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Fig. 6 - Case 1 Surface and Borehole Spectra

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Fig. 7 - Case 1 Partialy Completed MRI Building

Fig. 8 - Case 1 Pre- and Post-Construction Vibration Levels

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Fig. 9 - Case 2 Promblem Overview

Fig. 10 - Case 2 Vibration Spectra

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Fig. 11 - Case 3 Problem Overview

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Fig. 13 - Case 3 Top of Inertia Block

Fig. 14 - Case 3 Inertia Block Performance

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