Vibration in composite floors

A person walking across a floor imposes a series of repeated forces - periodic forces that build up a vibration response within the floor. Such vibrations are termed impulsive - they display a rapid build up to a peak followed by a slow decay. A series of events no more than 2 seconds apart can be considered impulsive and during normal walking foot contact with the floor can be expected to occur every 0.6 seconds, i.e. a frequency of 1.6 Hz (1.6 complete cycles per second).

During each cycle of loading, more energy is fed into the system and the magnitude of vibration grows until it reaches a maximum value - this is influenced by the amount of damping in the floor. When the 'forcing' cyclic frequency approaches that of the floor, resonance occurs. This is the point at which the vibrations are most severe. For impulsive vibrations, resonance occurs not only at the cyclic value but at multiples (harmonics) of it; for example, a person walking at 1.6Hz can produce resonance at 3.2Hz (2nd harmonic) and 4.8Hz (3rd harmonic). The higher the harmonic, the lower the magnitude of resonance. Since many floors have a natural frequency of 4-8Hz, problems are more likely to happen with the 3rd harmonic. By contrast, an aerobic class could produce resonance at 2.5Hz, 5Hz and the 3rd harmonic, 7.5Hz, making the 2nd and 3rd harmonics more critical.

To avoid vibration problems, the frequency of a floor should be greater than the highest frequency of the 'forcing' harmonic.