Union Pacific Center Vibration Monitoring

After many years without vibration concerns, some employees who worked on the 12th floor of the Union Pacific Center headquarters building began to complain about vibrations. The events were occurring sporadically throughout the day and made some of the employees nauseated. They also noticed rattling computer monitors, which made productivity difficult. 

To determine the cause of the vibrations, Walter P Moore used high-sensitivity accelerometers and data acquisition systems to monitor the floor vibrations. Almost immediately after testing began, a vibration event was recorded. This confirmed that the magnitude of vibrations was within the range of human perception. Given that the episodes sometimes lasted for over an hour, the diagnostic testing also confirmed that vibrations were truly bothersome and affected the employees’ work.

More testing needed to be done to find the source of the vibrations. First, sensitivity studies were used to assess the impact of major mechanical systems on floor vibrations, but none of these 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 12th floor 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 that closely mimicked 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.