Chapter 11Floor Vibrations on Healthcare Facilities: A Case Studyon a Surgical Microscope
Omer F. Tigli
Abstract The paper describes a vibration issue experienced on a surgical microscope and presents the results of a seriesof dynamic testing to resolve the issue. The microscope is located in an operating room (OR) on the fourth floor of a 10-story, steel framed, inpatient hospital building constructed in 2011. The fifth floor of the building is a mechanical space.On multiple occasions, neurosurgical cases have been disrupted due to the vibrations of the microscope. The microscopemanufacturer did not provide any specific vibration limits. Generic vibration criteria available in the literature are providedfor floors—with no reference to the eye vibrations of a microscope—and limit neurosurgery floor vibrations to 1,000 micro-in/s (mips) in RMS. An independent vibration criterion for the eye (3,500 mips) has been developed over the series of testsby comparing the subjective perception/tolerance of the OR personnel against the measured data. The largest vibration levelsmeasured at the eye reached 10,000 mips at 21 Hz and coincided with the motor speed of a condenser water pump operatingat 1,260 RPM on the fifth floor. Comparisons of floor and eye vibrations in the OR indicate that microscope amplifies thefloor vibrations three to four times at 21 Hz.
Recent advances in building materials and technologies allow more slender, lighter and economical structural systems forbuildings. Two main design requirements of buildings are (1) to safely carry the extreme loads that they may be exposedto during their life-time (strength criteria) and (2) to be serviceable to their users in their daily activities (serviceabilitycriteria). Modern buildings with slender structural systems can be shown to meet the strength and deflection criteria (thetraditional measure of serviceability checks), thanks to strict code requirements, expertise in the engineering community andwell-established, accurate analysis tools. However, buildings may fail to meet the second aspect of the serviceability checks:Vibration criteria.
Vibration criterion of a building floor defines the vibration levels that are tolerable by the occupants of that floor. Forinstance, vibration tolerance of people dining in a restaurant is different than the vibration tolerance of people workingout in a gym. However, the strength/deflection checks required for restaurant and gym floors are the same. Similarly,strength/deflection criteria for a paper office or an electronic office are the same. However, their vibration criteria are differentto the point that they may warrant different structural designs [1–3].
Buildings designed only for traditional strength/deflection criteria may have vibration serviceability problems. Unfortu-nately, if we go back and study the design documents of buildings with vibration problems, we rarely encounter calculationsthat reference vibrations. Although it is beyond the scope of this paper to study reasons behind this, in author’s humbleopinion, one contributing factor is a lack of vibration course/training in the engineering curriculum typically provided forstructural engineers.
This paper presents a case study involving vibration complaints on a surgical microscope used for neurosurgical cases onthe fourth floor of a 10-story, steel framed, inpatient hospital building. The fifth floor of the building is a mechanical space.
O.F. Tigli (�)McNamara/Salvia Inc. Consulting Engineers, 160 Federal Street, 5th Floor, Boston, MA 02110, USAe-mail: [email protected]
On multiple occasions, neurosurgical cases have been disrupted due to the vibrations of the microscope. About six vibrationcomplaints were reported within a year and each time disruptive vibrations lasted about 3 h after which the vibrations reducedto more tolerable levels.
From a source-path-receiver perspective, the receivers of the objectionable vibrations are the microscope and/or theoperating room (OR) personnel who are using the microscope. The source and the path of the vibrations are unknownand constitute the main objective of this study.
With the objective of finding the source of the vibrations, all nearby vibration sources that may be responsible for theobjectionable vibrations were considered first, including human walking, internal electronics of the microscope, the intensityof the air flow inside the OR, other medical equipment used in the building, and mechanical equipment located on the fifthfloor including four chillers, two air compressors and 18 pumps. The relatively long duration of the disruptive vibrationevents eliminated the sources that are transient in nature, such as human walking.
Next, a series of vibration tests were conducted in coordination with the continued use of the OR and the buildingengineering department. Ideally, the best approach would be to take vibration measurements while a vibration event isactually happening. However, the infrequent nature of the reported vibration events and pressing need to address this issueimmediately by the hospital administrative diverted us from this approach. Instead, each test consisted of taking accelerationmeasurements at the tip of the microscope eye and on the floor while a potential vibration source was turned on and off.The main challenges in this approach were high number of potential vibration sources and the inability of shutting off everysource except the one being tested.
After performing four vibration tests, a new vibration event was reported and we were able to take measurements duringthis event. After this last test, the magnitudes of the objectionable vibrations as well as corresponding frequencies weredetermined. This information was used to narrow down potential vibration sources and a final comprehensive vibrationtesting was conducted with a reduced set of vibration sources. It was found that when one of the pumps operated at certainspeed, the vibration levels measured during the vibration event were replicated.
