Vibration Monitoring Specification
Vibration monitoring specification...
Design Note Project
Phase 2 redevelopment of Battersea Power Station
Construction vibration monitoring specification
Project no Date
031063 20 January 2016 Revision
Design note for issue
Buro Happold provided Skanska with a proposal (dated 30 October 2015) which set out a method for preparing a specification for construction vibration monitoring at Phase 2 of Battersea Power Station redevelopment. The aim was to ensure that the specification was sufficient to protect an important building without allocating more resources to vibration monitoring than is actually necessary. The scope for the preparation of the specification was:
To make a site visit accompanied by a project engineer to identify assets (being sections of building fabric and or permanent fixings thereon) that could benefit from, and are suited to, continuous vibration monitoring. To undertake a desk top study of the structural drawings for the building to assess the structural connections between monitoring locations and use this information to reduce the number of monitoring locations as far as is reasonable, practicable and consistent with British Standards BS 5228-2 and BS 7385-2. The use of moveable stations as well as fixed should be considered to minimise number of monitoring stations. To return to site to confirm and photograph the finalised monitoring locations To discuss appropriate vibration thresholds for use in the reporting output from the vibration monitoring with key stakeholders vibration thresholds To secure agreement on vibration thresholds for use in the reporting To prepare a vibration monitoring specification, sufficient in detail to be the basis of tender submissions for the monitoring itself
This Design Note is the deliverable against the scope set out in Section 1.1. Sections 2-6 (including Appendix A) provide supporting evidence. The vibration monitoring specification is provided in Appendix B.
This report has been prepared for the sole benefit, use and information of Skanska for the purposes set out in the Design note or instructions commissioning it. The liability of BuroHappold Engineering in respect of the information contained in the report will not extend to any third party. All concepts and proposals are copyright © January 2016. Issued in commercial confidence.
Implications of vibration: cracking in buildings
Definition of damage categories in BS 7385-1: 1990
Vibration damage to buildings most commonly relates to the formation of cracks. Vibration damage to buildings is classified in: BS 7385-1, 1990 Evaluation and Measurement for Vibration in Buildings, Part 1. Guide for Measurement of Vibrations and Evaluation of their Effects on Buildings, British Standards Institute, British Standard (1990) This classification is echoed in ISO 4866: 2010, International Organization for Standardization, Mechanical Vibration and Shock - Vibration of Buildings -Guidelines for the Measurement of Vibrations and Evaluation of their Effects on Buildings (2010). The three classes of damage are reported as: Cosmetic: The formation of hairline cracks on drywall surfaces or the growth of existing cracks in plaster or drywall surfaces; formation of hairline cracks in mortar joints of brick/concrete blocks. Minor: The formation of large cracks or loosening and falling of plaster or drywall surfaces, or cracks through bricks/concrete blocks. Major: Damage to structural elements of the building, cracks in support columns, loosening of joints, splaying of masonry cracks, etc. These definitions have their equivalents in the intensity scales used by seismologists. In these standards, the term “threshold damage vibration level” is defined as the highest vibration level at which no cosmetic, minor, or major damage occurs.
Cracking in buildings
Causes of cracking
Cracking is an inevitable response to the inability of a structure to accommodate the movement to which it is subjected. Cracks arise because the real world is not static: whether at a macro or a micro level, materials respond to changes in their environment by trying to move. (www.buildingconservation.com)
A list of causes of cracking in a building would include, but not necessarily be limited to: Ground movement (beneath foundations): for example shrinkage in clay sub-soils, fine material washed away from granular material due to ground water / drain failure, land slip Foundation failure for example: consolidation of rubble foundations, decay of soft clay brick, chemical erosion of concrete Decay of superstructure for example: decay of timber wall plates in masonry, the corrosion of iron cramps in stone walls. Moisture movement for example: seasonal changes in timber, expansion of new clay bricks Thermal movement : due to expansion of materials as temperature rises, followed by contractions when temperature falls. In composite structures, differences in thermal expansion coefficient can increase crack formation Inherent defects : such as insufficient lateral restraints. Page 2 of 22
Inappropriate specification: for example use of modern repair techniques that are too rigid for the repair of structures built with pliable lime-based mortars. Deflection under load : for example suspended structures such as floors tend to deform under load, and even vertical elements subject to load will compress by a small amount. Any infill (which is by definition non-structural) must be detailed to accommodate such movements or cracking will occur. Vibration: the movement of super-structure through dynamic forcing which might be caused by seismic, wind, demolition, construction activities, operational activities (operation of machines), transportation sources, human activities Movement post vibration for example: the liquification of soils (describing the phenomenon where the strength and stiffness of a saturated / partly-saturated soil is significantly reduced following an applied cyclic stress), compaction of rubble foundations.
