The most frequent cause of vibration complaints comes from people walking through a space where others are working nearby - a common occurrence in modern office spaces fitted out with many cubicles and few full-height partitions. To make matters worse, computer monitors are increasingly mounted on flexible swing-arm supports that accentuate the vibration. We understand this problem and have designed tuned mass dampers for floor systems that significantly reduce the vibration.
Floor System Vibration Characteristics
A floor system has certain characteristic frequencies where it "likes" to vibrate. These are the floor structure's resonance frequencies. Likewise, people tend to walk at a constant pace of about 1.6 to 2.2 steps/second, which corresponds to frequency of 1.6 Hz to 2.2 Hz. Very few floor systems have resonance frequencies this low, so no problem, right? Wrong. Each step a person takes is an abrupt application of a force to the floor structure. Abruptly-applied forces at a frequency of, say, 2 Hz also result in harmonics at 4 Hz (2 times 2 Hz), 6 Hz (3 times 2 Hz), and so on. The magnitude of the force in each harmonic diminishes with each harmonic multiplier, but walking produces forces at these higher frequencies. Therein lies the problem.
Floor systems have resonance frequencies between about 4 Hz (for a long-span non-composite steel floor system) up to about 8 Hz (for a poured-in-place reinforced concrete floor system). These numbers vary from structure to structure, but the key point is that the 2nd or 3rd walking harmonic produces a frequency in the range where typical floor systems resonate.
If the floor is sufficiently flexible, the resonant magnification will be sufficient to cause vertical accelerations that can be perceived as annoying. The more frequently this happens during the workday, the more annoying it is.
During a vibration measurement survey, we will measure the vibration of the floor system as people walk across the floor during their normal day-to-day activities. A typical measured acceleration response is shown in the figure. We will also identify the primary structural modes of vibration of the floor system. These measurements are invaluable for fine-tuning our structural dynamics model of the floor structure (see above).
We use our portable data acquisition and analysis system to analyze the measured time history acceleration data to identify the critical frequencies present in the response. The example below shows the evidence of the walking harmonics discussed above and that the 3rd harmonic coincides with the floor's resonance frequency. In this case - a lightweight, flexible floor system - the floor's response is affected by the 1st walking harmonic at 1.9 Hz and the resonance-magnified response at 5.8 Hz. The 1.9-Hz vibration is not "vibration" per se; it is caused by the inherent flexibility of the floor system rather than its dynamic response characteristics.
Raised floors are becoming more common as the paper-less office paradigm expands. There is an ever-increasing need to accommodate the numerous electrical and data cables to support computers on each desk and multiple monitors at each computer. Raised floor systems offer an effective means of routing cables out of view but also tend to exacerbate vibration problems on some floor systems. The raised floor panels can be screwed down to the support frame, but are frequently left loose to facilitate ease of access to the cables below. As people walk across the raise floor, the combination of the very thin carpet panels and loose metal floor panels often produce a "thump" "thump" with each step. These repetitive "thumps" translate into higher-magnitude walking harmonics that transfers more energy into resonant floor modes and, hence, worse vibration. We have designed tuned mass dampers specifically for raised floor systems than can sit on the slab within the clear space below the floor panels.