The rest of the paper is outlined as follows. Section 11.2 gives the description of the microscope and the structuralfloor. Section 11.3 summarizes the vibration criteria selected for the microscope. Section 11.4 describes the vibration testsperformed and presents the results. Section 11.5 provides a critical review of the findings.
11.2 Description of the Microscope and the Structural Floor
The microscope is a portable Carl Zeiss OPMI Pentero surgical microscope (Fig. 11.1). The microscope has an articulatingarm of about 170 cm (�50-700) long. As expected, largest accelerations are observed at the tip of the arm where the eye islocated. The microscope is assigned to the operating room located on the fourth floor of the hospital building. Figure 11.2shows the plan view of the operating room. In a regular day, the microscope is first located at the calibration location (CL),and then it is moved next to the operation bed, operation location (OL), to be used during operations. The microscope islocated at CL in Fig. 11.1.
The fourth floor structure is framed with structural steel supporting a composite 4 ¼00 lightweight concrete slab on 300metal deck (7 ¼00 total thickness). The bay sizes are about 300-000 � 410-600 (9.1 m � 12.6 m), with columns occasionallystaggered in plan. The third floor and second floor are partially hung from the fourth floor framing via hanging columns atmid-bay. This unique structural feature increases the mass of the fourth floor.
The fifth floor is the main mechanical room for the Building. Potential sources of vibrations identified on the fifth floorare Chiller #3 (Ch3) located directly above the OR, Chillers #1 (Ch1) and #2 (Ch2) located to the west of Ch3, the Primary(PCHWP-1-4) and Secondary Chilled Water Pumps (SCHWP-1-4) located to the North of Ch3, Condensing Water Pumps(CWP-1-4) located to the south of the OR, the compressor pumps located in the northwest corner of the mechanical room.
11.3 Vibration Criteria
The microscope manufacturer did not provide any specific vibration criteria other than stating that the floor should meet the“neurosurgery” requirements. Generic vibration criteria of healthcare facilities are available in the literature [4, 5] where themaximum allowable vibration velocity of neurosurgery floors is limited to 1,000 mips.
The recommended vibration levels are defined on the floor, not on the tip of the microscope, which may be a more relevantlocation for assessing vibration performance of microscopes. Different microscopes may amplify floor vibrations differently
11 Floor Vibrations on Healthcare Facilities: A Case Study on a Surgical Microscope 93
Fig. 11.1 Photo of the surgicalmicroscope during a vibrationevent
Fig. 11.2 Architectural planview of the operating room
and a single floor vibration criterion may not control the actual vibration levels experienced by the user. The author failed tofind any published vibration criteria defined at the microscope eye.
In order to establish a vibration criteria for the eye, we resorted to subjective evaluations of OR personnel. Through thevibration tests performed at six different occasions, we observed that OR personnel did not mind scope tip vibrations up tothe range of 3,000–4,000 mips levels. Therefore, a scope tip vibration criterion of 3,500 mips is proposed in this paper.
94 O.F. Tigli
11.4 Dynamic Testing
Vertical accelerations on the floor and on the microscope tip were measured using PCB Piezotronics Model 393B05accelerometers at a sampling rate of 2,048 Hz. Digital data acquisition was performed using a National Instrument USB-9234DAQ card. In the post-processing, measured accelerations were first segmented into 5-s long blocks with 50 % overlappingallowed. Each data set was low-pass filtered with a cut-off frequency of 50 Hz, and then resampled at a sampling rate of100 Hz. The power spectrum of each data block was calculated with a peak conversion set to root-mean-square (RMS).Then, the final power spectrum of each measurement was obtained by averaging the power spectra corresponding to eachdata block.
11.4.1 Test #1
The main objective of the first test was to investigate whether Ch3 had anything to do with the objectionable microscopevibrations because Ch3 is located directly above the OR. It was found that the vibration levels on the floor and at the scopetip did not change significantly when Ch3 was on or off (Fig. 11.3). Therefore, we conclude that Ch3 was not the source ofthe reported vibration issue.
11.4.2 Test #2
Our next objective was to investigate the following potential sources of vibrations: Ch1, (Ch2 was out of service), Instrumentcompressor and Medical Vac. First, we took a baseline measurement when all the equipment mentioned above was running.Then, we took successive measurements when only one of the equipment was turned off. Figure 11.4 presents floor and scopetip vibrations for various combinations of mechanical equipment. None of the selected mechanical equipment combinationssignificantly changed the floor or scope tip vibrations. Therefore, we conclude that none of the mechanical equipment testedis responsible for the vibration issue.
