Vibration in composite floors

Floor plates constructed from steel permanent shuttering and lightweight or conventional concrete are very popular. Problems of vibration in office floors are most often encountered in long-span steel floors with composite decks. Precast concrete floors are also vulnerable in long span configurations.

However, a by-product of this 'slimming down' is vibration within a floor. Vibration is generally not harmful, but can be very disconcerting to occupants. Generally, the psychological effects are worse than the physical problem and can affect a person's ability to work (see next page).

Workers such as traders, who rely upon small lines of text on visual display units, can suffer difficulties due to movement of the screens, so damping devices fitted to screen arms are often required (see Damping effects).

Vibrations are usually measured in terms of acceleration - sometimes expressed as a percentage of the normal acceleration due to gravity - 9.8m per second per second. Low vibrations levels can be unacceptable to persons at rest, while those engaged in aerobics are much more likely to be tolerant of higher levels (see Site analysis).

A person walking across a floor imposes a series of repeated or impulsive loads that build up a vibration response within the floor. 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 1.6Hz (1.6 complete cycles per second) (see Harmonics).

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 vibrations are most severe.

The physical response of a floor to imposed repetitive impact loads will depend upon a number of factors - span, deflection, stiffness, mass, etc. Floor coverings make little difference although some slight improvements can be derived from partitions, heavy furniture and so on.