In accordance with the scope set out in Section 1.1, the this Design Note focusses on the last two in the list above (although these cannot be viewed entirely in isolation from the other items in the list).
Propensity to crack
Different building materials have differing propensity to crack. This is due to the sensitivity of materials to higher levels of strain. For example, relatively malleable materials such as
Copper Steel Fir timber
would only crack at very high levels of strain. However, the most brittle materials such as
Hard wood panelling Plaster Ceramics
crack more readily under strain. There are intermediate building materials:
Brick Concrete Thick glass
where vibration induced strains might leads to cracking but it is not commonly expected.
Cracking caused directly by vibration
Cracks are formed (but are not necessarily visible) in a material when the localised strain exceeds the failure strain for the material. Cracks are encouraged in areas of stress-concentration such as:
At a free surface In corners and in the crux of angles At the tip of a pre-existing crack of other defect
Cracks can form due to static loading alone or due to cyclic loading and unloading of a structural element. When the loading period is short compared with the rate at which strain energy is dissipated in the material, a strain wave is formed. Strain waves propagate rapidly through structural elements in the form of longitudinal (compression) waves and rather more slowly in the form of bending waves. In soils, the analogous situation is the propagation of longitudinal (body) waves and shear waves at depth, and the relative dominance (70% of energy) of shear (Rayleigh / Love) waves near the surface of the soil. The amplitude of a strain wave can be magnified through resonance. This effect is most serious for the case of bending waves in structural elements that are wider than they are thick, and where the bending wavelength is the same order of magnitude as the length or width (or both) of the element.
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Cracking caused directly by strain waves (i.e cracking that is directly caused by vibration) is best assessed by measurement / prediction of dynamic strain (or dynamic deflection). However, it is more experimentally convenient to measure vibration velocity and set thresholds that are equivalent to a threshold value of dynamic displacement (noting that displacement is the time integral of velocity, and for simple harmonic motion at a single frequency, is equal to vibration velocity divided by radial frequency). Because of the nature of strain and ground waves, being a combination of bending / shear and compression waves, assessment of vibration should be undertaken in all three axes (longitudinal, lateral and vertical). Peak particle velocities (PPV) are normally reported in terms of the maximum orthogonal component of vibration (for example, if the vertical component of vibration is the greatest, then the vertical PPV is commonly reported).
Cracking caused post vibration
High amplitude vibration can increase the rate at which ground settles beneath foundations and can cause movement of the building sometime after the vibration has ceased. The most dramatic form of this is due to liquification of soils. This only applies to certain types of soils and strata of soil types that are highly saturated with water. In situ measurement of the liquification potential of soils made with a shear-wave vibrator are reported in Tomio Inazaki, Public Works Research Institute, Japan Proceedings of IWAM04 Mizunami, Japan 2004 This paper reports liquification effects in soils, critically depending on soil conditions , at acceleration levels in the range 2 20Hz typical frequency) PPVref = 6 mm/s at 7.5m distance for hydraulic breaker (>20Hz typical frequency) PPVref = 2 mm/s at 7.5m distance for bulldozer (>20Hz typical frequency) PPVref = 1 mm/s at 7.5m distance for a jackhammer (>20Hz typical frequency)
suggests that PPV at the foundations of a building could approach 15mm/s when a source of construction vibration is within 10 m of the perimeter of the building. This is the first damage threshold for the general fabric of the building in Table 1. A PPV of 3 mm/s at the foundations (that associated with the damage threshold for decorative finishes in Table 1) could occur when piling at a distance of 40m. In addition, PPV in this 40m zone are likely to drop by a factor of around 2 over a propagation distance of 10m (except when within 10m of the source when the rate of level decay will be significantly greater).