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Fig. 11.3 Vertical floor (a), and scope tip (b) vibrations when Chiller #3 (Ch3) was on and off (Test #1)
11 Floor Vibrations on Healthcare Facilities: A Case Study on a Surgical Microscope 95
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Fig. 11.4 Vertical floor (a) and scope (b) vibrations when Chiller #1 (Ch1), Inst. Comp. and Med. Vac were on/off (Test #2)
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Fig. 11.5 Vertical floor (a) and scope tip (b) vibrations when the scope was ON and OFF (Test #3)
11.4.3 Test #3
In this test, the microscope was turned on and off and the background vertical accelerations on the floor and on the microscopewere recorded. As Fig. 11.5 indicates, no significant change in the vibration levels was observed, so the scope itself (i.e., it’sCPU) was not the source for the objectionable scope vibrations.
11.4.4 Test #4
Next, the effect of air pressure/flow within the OR was investigated for three conditions: When the air flow in the OR wasturned off completely, when it was maximized and when it was minimized. The data indicate that there were no appreciable
96 O.F. Tigli
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Fig. 11.6 Vertical floor and scope vibrations when the air flow in the OR is OFF, maximized and minimized (Test #4)
differences in the measurements for all three cases (Fig. 11.6), so the air flow was not responsible for the objectionable scopevibrations.
Surprisingly, the vibrations on the floor and at the scope tip exceeded their respective vibration criteria (1,000 and3,500 mips) for the first time since our first testing (see the peak at �21 Hz with a magnitude of �2,250 mips on the floor and�8,500 mips at the scope tip). This was the case regardless of the air flow condition in the OR. This unexpected result could beattributed to an unknown source that became active during Test #4 but was inactive other times. The same source might havebeen responsible for the scope vibrations reported previously. Note that measured scope tip vibrations were three to four timeslarger than what was measured on the floor (Fig. 11.6). This indicates that the microscope has a resonant frequency at 21 Hz.
11.4.5 Test #5
This test was performed due to an urgent call informing us about an ongoing vibration event in the OR during a neurosurgicalcase. The author volunteered to go into the OR and took measurements on the floor and the scope tip, when the scope waslocated at CL (Fig. 11.1). About a 10-min long data was collected, which was broken into three pieces in the post-processingstep for the purposes that will be described below. RMS averaged power spectra of these three data sets measured on the floorand at the scope tip are presented in Fig. 11.7. The largest scope vibrations were observed in Set 1 as 10,185 mips at 21 Hz,which reduced to 4,980 mips in Set 2 and then to 250 mips in Set 3. Similarly, the largest floor vibrations were observed inSet 1 as 3,030 mips at 21 Hz, which reduced to 1,480 mips in Set 2 and to 75 mips in Set 3. This indicates that the vibrationsource that was responsible for the reported complaint either stopped or changed its frequency a few minutes after we startedour recording. Because measured vibrations both on the floor and at the scope exceeded their respective vibration criteria, thecomplaints and the selected vibration criteria were confirmed. It is also interesting to note that excessive vibrations measuredduring Tests #4 and #5 occurred at the same frequency of 21 Hz.
11.4.6 Test #6
The next task was to track the vibrations’ source. The most valuable information identified about the source was its frequency.The unknown vibration source must be operating and generating vibrations at 21 Hz. After studying datasheets for variousmechanical equipment located on the fifth floor, we found that all of the pumps and chillers were equipped with variable
11 Floor Vibrations on Healthcare Facilities: A Case Study on a Surgical Microscope 97
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Fig. 11.7 Vibration spectra on the floor and at the scope tip for three data sets (Test #5)
Table 11.1 Engineeringproperties of the pumps located atthe fifth floor
Pump tags Motor HP Motor RPM Weight (lbs)
CWP-1,2,3,4 150 1,780 3; 611
PCHWP-1,2,3,4 100 1,780 2; 241
SCHWP-1,2 100 1,780 2; 965
SCHWP-3,4 50 1,780 1; 567
HWP-1A,1B 30 1,750 725
HWP-2A,2B 40 1,750 1; 075
HRC-1,2 1:5 1,750 175
frequency drives (VFDs), designed to change the pumps motor speed automatically in order to optimize the efficiency of thewhole system. We found that when operated at 100 %, all pump motors generate vibrations at around 30 Hz. When VFD isset to 70 %, motor speeds are adjusted to around 1,290 RPM, which generates vibrations at �21 Hz. Table 11.1 lists all 18pump motors located on the fifth floor and some of their engineering parameters.
