Damage to Structures from Ground-borne Vibration - BRE Digest 403.pdf

Digest 403 March 1995

CI/SfB (J6)

Damage to structures from ground-borne vibration Ground-borne vibrations from civil engineering, blasting or traffic often cause noticeable vibrations in buildings. Householders are occasionally worried that vibrations might damage their property and this can be a significant cause of distress. Guidance levels for damage from ground-borne vibration were introduced in two British Standards in 1992 and 1993. This Digest gives information on the current UK position concerning damage to buildings. A guide to evaluation of human exposure to vibration is given in BS 6472 but is not considered here. This Digest replaces Digest 353 which is now withdrawn.

Although vibrations induced in buildings by groundborne excitation are often noticeable, there is little evidence that they produce even cosmetic damage (such as small cracks in plaster). This lack of data is one of the reasons why the British Standards Institution (BSI) did not provide guidance before 1992 and why there is still no International Organization for Standardization (ISO) guidance limits. It also indicates that damage solely attributable to vibrations is not common. Some European countries have provided quantitative guidance in their codes for some years; however, it is not strictly valid to apply these limits in the UK because the data on which they are based relate to different structural types and settings. If there is concern that damage may occur (for example, from piling close to a building), it is important to survey the property before exposure to the vibrations (or at the earliest opportunity), measure the vibration levels induced and finally check if any damage is evident. It is not enough simply to examine a structure after exposure and assume that any previously unnoticed cracking is a result of the vibrations. With this type of problem, accurate measurements and records are important. This Digest provides some guidance about what parameters should be measured and where measurements should be made.

DEFINITIONS ● Particle velocity: velocity of particles set into motion by the propagation of a disturbance through the ground and a structure by a source of vibration. ●

Frequency: the frequency of vibration of such particles.

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Peak particle velocity (ppv): the maximum instantaneous particle velocity at a point during a given time interval.

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Peak component particle velocity: the maximum value of any one of the three orthogonal component particle velocities measured during a given time interval.

Particle velocity in the soil is therefore distinct from wave velocity; whilst the disturbance due to a source propagates away from the source with a certain wave velocity, the ground particles oscillate with a variable particle velocity.

Technical enquiries to: Building Research Advisory Service Garston, Watford, WD2 7JR Tel: 01923 664664 Fax: 01923 664098

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BRITISH STANDARDS ON GROUND-BORNE VIBRATION AND DAMAGE There were no British Standards giving guidance on potential damage to structures from ground-borne vibrations until 1992. The first to be published was BS 5228: Part 4 in May 1992; this deals with piling operations. Then, in November 1993, BS 7385: Part 2 was issued, covering most forms of ground-borne vibration. Since these two codes of practice do not give exactly the same guidance, some explanation is needed. BS 5228: Part 4 Table 1 (opposite) is abstracted from the standard. The first thing to note is that the values are suggested as providing a conservative threshold for minor or cosmetic (non-structural) damage. As such, they can be thought of as representing good practice for the piling industry. Unlike BS 7385, they make use of peak particle velocity (ppv) measurements, thus ensuring that the maximum velocity is used in any assessment. The code says that special consideration should be given to ancient ruins and listed buildings. It uses the term intermittent vibrations, defined as being a sequence of transient vibrations with sufficient intervals between successive events to permit the amplitude to diminish to an insignificant level in interim periods. BS 7385: Part 2 The recommended vibration levels are given in Table 2 and Fig 1. These levels were derived following an extensive review of UK data (which yielded very few cases of vibration-induced damage), and include the results of experimental investigations carried out in other countries into vibration-induced damage thresholds. Because they are based on experimental work, which primarily used the peak component particle velocity, the guidance given is in terms of this measured value. The levels suggested are judged to give a minimal risk of vibration induced damage – see BS 7385: Part 1. However, it is recognised that ppv should also be evaluated, because this is scientifically the correct measurement to use (it does not depend upon orientation) and records quoting frequency and ppvs should provide a robust UK database for future codes. The guide values in Table 2 have been determined for transient vibrations. Although there is less experimental data for continuous vibrations, it is recognised that they may give rise to magnification due to resonance. Consequently, for continuous vibrations the guide values in Table 2 may need to be reduced by up to 50%, especially at the lower frequencies where the lower guide values apply. Important buildings which are difficult to repair may require special consideration on a case-by-case basis. A building of historical value should not (unless it is structurally unsound) be assumed to be more sensitive. As might be expected from the derivation of the standards, BS 5228 generally recommends lower levels than does BS 7385. The exception is that BS 5228 allows a higher velocity for heavy and stiff structures for commercial and industrial buildings at frequencies above 50 Hz. This is outside the main frequency range expected for piling.