Vibration warning thresholds versus vibration damage thresholds
The vibration levels shown in Table 1 are damage threshold levels being the highest vibration level at which no cosmetic, minor, or major damage occurs. It is common practice to set warning vibration thresholds that are typically two-thirds of the value assigned to each damage threshold. This has been the case for the Phase 2 building to date where the specifications issued for the works to the Wash Towers and for the chimney demolition set a maximum vibration level of 10mm/s on those structural elements during the works. The 10mm/s is two thirds of the 15 mm/s set out in Table 1. Such a maximum is commonly referred to as a red warning level – that which shall not be exceeded, and if breached, would cause the cessation of works until mitigation is put in place. Commonly, red warning levels are accompanied by amber warning levels – usually set at two-thirds of the red level. The recommendation for the Phase 2 building, maintaining alignment with the damage thresholds set out in Table 1, is therefore
RED warning level – 10 mm/s (peak particle velocity in any direction) AMBER warning level – 7 mm/s (peak particle velocity in any direction)
Jones and Stokes, Transportation and construction vibration guidance manual, California Department of Transport, 2004
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Maximum spacing of vibration monitors
Interpreting the implications of the modelling reported above in the context of vibration monitoring at the Phase 2 building: 1.
There is a 40m wide zone around the perimeter of the building fabric, outside of which where it is reasonable to expect levels of construction vibration at the foundations to be below 3mm/s, being the lowest damage threshold vibration level recommended for the general fabric of the building in Table 1. Works in this zone require vibration monitoring but there is low probability of a need to significant vibration mitigation.
Vibration levels could be above either the amber of red warning thresholds when mechanised works (piling in particular) are scheduled within 10m of the building fabric. This zone requires active vibration management.
When dealing with PPV in the range 3-20 mm/s (as per Table 1) a doubling or halving of vibration level is a significant change and this typically takes place over a 10m propagation distance. Over a 20m propagation distance, levels change by a factor of 3.
On the basis of this, the following rationale for the maximum spacing between vibration monitors on the fabric of the building is offered:
In general, monitors should be spaced on a 20m grid pattern around the base of the building when working with vibration intensive equipment. This is to provide a tool for use in active vibration management in the 10m zone, on the basis that vibration intensive works will always be within 10m of a monitor. In this case, an amber warning level detected by one monitor suggests that the maximum level anywhere in the vicinity will be lower than the damage threshold level.
In addition, should be distributed at high levels around the fabric to provide horizontal components of PPV (Table 1). Typically the spacing should be no more than 20m as before.
In addition, at least one monitor should be placed in the corner of particularly vibration sensitive rooms (Control Room A, Control Room B, the closed-up offices) for use in ongoing vibration vigilance.
Recommended number of monitors
It is understood that the full perimeter of the building will be subject to works for the first two years of construction as the main piling and other vibration-intensive works take place. Therefore the full building should be continuously monitored, with the total number of monitors implied by the assessment in Section 6.2 being 51. An example layout of the monitors is shown in Figure 7. However, after this period, the number of monitors could be reduced by increasing the spacing to a maximum of 40m in certain locations so works are within 20m of the nearest monitor. In this case, the amber warning level should be reduced to 50% of the red level (5mm/s rather than 7mm/s). This ensures that an amber warning level detected by one monitor suggests that the maximum level anywhere in the vicinity will still remain lower than the damage threshold level as before. An example layout of the monitors for later years of construction is shown in Figure 8.
Recommendations on level warnings, alarms and analysis
The operational implications of amber and red warnings need to be agreed as part of the process of determining a vibration monitoring contract. As discussed in Section 6.4, the amber and red warnings are dependent of the damage thresholds and the spacing of monitors. Amber warnings indicate the probability that the damage threshold could be approached for any fabric close to the vibration source. Red warnings indicate the high likelihood that the damage threshold will be approached for any fabric close to the vibration source. Typically, amber warnings get communicated to machine operators so they can proceed with increased vigilance and caution and red warnings cause works to be halted whilst vibration mitigation is put into place. There is also the opportunity to have visual alarms associated with individual monitors so that a light flashes when a preset vibration level is exceeded. These should be considered for provision of amber warnings when working in close proximity to building fabric in the close vibration management zone. The number of monitors required on the Phase 2 building produce a significant volume of data each day. The significance of the Phase 2 building means that a heightened vigilance is required for the vibration monitoring in order to ensure appropriate protection of the building fabric and finishes. Procedures for the analysis and dissemination of this data, Page 12 of 22
including routine and ad-hoc meetings to discuss the data (as required) should be agreed as part of the process of determining a vibration monitoring contract.
Figure 7 Example layout of monitors with typical 20m spacing for use in first two years of construction. Suitable for an amber warning level set at 67% of the red warning level
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Figure 8 Example layout of monitors with up to 40m spacing for use in first two years of construction. Suitable for an amber warning level set at 50% of the red warning level
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Vibration monitoring protocols
Basis of specification
The recommended clauses for a vibration monitoring specification of are set out in the Sections 2-6 of this document, including Appendix A. Particular attention is drawn to:
The building classification described in Section 5.1 The vibration damage thresholds recommended in Section 5.2 The vibration warning thresholds recommended in Section 6.2 The number of monitors and their locations recommended in Section 6.4 and the impact this has on amber warning levels
Vibration monitoring specification
The recommended clauses for a vibration monitoring specification are set out in Appendix B.
