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:
Noticeable vibrations were present but not consistent.
No current or pending equipment malfunctions were identified.
Second floor coffee was delicious and strong.
No workers or aerobics classes were discovered on the third floor.
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!
Crane is a mechanical engineer and regional property manager with Childress Klein Properties, a leading real estate developer and property management services provider in the Southeast.