Appendix A Screening method
A1 Continuous and impulsive vibration
As a screening method, the overall unweighted rms acceleration could be assessed against the preferred values contained in Tables 2.2 and 2.4. This represents a conservative approach to demonstrating compliance or the need to use the more precise approach using appropriate frequency weightings.
Alternatively, overall unweighted vibration velocity could be used for assessment. For vibration predominantly in the frequency range 8 to 80 Hz, rms velocity can be used as a screening parameter, as this closely represents the weighting curve. The relevant assessment criteria for velocity are listed in Appendix C.
Any assessment which uses the screening method rather than comparison of the weighted values against the criteria should clearly state that the data are based on the screening method and present the measurement parameter used (such as rms velocity or rms acceleration). Sufficient justification should accompany whichever approach is used in an assessment.
A2 Intermittent vibration
The crest factor is the ratio between the peak level (without time response) and the rms value of a signal. As a screening method for intermittent vibration, the eVDV equation can be used for crest factors below 6. The eVDV is obtained as follows:
eVDV = k × a rms × t 0.25
where k is nominally 1.4 for crest factors below 6, a rms = weighted rms acceleration (m/s2) and t = total cumulative time (seconds) of the vibration events(s) or period(s) of vibration.
For crest factors above 6, the eVDV equation might no longer accurately represent the vibration dose, so the VDV equation should be used instead (see Section 2.4.1).
Where the vibration varies in magnitude over time in a more complex fashion, the rms acceleration value should be measured over representative time periods. Each individual time period should have a crest factor of less than 6. The eVDV can then be calculated with the above formula.
A vibratory roller is being used during construction of a car park near an office block. The vibration values on the office floor are constantly varying, and cannot be segregated into groups or periods of similar vibration value. The weighted acceleration value a was measured to be 0.032 m/s2 over a 3-hour period of roller work in the morning, and then 0.055 m/s2 over 2 hours in the afternoon.
The estimated vibration doses, eVDV, are given as follows.
- For the 3-hour morning exposure:
eVDV = 1.4 × a × t 0.25
= 1.4 × 0.032 × (3 × 60 × 60)0.25
= 0.46 m/s1.75
- For the 2-hour afternoon exposure:
eVDV = 1.4 × 0.055 × (2 × 60 × 60)0.25
= 0.71 m/s1.75
The total estimated vibration dose over the full day (5 hours' vibration) is:
eVDV = (0.464 + 0.714)0.25 = 0.74 m/s1.75
Reference to Table 2.4 indicates that this vibration dose clearly exceeds the preferred value of 0.4 m/s1.75. Hence, adverse effects are expected, and mitigation measures should be investigated to reduce the effects of vibration.
Page last updated: 12 June 2013