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AURACLE ACOUSTICS  -  Vibration

Vibration

The central reference document in the UK for the assessment of environmental vibration is BS6472: 1992: 'Evaluation of Human Exposure to Vibration in Buildings (1 Hz to 80 Hz)' which deals with the assessment of continuous, intermittent and impulsive vibration, and vibration from blasting.  It is commonly applied to vibration from sources such as railways and demolition & construction activities. This current edition of the document replaced the 1984 edition, but is itself currently under review.

As its title indicates, the document is concerned manily with the human response to vibration, and does not comment on the potential for damage to be caused to buildings and other structures. However, it does differentiate between four categories of buildings or spaces which are identified as: critical working areas (eg hospital operating theatres and precision laboratories); residential properties; offices; and workshops.  Certain precision activities may be adversely affected by levels of vibration which are below those which cause disturbance to humans.

A key feature of BS6472 is the concept of the Vibration Dose Value (VDV) .

VDV is calculated by taking the fourth root of the integral of the fourth power of acceleration after it has been frequency-weighted.  The frequency-weighted acceleration is measured in m/s2 and the time period over which the VDV is measured is in seconds.  This yields VDVs in m/s1.75.

An estimated VDV (eVDV) can be calculated approximately for a given time period, for example a 16-hour day, using the expression: eVDV =  1.4 x a(rms) x t0.25, where a(rms) is the the rms acceleration in m/s2 and t is the total duration of vibration exposure occurring the total time period in seconds.  However, BS6472 notes that this procedure will underestimate the true VDV where the crest factor of the vibration exceeds a value of about 6.

Table 7 of the document sets out a set of VDV assessment criteria, predicting the likelihood of adverse which may be expected for various levels of VDV for the daytime and night-time periods. These criteria are frequently adopted by local planning authorities in the setting of standards and targets in respect of vibration from sources such as railways and construction activities.

While levels of vibration corresponding to the assessment the criteria set out in Table 7 of BS6472 may be sufficient to cause disturbance to humans, they would generally be well below the levels of vibration which have the potential to damage buildings and other structures.

Paragraph 4.1 of the document states:  "Comments regarding building vibration in residential situations are likely to arise from occupants of buildings when the vibration levels are only slightly in excess of thresholds of perception."  Levels of vibration which are sufficient to cause damage to buildings and other structures are typically well above the thresholds of himan perception.

Auracle Acoustics is able to provide advice on the interpretation and implementation of BS6472: 1992, and is able to assist in the achievement of its VDV targets where appropriate.

 

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