A Natural (FM) Frequency
A strange phenomenon proves that facility management
isn't always talkin' about good vibrations.
By Jeff Crane, P.E.
Have
you ever encountered a facilities phenomenon that
began as a mystery and ended as a valuable educational
experience? My team recently responded to a work order
reporting an annoying second floor vibration. With
this type of report, most facility managers would
logically expect to find one of the following circumstances:
a. Since slight floor vibrations in multi-story,
concrete, and steel buildings are common, a new employee
on the second floor probably needs an orientation
in FM101.
b. A piece of rotating equipment, most likely an
HVAC fan, has a balancing problem or is losing a belt
or pulley.
c. Decaf coffee on the second floor was accidentally
brewed in all carafes and caffeine withdrawal headaches
are causing illusionary vibrations.
d. Unauthorized work (hammering, drilling, etc.)
or an aerobics class is being conducted on the third
floor.
Since these black and white diagnoses are the most
common reasons for floor vibration, they can be quickly
checked out and resolved. But what if there isn't
a black and white answer? What about a hazy shade
of gray? What if two, three, or even four people have
different opinions about perceived vibrations and
can't decide whether or not they're normal?
Back to our story. Our maintenance technician conducted
an initial investigation and reported:
Since
we couldn't close the work order, a more comprehensive
investigation was initiated. The chief engineers worked
with our suppliers and carefully inspected elevators,
compressors, kitchen equipment, exhaust systems, outside
air fans, and anything else in the building that rotated
or moved. Several of us made multiple visits at various
times of the day (including early morning and late
evening), hoping to recognize a pattern or consistency
that would help us identify and eradicate the annoying
vibration. We were stumped. We needed outside assistance.
After a final round of unsuccessful experimentation
with the HVAC fans' variable speed drives (ramping
them up manually in an attempt to identify a specific
speed where a vibration became pronounced), we contacted
a vibration testing firm. We talked to the operations
manager and got a crash course on vibrations, floor
stiffness, excitation forces, and natural frequency.
I began having terrible flashbacks to a differential
equations course (that I had to take twice) and a
semester studying machine design that included several
weeks of vibration analysis. As I recalled why I had
decided not to pursue a career building bridges like
the one over Tacoma Narrows (you didn't expect a vibration
story without that reference, did you?), we discussed
the fundamentals of structural vibrations focusing
on mass, stiffness, and natural frequency.
Mass and floor stiffness are pretty self-explanatory-modular
furniture, people, and file cabinets are all associated
with mass while the steel/concrete design and component
assembly result in the stiffness of a structure. Perhaps
the best way to understand natural frequency is to
understand that structures are always in motion or
vibrating, even though we can't always feel it. That
motion/vibration is known as a structure's natural
frequency.
My dusty old textbook-Mechanical Vibrations, Second
Edition (Singiresu S. Rao, 1996, p. 37)-offers the
following assistance: "If a system, after an
initial disturbance, is left to vibrate on its own,
the frequency with which it oscillates without external
forces is known as its natural frequency."
In our situation, detailed testing confirmed that
this specific area of the second floor had a natural
frequency that was unique when compared to other parts
of the building and even other areas on the second
floor. The natural frequency of our studied area was
in a range (in units of cycles per second or Hz) that
was understandably annoying to the occupants. When
people walked through this area, the vibration amplitude
(think about amplitude like the height of a wave)
increased and became even more noticeable.
With testing completed, we contacted the building's
original structural engineer for advice. As expected,
we had another vibration lesson regarding mass, floor
stiffness, and natural frequency. Instead of trying
to manipulate the mass related to the structure (reconfiguring
workspaces, adding or removing mass, etc.), we agreed
with the engineer's suggestion to modify the stiffness
of the floor. Since adding a vertical column in the
middle of the first floor to stiffen the second floor
would not be practical, he recommended welding steel
bracing between the horizontal steel support beams
under the second floor. This would increase the floor's
stiffness and change the area's natural frequency
to something more acceptable.
We met with a steel fabricator, surveyed the area
above the first floor ceiling tile, and scheduled
weekend work to avoid business disruption. It's now
Saturday night. The bracing is done and the cleanup
is finished. Over the next several days, we'll determine
if sufficient improvement has been achieved or if
we need to consider additional bracing.
Although more time consuming and challenging than
typical work orders, out of the ordinary projects
can spice up our jobs and develop our mental toolboxes.
Solving the more difficult facility riddles can also
improve our confidence, priming us for the next mysterious
work order!
Feedback about this article? E-mail
your comments to bkraemer@groupc.com.