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ISO 4866: 2010 classification of resistance to vibration
Ancient and historical buildings or traditionally built structures
The types of buildings considered in this group can be divided into the following two subgroups: a) ancient, historical or old buildings; b) modern buildings constructed in older, traditional style using traditional kinds of materials, methods and workmanship. Generally, this group is of heavier construction and has a very high damping coefficient due, for example, to soft mortar or plaster. This group also includes traditionally resilient structures in earthquake zones. Buildings in this group seldom have more than six storeys.
Group 2: Modern buildings and structures The types of buildings considered in this group are all of modern structure using relatively hard materials connected together in all directions, usually lightweight overall, and with a low damping coefficient. This group includes frame buildings as well as calculated load-bearing wall types. Buildings vary from single to multistorey. All types of cladding are included.
Category of structure
The category of a structure provides a measure of resistance to vibration. Buildings and structures are classified in accordance with Table B1 in ISO 4866: 2010 (reproduced as Figure 9 below).
Category of foundation
The class of foundation A,B,C with A resulting in structures with the highest levels of acceptable vibration and C having the lowest levels. Class A includes the following foundation types:
linked reinforced concrete and steel piles; stiff reinforced concrete raft; linked timber piles; gravity retaining wall
Class B includes the following types: independent reinforced-concrete piles that are usually connected only at their pile caps; spread wall footing; timber piles and rafts. Class C includes the following types: light retaining walls; large stone footing; strip foundation; plate foundation; no foundations (walls directly built on soil).
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Figure 9 Category of structure
ISO 4866: 2010
Type of soil
Soils are classified into: type a: unfissured rocks or fairly solid rocks, slightly fissured, or cemented sands; type b: horizontal bedded soils, very firm and compacted non-cohesive soils; type c: horizontal bedded soils, poorly compacted firm and moderately firm non-cohesive soils, firm cohesive soils; type d: all types of sloping surfaces with potential slip planes; type e: loose non-cohesive soils (sands, gravels, boulders), soft cohesive soils (clays), organic soils (peat); type f: fill.
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Figure 10 Classification of buildings
ISO 4866: 2010
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Vibration monitoring specification: sample clauses
A. The Work of this specification includes furnishing, installing and maintaining vibration monitoring instrumentation; collecting vibration data; and interpreting and reporting the results. The purpose of the vibration-monitoring program is to protect the following properties from excess vibration during demolition and construction activities associated with the [ ] Project: 1. Building name and address 2. Building name and address B. [ ] is not responsible for the safety of the Work based on vibration-monitoring data, and compliance with this specification does not relieve the Contractor of full responsibility for damage caused by the Contractor’s operations.
RESPONSIBILITIES OF CONTRACTOR
A. Furnish and install vibration-monitoring instrumentation. B. Protect from damage and maintain instruments installed by the Contractor and repair or replace damaged or inoperative instruments. C. Collect, interpret and report data from instrumentation specified herein. D. Implement response actions.
QUALIFICATIONS OF VIBRATION MONITORING PERSONNEL
A. The Contractor’s vibration-monitoring personnel shall have the qualifications specified and the appropriate training and certification required for the [ ] site . These personnel may be on the staff of the Contractor or may be on the staff of a specialist subcontractor. However, they shall not be employed nor compensated by subcontractors, or by persons or entities hired by subcontractors, who will provide other services or material for the project. B. The Contractor’s vibration-monitoring personnel shall include a qualified Vibration Instrumentation Engineer who is a chartered or incorporated engineer (or professional equivalent) who has a minimum of [ ] years of experience in the installation and use of vibration-monitoring instrumentation and in interpreting instrumentation data. The Vibration Instrumentation Engineer shall: 1. Be on site and supervise the initial installation of each vibration-monitoring instrument. 2. Supervise interpretations of vibration-monitoring data. C. The Contractor’s vibration-monitoring personnel shall be subject to the review of [
A. A record of laboratory calibration shall be provided for all vibration-monitoring instruments to be used on site. Certification shall be provided to indicate that the instruments are calibrated and maintained in accordance with the equipment manufacturer’s calibration requirements and that calibrations are traceable to the UKAS.
