Floor Vibration

Floor vibration is the up-and-down movement of the floor typically caused when subjected to dynamic forces. These forces can be applied directly to the floor by human movement or by machinery. Common examples of vibrations caused by human movement include walking, dancing, jumping; examples of vibrations caused by machinery include washing and drying machines, air-conditioning systems, heating. During the last two decades, there has been an increasing concern on the movements caused by people in building floors. The use of lightweight concrete and high strength materials in modern buildings has led to a decrease in the natural frequency at which buildings vibrate, hence causing a serviceability issue (Setareh, 2002). It is known that the greater the flexibility of the floor, the greater their response of vibration when impacted. However, regardless of the type of construction, all floor systems are flexible; the problem arises when the vibration is of an intensity that annoys the occupant.

Vibrations in building floors typically have two sources; (1) 'external', and (2) 'internal'. ISO 10137 (2007) gives the following examples for external sources: construction activity, mining or quarry blasting; road and rail traffic; punching presses or other machinery in nearby buildings. The one with increasing concern and focus is the internal vibration sources. ISO 10137 (2007) states the following as examples for internal vibration sources: Human excitation; moving machinery (trolleys, overhead cranes, etc.); impact machinery (punches, presses, etc.); rotating machinery. All of these vibration sources have vibration limits. Vibration limits are expressed in terms of acceleration, as a percentage of acceleration due to gravity (g). This limit is dependent on what the individual is doing at the time of experiencing vibration. For example, people who are relaxed and not involved with much movement find acceleration of 0.5%g to be unacceptable, whereas those who are involved in various activities such as dancing or aerobics tend to accept accelerations as high as 10%g. Researchers in the past have conducted various experiments regarding human perceptibility to vibrations, which will further be discussed in the literature review section.

It has been proven and concluded by numerous scholars that floor damping and stiffness are two of the most important factors highly associated with the vibration of floors. Floors with low damping increase vibration, whereas floors with greater member depths increase stiffness. Damping of a floor is difficult to predict because the damping properties are not only dependent on the structure, but also dependent on non-structural components. These non-structural components include partitions, suspended ceilings, false floors (also known as raised access floor), and furniture such as cabinets and bookshelves, all of which affect the damping properties significantly. On the other hand, increasing the stiffness of the floor increases the natural frequency, thus moving resonance to a higher harmonic and as a result reducing floor vibration experienced by the occupant. However, in the case of solid concrete floors, which have high damping and stiffness, walking vibration is rarely a problem.

Accurate prediction of the vibration problem is not available due to every person having their own idea of personal comfort. Current methods of predicting vibration in floor systems range from hand calculations of a simplified model to complex finite element models. However, it shall be noted that each prediction method would, inevitably, yield different results due to different simplifying assumptions. Typically, human induced footfall loading is known to be the major source of vibration disturbances due to its frequent occurrence and, cannot be isolated in practice (Murray, 1975; Ohlsson, 1988; Hanagan & Murray, 1997). Murray (1998) characterised this as "probably the most persistent floor serviceability problem encountered by designers". The field of floor vibration is one which requires attention. The number of complaints received by building owners and occupants concerning floor vibration has remarkably increased; generating the second most frequent source of complaints, second only to roofs. It has been previously recommended by Ohlsson (1998) and Erikson (1994) that vibration of the floor system may be improved by including non-structural elements, as mentioned earlier.