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Table 1

Peak particle velocities which provide conservative thresholds for minor damage from piling operations Derived from BS 5228: Part 4 and applicable to a frequency range 10 – 50 Hz

Status Soundly constructed residential property and similar property in good repair Structures where preliminary surveys reveal existing significant defects of a structural nature

Intermittent mm/s

Continuous mm/s

10

5

Reduction of the above values by up to 50%

Light and flexible structures for commercial and industrial use

20

10

Heavy and stiff structures for commercial and industrial use

30

15

At low frequencies (below 10 Hz), large displacements and associated large strains necessitate lower ppv values (50% lower). At high frequencies (above 50 Hz), much smaller strains allow the ppv limits to be increased (100% higher).

Table 2

Transient vibration guide values for cosmetic damage: from BS 7385: Part 2

Line see Fig 1

Type of building

Peak component particle velocity in frequency range of predominant pulse 4 Hz to 15 Hz

1

Reinforced or framed structures Industrial and heavy commercial buildings

2

Unreinforced or light framed structures Residential or light commercial buildings

15 Hz and above

50 mm/s at 4 Hz and above

15 mm/s at 4 Hz increasing to 20 mm/s at 15 Hz

20 mm/s at 15 Hz increasing to 50 mm/s at 40 Hz and above

Values are at the base of the building. For line 2, at frequencies below 4 Hz, a maximum displacement of 0.6 mm (zero to peak) should not be exceeded.

Fig 1 Transient vibration guide values for cosmetic damage

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MEASURING VIBRATION HISTORICAL The basic philosophy has been that it is impracticable to measure vibrations in a large number of buildings and that it is desirable to use some measurement of the ground vibrations as an indicator of structural damage. Several different parameters have been suggested to indicate damage but it is now generally accepted that the best single descriptor is particle velocity. It is also one of the easiest parameters to measure and it is convenient to use such a measurement as a limit to acceptable vibrations levels at, say, the boundary of a construction site. Obviously, taking measurements outside a building can, at best, be only a crude indicator of damage potential inside the building. The type of structure, design details and state of repair are all important factors and measurements inside the building are required to provide a reasonable picture of what is happening. MAIN SOURCES OF VIBRATION The main sources of man-made, ground-borne vibrations are blasting (quarry, mining or construction), traffic, machinery and civil engineering work, particularly piling. These sources have certain differences. For example, blasting may produce high levels of vibration for a short period of time, whilst traffic may produce low level, almost continuous vibration. DURATION OF VIBRATION The duration of the vibration can have a marked effect on structural response. Continuous vibration may produce a significantly higher response due to dynamic magnification if the excitation frequency is close to a resonance frequency of a structural element. This possibility of dynamic magnification is the reason why different limits are suggested for continuous and transient vibrations. WHAT TO MEASURE AND WHERE TO MEASURE IT The three orthogonal components of particle velocity (usually one vertical and two horizontal) should be measured simultaneously. Ideally, the response time-histories of each component velocity at each measurement position should be recorded. This will allow the ppv (or component ppv) to be calculated, and the predominant frequency of vibration to be determined. Vibration measurements should be made at the base of the building facing the source of vibration. One of the directions of measurement should be parallel to one of the side walls of the building and measurements should be made on the side of the building facing the source of vibration. If this is not possible, measurements should be taken on the ground outside the building, with the transducers arranged so that one is in line with the vibration source and at the part of the building nearest the source. If detailed measurements of the building response are required, they should be taken at the foundation and at the uppermost storey and at any positions where damage is expected. The vertical component of particle velocity may also be measured at the centre of floors of intermediate storeys. If the building covers a large area, measurements should be taken at several points simultaneously.