Test #6 was designed so that floor and scope vibrations were recorded continuously while each motor pump was setto operate at 70 % (21 Hz) and then at 90 % (27 Hz) in a predetermined order. The objective was to find the mechanicalequipment, which when operating at 21 Hz, would reproduce the scope vibrations that were measured during Test #5. Welocated the microscope at OL during Test #6, because it will always be located at this position during surgical cases.
Figure 11.8 presents the measured vertical vibration levels on the floor and at the tip of the microscope when condenserwater pump 4 (CWP4) operates at 70 % (21 Hz) and then at 90 % (27 Hz). Both floor and scope vibrations exceed thevibration criteria when CWP4 operates at 21 Hz. Measured vibration levels on the floor and on the scope tip never exceedthe vibration criteria when other pumps listed in Table 11.1 operate at 21 Hz and 27 Hz. These measurements are skippedin this paper due to space limitations, except measurements associated with condenser water pump 3(CWP3) are presentedin Fig. 11.9 for comparison purposes. CWP3 is identical to CWP4 and located next to CWP4. However, measured floor andscope vibrations in the OR during CWP3 operating at 21 and 27 Hz are all within the vibration criteria.
Because the main objective of Test #6 was to find the mechanical piece that would replicate the vibration levels measuredduring Test #5, it is worth comparing Figs. 11.5 and 11.8. Note that the magnitudes of the vibrations at 21 Hz measured onthe scope tip (10,185 mips on Test #5 and 6,075 mips at Test #6) are quite different. This difference may be explained by themicroscope being located at two different locations in the OR (at CL during Test #5, at OL during Test #6, see Fig. 11.2).
To investigate the effect of scope location (OR vs CL) on the vibration amplitudes within the OR, we placed an additionalaccelerometer on the floor at CL during Test #6 when CWP4 was operating at 70 %. Also, we calculated the transmissibilityfunctions relating the response measurements taken on the floor to those on the scope tip from the data collected during Test#5 (the scope located at CL). Therefore, we were able to estimate the scope tip vibrations from the transmissibility functionsand the measurements taken on the floor at CL during Test #6.
98 O.F. Tigli
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Fig. 11.8 Vertical vibration spectra on the floor (a) and on the scope (b) when CWP-4 operates at 70 and 90 % (Test #6)
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Fig. 11.9 Vertical vibration spectra on the floor (a) and on the scope (b) when CWP-3 operates at 70 and 90 % (Test #6)
The estimated scope vibrations from Test #6 were compared against the measurements taken during Test #5 in Fig. 11.10.The magnitudes of vibrations at 21 Hz were found to be 10,185 and 10,235 mips in Test #5 and #6 respectively.This confirmed that the scope tip vibration amplitudes were a function of where the scope was located within the OR.Furthermore, because the vibration levels measured during Test #5 were replicated only when CWP4 operating at 70 %, thesource of the objectionable vibrations was confirmed.
Based on these findings, we recommended the hospital engineering team to avoid using the pump until its isolationsystem, motor-pump alignments and other internal operations can be investigated for deficiencies. Alternatively, it shouldbe programmed to operate at “safe” speeds that should be determined in coordination with additional vibration testing.The hospital engineering team had the pump checked for any mechanical deficiencies and reprogrammed its motor to skipfrequencies between 20 and 22 Hz and employed a wait-and-see strategy. No vibration complaints have been reported by theOR personnel since then.
11 Floor Vibrations on Healthcare Facilities: A Case Study on a Surgical Microscope 99
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Fig. 11.10 Comparison of theestimated (Test #6) and measured(Test #5) vertical vibrationspectra at the microscope eye
11.5 Conclusions
A case study involving objectionable vibrations on a surgical microscope was presented. After extensive testing, it was foundthat the source of the disruptive vibrations was a condenser water pump located on the floor above the OR. When the pumpoperated at 21 Hz, it generated floor vibrations that travelled through the building columns and other structural elements tothe floor below. The microscope amplified the floor vibrations further, indicating that the frequency of the floor vibrationscoincided with one of the natural frequencies of the microscope. Re-programing the pump so that it never operates at theproblematic speeds was our suggested and successfully implemented solution.
A vibration criterion of 1,000 mips on the micro-surgery floors was acceptable for the case presented. However, becausedifferent microscopes may amplify floor vibrations differently, a more direct vibration criterion may be established right onthe eye of the scope. Based on the subjective comments of the OR personnel and the limited data collected in this study, itis suggested that a vibration level of 3,500 mips at the microscope eye may be an appropriate vibration criterion for surgicalmicroscopes.
Acknowledgments Mr. John Tracy, Associate at McNamara/Salvia Inc. is gratefully acknowledged for his assistance for the scheduling andperforming of the dynamic tests.
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