Vibration in composite floors

Floors prone to vibration are best considered prior to fitting-out works. A detailed analysis will identify areas most at risk so that the design and location of corridors or walkways can be configured to avoid these.

Over the last few years the construction of multi-storey office buildings has almost become standardised, with the use of profiled steel metal decking and concrete slabs spanning between steel beams the accepted norm. Such construction is highly engineered, designed to reduce self-weight, economical and capable of delivering large uninterrupted spans.

Coupled with the development of composite floors is the realisation that vibrations in the floor can create unfavourable psychological responses from the occupants. Vibrations are simply up and down motions of a floor caused by the application of forces - either the activities of the occupants or arising from external influences such as traffic, underground trains, etc. Vibration of this nature is not indicative of structural failure and may be within accepted building limits but to a lay person regular vibration can be disturbing and can affect their ability to work.

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. Vibrations as low as 50mm/s2 can be unacceptable to persons at rest, while those engaged in aerobics are much more likely to be tolerant of vibrations of 1,000mm/s2or more. Generally, it is not so much the person causing the vibration as the person who may be watching or sitting near by who will notice the effects more.

Vibration that occurs for more than 16 hours during the day or an uninterrupted period of 8 hours during the night can be very disturbing at a low threshold - this is termed continuous vibration. These forms of vibration are usually due to a physical factor - perhaps a worn bearing on a heating pump or a failure in the anti-vibration mounts in a plant installation. This type of vibration is not discussed here.

All buildings are subject to external vibration influences, for example heavy road traffic, adjoining demolition work, underground trains, etc. Most people will be familiar with the slightly bouncy feel of a domestic timber floor but will not be troubled by it. However, the vibrations that occur within office floors of composite construction can be created by something far less dramatic - the occupants themselves. Often vibrations can be traced back to the simple action of a person or group of persons walking purposefully across a floor or undertaking normal activities such as dropping a box of copy paper onto a floor.

The problem of vibration in composite floors should not be compared with the vibrations that can be created in dance floors, grandstands, etc., where large numbers of people congregate and where they may jump together in tune to a regular musical beat. If the resonant response of the floor approaches or mirrors that of the natural frequency of the floor, loads can be magnified significantly and can lead to dangerous structural conditions such as fatigue (similar to bending a wire back and forth until it breaks). In these conditions, vibrations in excess of 2,000mm/s2are likely. The change of use of a building to a fitness studio or gym may need to take into account the effect of a number of people jumping or running in unison, for example during an aerobics class.

Human perception to vibration is a complex subject. Very low levels of vibration are usually identified quickly, so perception is fairly acute. However, changes in the magnitude of vibration are more difficult to identify and will never be identical between any 2 individuals.

While the implications in structural terms of vibration may be insignificant, an office floor that is perceived to be 'lively' could engender feelings of insecurity even at low frequencies. At higher frequencies of vibration, human reaction is generally linked to associated noise levels rather than the physical vibration itself.