4 Vibration measurement and prediction
4.1 Units of measurement
The criteria presented in Section 2 are expressed as (or in the case of Table 2.4, based on) rms acceleration. Overall weighted rms acceleration values are used to assess compliance with the criteria. Building vibration may also be measured in rms velocity or peak velocity. Appendix C contains equivalent criteria presented in these terms. Sufficient justification should accompany whichever approach is used in an assessment.
4.2 Location and direction of vibration measurement
As far as is practical, vibration measurements and assessment criteria should refer to the place at which the vibration affects people, which, for this guideline, is inside buildings.
Depending on whether occupants are standing, sitting or lying down, vibration may enter the body in the x-axis, y-axis or z-axis. People are more sensitive to z-axis vibration than to x- and y-axis vibration. However, human exposure to vibration should usually be measured in all three axes, so that the results can be combined and compared to the criteria.
Restrictions on access to buildings often make it necessary for measurements to be made outdoors or at a point other than where the vibration effects are experienced. In such cases, an appropriate transfer function should be used to convert levels at the measurement point to those likely to occur at the assessment point inside the building. As far as is practical, testing should be carried out, or the results of existing studies should be used, to determine the appropriate transfer function. As a screening method, if criteria are met in the ground outside they may also be met inside, and therefore no further work would normally be required in this case.
However, special care should be taken in situations where there may be resonance effects in floors and walls, as vibration values inside buildings may be higher than in the ground immediately outside. Although there is wide variation from structure to structure, in these situations vertical vibration could be at least twice as high. The multiplying factors for horizontal vibration are often lower.
In multi-storey buildings, ground vibration values from external sources tend to diminish with increasing height. The rate of decrease per floor is highly dependent on such factors as distance to the vibration source, the type of building structure and the plan area of each level of the building.
Care also needs to be taken when people are sitting or lying on soft furnishings. High-frequency floor vibration can be reduced by the furnishings, but situations can occur where low-frequency vibration (usually below 10 Hz) is amplified by resonance effects caused by the combined effects of body weight and support stiffness.
4.3 Measurement instrumentation and techniques
A single instrumentation system is unlikely to meet all the requirements of frequency and dynamic range under the wide range of situations for which this guideline applies. In general, a vibration measuring system usually includes the following instrumentation:
- transducers, typically piezoelectric accelerometers or geophones
- signal-conditioning equipment
- a data recording and analysis system.
Performance characteristics for the measurement instrumentation should meet the requirements set out in BS 6841 and BS 7482 Parts 1 and 3.
A dynamic range of 40 dB is adequate for most purposes, but 50 dB is preferred. The signalto- noise ratio (with respect to the background vibration) should generally be not less than 5 dB.
If the signal-to-noise ratio is between 10 dB and 5 dB, the measured value should be corrected (i.e. diminished), and the correction method should be reported. Background vibration, in relation to vibration measurement, is defined as the sum of all the signals not due to the phenomenon under investigation.
4.4 Mounting of vibration transducers on buildings
The mounting of vibration transducers to vibrating elements or substrate should generally comply with Australian Standard AS 2775-2004 Mechanical vibration and shock-Mechanical mounting of accelerometers. The aim should be to reproduce faithfully the motion of the element or substrate without introducing additional response. Care should be taken with triaxial assemblies to avoid rocking or bending.
The mass of the transducer and monitoring unit (if any) should not be greater than 10 per cent of that of the building element to which they are fixed. Mountings should be as stiff and as light as possible. Care should be taken with some velocity transducers (geophones), which are often heavier than accelerometers.
Avoid brackets. It is better to fix three uniaxial transducers to three faces of a metal cube rigidly mounted by means of studs or quick-setting, high-modulus resin. The transducer mounting can be secured to the structure of the building by expansion bolts or rigid-type adhesives.
In special circumstances, it is acceptable to glue the transducer or attach it using magnetic attraction. Do not use double-sided foam tape under any circumstances.
Measurements on floors having soft coverings such as carpet or soft vinyl tiles tend to give distorted results and should be avoided. Where it is not possible to relocate the transducers, make comparative measurements with different mass and coupling conditions for the mounting block to evaluate the effect of the soft coverings.
More information can be obtained from Scannell (1995).
4.5 Mounting of vibration transducers on the ground
The transducer should be firmly mounted so that the ground vibration is accurately measured. The mounting method that is required is related to the magnitude of vibration as well as the frequency. Refer to Australian Standard AS 2775-2004 Mechanical vibration and shock-Mechanical mounting of accelerometers. Incorrect coupling of the transducer to the ground may lead to erroneous recordings.
When the measurement surface consists of rock, asphalt or concrete, the transducer should be fastened to the measurement surface with a bolt or with epoxy or other quick-setting, rigid cement.
Where soil conditions permit, the transducer may be fixed to a stiff steel or aluminium 'star-stake' driven through a loose surface layer. This stake should not project more than a few millimetres above ground surface. Take care to ensure close contact between the transducer star-stake and the ground.
Where transducers have to be mounted in softer ground, to minimise coupling distortion they may be buried (and well tamped) to a depth at least three times the main dimension of the transducer or mounting unit. Alternatively, they can be fixed to a rigid surface plate (e.g. a well-bedded paving slab).
The transducer should be located at a sufficient distance from any structure (including large trees) so as to avoid undue interference from vibration 'feeding back' from the structure. The required distance away will be specific to site and structure and would need to be determined for each case. The transducer should be orientated as recommended by the manufacturer in the direction of the vibration source.
Procedures currently used for predicting groundborne vibration are based on a combination of measurement and the use of formulas derived from actual experience. Examples of such assessment procedures are included in documents such as the US Federal Transit Administration's Transit noise and vibration impact assessment (1995) and the Transport Research Laboratory's Groundborne vibration caused by mechanised construction works (Hiller & Crabb, 2000). It is important that any method or procedure used to predict vibration be clearly described and validated before use (e.g. via test measurements and calculations, published studies, comparison with existing databases etc). All assumptions used in the prediction should be clearly stated, and the expected accuracy should be quoted along with the final predicted values.
Where new projects are proposed, predictions should be conducted for two scenarios: proposed start date and 10 years from the date of commencement. Where mitigation measures are being proposed, predictions should be conducted for both the before and after mitigation situations, and a mitigation strategy should be prepared and based on the worst case from both the date of commencement and 10 years hence.
4.7 Information provided in a vibration assessment report
A vibration assessment report should include at least the following information:
- project description
- relevant guideline or policy that has been applied
- details of any background measurements that have been undertaken
- details of instruments and methodology used for measurements (including reasons for settings and descriptors used and calibration details)
- a site map showing location of vibration sources, measurement locations and receivers (where appropriate)
- vibration criteria applied to the project
- vibration predictions for the proposed activity
- a comparison of predictions against vibration criteria
- a discussion of proposed mitigation measures, the vibration reduction likely and the feasibility and reasonableness of these measures
- how compliance can be practically determined.
Page last updated: 12 June 2013