Vibrations in buildings

Vibrations are the oscillatory motions that can be experienced by a building, usually through its floors. Vibrations are regular cyclic motions of a given frequency and amplitude, typically being vertical vibrations, although horizontal vibrations are possible.

The length of a wave vibration is measured from the beginning of one point on a wave to the same point on the next wave and is known as the frequency. This is expressed as Hertz (Hz). The height of a wave vibration is measured from the centre line and is known as the amplitude. This is expressed in metres. The larger the amplitude, the slower the wave is as it moves from peak to trough (oscillation).

The consequences of building vibrations are determined by the source of the motion, its duration, and the building’s construction and layout. They may include:

• Presenting a nuisance to occupants.
• Disturbing sensitive equipment.
• Causing fixture and fittings damage
• Damaging structural integrity.
• The consequences will be .

Vibrations that affect buildings can be produced by a variety of sources and most are felt through the floor system. Vibrations can originate directly in the floor and then travel out from the source, or can be propagated through building members from other sorces that originate in the ground or outside the building.

Internal sources include:

• HVAC equipment.
• Lift and conveyance systems.
• Fluid pumping equipment.
• Human activity, e.g. walking, dancing, aerobic exercises, etc.

External sources include:

• Seismic activity.
• Road, rail, subway systems.
• Industrial activities.
• Construction activities, e.g. demolition works.
• Wind buffeting.

As even very low amplitudes of vibration can be perceived by occupants, building designers must consider how to avoid nuisance being caused through vibration. They should also consider the structural strength to ensure it is enough to resist the peak dynamic forces acting on it. Structural members and their connections must be designed to resist such forces, and connections must be designed to ensure that fatigue does not occur due to repeated cyclic loading.

Frequency and amplitude are taken into building designers’ calculations to determine where extra stiffness is required to dampen the vibration. Damping refers to the minimising of amplitude in a vibration or mechanical energy loss.

Anticipated usage of the floors is a critical consideration that should inform the concept design. Similarly, if a building is being constructed near a busy road or above a subway system, the foundations should be carefully detailed to accommodate the likely vibrations.

During the initial design of a building, the likelihood of perceptible vibration in sensitive spaces should be carefully assessed. This is particularly important for buildings that are designed with an unusual geometry or complicated structural systems. As the complexity of a structure increases, the impact of vibration becomes more difficult to accurately predict and effectively minimise.

It is important that building designers properly consider levels of acceptable vibration at the concept design stage as it can be very difficult, and costly, to modify an existing floor to reduce its susceptibility to vibration. Remediation may require major changes to the mass, stiffness or damping of the floor system.

Vibrations are sometimes dealt with most effectively at source. For instance, machinery-induced vibrations can be minimised by using isolating mounts or motion-arresting pads. An inexpensive approach is to increase the floor loading within the building. Weight can be placed under a raised floor to minimise movement from foot traffic for instance, and dissipate vibration.

NB A vibration dose value is defined in BS6472-1(2008) as a relationship that yields a consistent assessment of vibration which correlates well with subjective response. It is used to estimate the probability of adverse comment which might be expected from human beings experiencing vibration in buildings. Consideration is given to the time of day and use made of occupied space in buildings, whether residential, office or workshop.

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