Union Pacific Center Vibration Monitoring
Walter P Moore was commissioned by Union Pacific Railroad’s corporate headquarters in Omaha to determine the source of a troublesome intermittent vibration. After many years without vibration concerns, some employees on floor 12 of the building began to complain about vibrations. The events were occurring sporadically throughout the day, and made some of the employees nauseated, as well as rattled their computer monitors, thus making productivity difficult. We used high-sensitivity accelerometers and data acquisition systems to monitor the floor vibrations and almost immediately after testing began, a vibration event was recorded. This confirmed that the magnitude of vibrations was within the range of human perception, and given that the episodes sometimes lasted for over an hour, the vibrations were truly bothersome.
Evaluating all possible sources, we first performed sensitivity studies to assess the impact of major mechanical systems on floor vibrations, but none were driving the events observed. As testing extended into the nighttime hours, we determined the zone of high influence. On a hunch and relying on previous observations, we identified the likely source as an employee positioned in the center of the floor span who was vigorously wobbling his legs. While most of us have had a “nervous knee” at one point or another, this gentleman was bouncing both knees in an alternating pattern closely mimicking the 4-hertz natural frequency of the floor. Coupled with low damping from an open office concept, the bouncing was causing resonance. An unlikely culprit, indeed, but extremely distracting nonetheless.