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INSTRUMENTATION REQUIRED As particle velocity is used as a damage indicator, it is preferable to monitor it directly using a velocity transducer. The most common type is the geophone. Particle motion in three orthogonal directions should be monitored at each position in the ground, so three single-axis transducers (or one triple-axis transducer) are required. In the building, the transducers should be fixed to the measurement position rigidly so that they give a true record of the particle motion. In soil, the transducer should be fixed to a stiff, steel rod, 10 mm diameter or larger, driven through the loose surface layer so that it does not project more than a few millimetres above the surface layer. Further guidance and alternative fixing methods are given in BS 7385: Part 1. The transducer must have a sensitivity and frequency range to cover the anticipated range of vibration frequencies and velocities. BS 7385: Part 1 suggests ranges of 1 to 1000 Hz and 0.2 to 500 mm/s. These are extreme: ranges of 4 to 250 Hz and 0.2 to 50 mm/s are more likely. If very low frequency vibrations are encountered, it may be difficult to obtain velocity transducers with the required frequency range and it may be necessary to use accelerometers. Of all man-made ground vibration, blasting produces the largest frequency and velocity ranges. The signals from the transducer should be displayed and recorded. Displaying the signals directly on an oscilloscope will give an indication of the vibration levels; recording the response on tape or directly on to a computer will enable the data to be analysed fully. It may be necessary to use amplifiers; anti-aliasing filters will be needed if the data are recorded digitally. Instrumentation must be calibrated. Individual instruments can be calibrated in the laboratory using a known source or the whole measurement chain can be calibrated on site. Recommended procedures for calibrating transducers (or pick-ups) are given in BS 6955. OTHER MEASUREMENTS After taking measurements of the ground motion and the response of the structure to the excitation, the measurements should be repeated when the source is absent; this provides data on the background noise level. Measurements of vibration levels in buildings should also be taken with people walking near the measurement areas and slamming nearby doors. This will give an indication of the vibration levels which are encountered normally.

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GUIDELINES FOR SENSITIVE EQUIPMENT ISO 8569 gives provisional guidelines for the measurement and evaluation of shock and vibration affecting sensitive electronic equipment. This includes computers, telecommunication equipment and laboratory instruments. Data from these measurements can be used to establish a data base which can be used to give guidance on vibration levels to constructors, users, suppliers and manufacturers of sensitive equipment. There is no explicit guidance on acceptable vibration levels for such equipment so recommended vibration levels should be obtained from instrument manufacturers.

REPORTING The data must be recorded and reported correctly. ISO 8569 provides guidelines for the measurement and reporting of shock and vibration effects in buildings. Ideally, the report should include the following: ●

name, affiliation and professional standing of person taking measurement

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dates of measurement and weather conditions

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information on source of excitation, including any technical details

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type of soil and any measured soil parameters (especially wave velocity)

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distance of source from structure, horizontal and vertical

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description of structure: room sizes, layout and site location

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building construction type and floor plan

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general structural condition, including list of defects

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transducers, operating ranges and calibration factors

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amplifiers, recorders, analysers and calibrating equipment

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calibration procedures and results

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measurement positions and axes

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individual recorded time histories and maximum calculated particle velocities

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predominant frequencies in time histories

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background noise levels and normal vibration levels

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records of any damage, including photographs.

More details are given in BS 7385: Part 2.

REDUCTION OF VIBRATIONS There are three components to any vibration system: ● ● ●

the source the transmission path the receiver (the structure).

