Subjective assessment of vibrations in lightweight floors

Abstract: Lightweight steel floors arise a lot of attentions because of its advantages, such as cost effective, recyclable, and having a short construction period. On the other hand, low frequency vibrations induced by normal human activities get more and more complains for these types of structures. This licentiate thesis studies human perception to floor vibrations, which are in the frequency range of 5 ~ 30 Hz in the vertical direction (foot to head), and human acceptance criteria. From the structural dynamic point of view, the human-floor system is a dynamic system and it can be treated as the following model, VIBRATION RESPONSE = DYNAMIC PROPERTIES × INPUT FORCE. Humans play two roles in this system: both the source and the sensor. Human walking, running and jumping exert forces on the floor system. At the same time, human bodies receive floor vibrations through the feet, the bottom, visual impression or sound. The dynamic properties of the floor system can be calculated theoretically or measured experimentally. The walking force is reviewed since walking induced vibrations cause most of the complaints, particularly for the single person walking case. Commonly used design criteria for solving floor vibrations problem concern static measures. New approaches take into account only the fundamental natural frequency of the floor system and categorize lightweight steel floors into high/low-frequency floors depending on its fundamental natural frequencies. Generally, for high-frequency floors, the deflection criterion is suggested while acceleration criterion is applicable for low-frequency floors. The reason why only the fundamental natural frequency is concerned is due to the fact that the walking force is dominated by the walking frequency and its first 3 harmonics. If the fundamental frequency of a lightweight floor coincides with the harmonics of the step frequency, an amplification effect will happen, which should be avoided. The most often cited reference ISO base curve (ISO 2631-2) represents magnitudes of approximately equal human response with respect to human annoyance and/or complaints about interference with activities in z-axis. Others provided different suggestions for human perception to vertical vibrations. A series of studies with human subjects were conducted, with the objectives to 1) test the absolute threshold value of single frequency vibration in the vertical direction, 2) masking effect of a base frequency in the vertical direction, 3) characterization of annoying dual frequency vibration. The method of adjustment was used for study one and two. 7 subjects participated in study one and two, 11 subjects took part in study three. In study one, 9 different single frequency vibration signals in the vertical direction were tested. In the second study, in the presence of the base frequency (8 Hz, 35mm/s2), the subjects adjusted the amplitude of the test frequencies until they just felt the difference. In study three, the annoyance of 26 vibration signals were tested. The absolute threshold values were found and complied with base curve for z-axis well. A main difference was a level difference, which might be due to different test procedures. This test provides a more realistic application than ISO 2631-2 considering the vertical vibrations in buildings. Study two revealed that the second frequency component always was important. Further annoyance studies showed that adding a frequency component to the vibration signals contributed to an increase of annoyance; the frequency component close to the base frequency was generally more annoying than other frequency components; the amplitude of the 2nd frequency component also contributed to annoyance, which is widely accepted. As for the application to lightweight floor serviceability, the studies provided strong arguments for the considering of the effect of the natural frequencies lying close to the fundamental one, which might be excited by high harmonics of walking forces.