To reduce vibration levels at a receiver, it is usually cheaper and easier to modify the energy input or frequency of the source. Changing the transmission path is the most difficult option, although excavating a trench between the source of vibration and the receiver may appear to provide a physical barrier. Because the trench must be at least 0.3 times the wavelength of the ground wave, this is not normally practicable for low frequency vibrations. Changes to the receiver can be made, either at the design stage if a vibration problem is envisaged, or by modifying the existing structure. A structure may be mounted on supports or bearings to attenuate vibrations. Guidance on the selection and use of elastomeric bearings is given in BS 6177. Alternatively, the resonance frequencies of a structure or structural element may be altered by structural changes to avoid problems due to resonance at a particular excitation frequency. 6

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RECOMMENDATIONS Anyone undertaking work which will create vibration near housing must be aware that the vibration will be a cause of concern to local residents. Reasonable steps must be taken to control the vibration levels and consequent nuisance. Section 61 of the Control of Pollution Act 1974 provides a means whereby an application can be made to the local authority to agree the level of vibration which should be acceptable on a given site. In itself this may not allay the concern of local residents but following the approved procedures should help to minimise problems. Also, if the local authority believe the noise or vibration constitutes a nuisance and bring an action it would be a defence to prove that the alleged offence was covered by a relevant consent given under the Control of pollution Act 1974. Householders should realise that construction works do create some vibration. Although people often feel them, these vibrations rarely cause damage to property. However, if it is anticipated that human annoyance or structural damage may occur as a result of civil engineering works, or other sources of ground-borne vibration, the following steps should be considered. ●

If the vibration constitutes a nuisance to occupiers of premises and negotiation of removal or reduction of the nuisance fails, an action may be taken in court for an order to abate the noise or prohibit its occurrence under the Environmental Protection Act 1990.

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If damage is suspected, measurements of the vibration levels, in terms of particle velocity, should be made and photographs taken of any damage. Photographic evidence should ideally be of a ‘before and after’ type. In addition, measurements of vibration levels should be made when the source of vibration is absent so that a comparison of vibration levels due to the source of vibration can be made with levels due to everyday activities.

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The Environmental Protection Act 1990 provides that an occupier of property aggrieved by noise (including vibration) which constitutes a nuisance may apply to the magistrates’ court for an order to be issued to abate the nuisance or prohibit its occurrence. Common law may also provide a remedy for a person affected by a noise or vibration, particularly the law of nuisance and, where there is damage, the law of negligence. Of course, any person who complains about noise or vibration should seek legal advice on the steps that may be taken to secure a remedy.

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FURTHER READING Control of Pollution Act, 1974, Part III. London, HMSO. Land Compensation Act, 1973, Chapter 26. London, HMSO. ISO 8569. Shock and vibration sensitive equipment. Methods of measurement and reporting data of shock and vibration effects in buildings. (Draft). British Standards Institution BS 5228:— Noise control on construction and open sites Part 4: 1992 Code of practice for noise and vibration control applicable to piling operations BS 6177: 1982

Guide to the selection and use of elastomeric bearings for vibration isolation of buildings

BS 6472: 1992

Guide to the evaluation of human exposure to vibration in buildings (1 Hz to 80 Hz)

BS 6955:— Calibration of vibration and shock pick-ups Part 0: 1988 Guide to basic principles BS 7385:— Evaluation and measurement for vibration in buildings Part 1: 1990 Guide for measurement of vibration and evaluation of their effects on buildings (also ISO 4866 1989) Part 2: 1993 Guide to damage levels from ground-borne vibrations Other BRE Digests 343 Simple measuring and monitoring of movement in low-rise buildings Part 1: cracks 344

Simple measuring and monitoring of movement in low-rise buildings Part 2: settlement, heave and out of plumb

Printed in the UK and published by Construction Research Communications Ltd for the Building Research Establishment. Available by subscription. Current prices from: Construction Research Communications Ltd, 33 - 39 Bowling Green Lane, London EC1R 0DA. Tel: 01923 664444 Fax: 01923 664400. Full details of all recent issues of BRE publications are given in BRE News sent free to subscribers. Crown copyright 1995

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