A. As soon as feasible after the Notice to Proceed, submit manufacturer’s product data describing all specified vibrationmonitoring instruments to [ ] for review, including requests for consideration of substitutions, if any, together with product data and instruction manuals for requested substitutions. B. Within 3 weeks after the Notice to Proceed, submit to [ ] for review the resumes of the Vibration Instrumentation Engineer and any vibration monitoring technical support personnel, sufficient to define details of relevant experience. C. Within 5 workdays of receipt of each instrument at the site, submit to [ laboratory calibration and test equipment certification.
] a copy of the instruction manual and the
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D. Prior to the start of construction and prior to performing any vibration monitoring, the Contractor shall submit to [ for review a written plan detailing the procedures for vibration monitoring. Such details shall include:
1. Description of the instrumentation and equipment to be used. 2. Measurement locations and methods for mounting the vibration sensors. 3. Procedures for data collection and analysis.
4. Means and methods of providing warning when the Response Values, as specified in section 9.3.7, are reached. E. Submit data and reports as specified in Section 9.3.4.
A. Whenever any product is specified by brand name and model number, such specifications shall be deemed to be used for the purpose of establishing a standard of quality and facilitating the description of the product desired. The term “acceptable equivalent” shall be understood to indicate a product that is the same or better than the product named in the specifications in function, quality, performance, reliability, and general configuration. This procedure is not to be construed as eliminating other manufacturers’ suitable products of equal quality. The Contractor may, in such cases, submit complete comparative data to [ ] for consideration of another product. Substitute products shall not be used in the Work unless accepted by [ ] in writing. The Engineer will be the sole judge of the suitability and equivalency of the proposed substitution. B. Any request from the Contractor for consideration of a substitution shall clearly state the nature of the deviation from the product specified. C. The Contractor shall furnish all installation tools, materials, and miscellaneous instrumentation components for vibration monitoring.
A. Provide portable seismographs for monitoring the velocities of ground vibrations resulting from construction activities. The seismograph shall be compliant with the requirements of ISO 4866: 2010. The Contractor shall submit evidence of compliance to [ ] for review. In addition the monitoring system have the following minimum features: 1. Seismic range: 0.01 mm/s to a minimum of 60 mm/s peak particle velocity or better at frequencies between 0.1 Hz and 100 Hz, and with a resolution of 0.01 mm/s or less. 2. Frequency response (+3 dB points): 2 to 200 Hertz. 3. Three channels for simultaneous time-domain monitoring of vibration velocities in digital format on three perpendicular axes. 4. Two power sources: internal rechargeable battery and charger and 240 volts AC. Battery must be capable of supplying power to monitor vibrations continuously for a minimum of [ ] days. 5. Capable of internal, dynamic calibration. 6. Continuous monitoring mode must be capable of recording single-component peak particle velocities, and frequency of peaks with an interval of one minute or less. 7. There must be a triggering facility, whereupon exceedance of a threshold ppv, the monitor gathers vibration data as a minimum sample rate of 500 samples per second for at least 30 seconds.
INSTALLATION OF SEISMOGRAPHS
A. The Contractor shall install seismographs at points agreed with [
] in writing.
C. The seismograph vibration sensors shall be firmly mounted on the surface slab of concrete or asphalt, or firmly set in undisturbed soil, or bolted to appropriate walls, or mounted on a stiff bracket of design approved by [ ]. Page 20 of 22
FIELD CALIBRATION AND MAINTENANCE
A. The Contractor’s instrumentation personnel shall conduct regular maintenance of seismograph installations. B. All seismographs shall have been calibrated by the manufacturer or certified calibration laboratory within one year of their use on site. A current certificate of calibration shall be submitted to [ ] with the Contractor’s data.
A. The Contractor shall collect seismograph data prior continuously at each monitoring location. This monitoring shall consist of a continuous recording of the maximum single-component peak particle velocities for one-minute intervals. B. During the monitoring, the Contractor shall document all events that are responsible for the measured vibration levels, and submit the documentation to [ ] with the data as specified in Section 9.3.4. C. All vibration monitoring data shall be recorded contemporaneously and plotted continuously on a graph by the data acquisition equipment. Each graph shall show time domain wave traces (particle velocity versus time) for each transducer with the same vertical and horizontal axes scale. D. No significant vibration-producing activity shall occur within any 40m x 40m zone unless the local monitoring equipment is functioning properly. E. The equipment shall be set up in a manner such that an immediate warning is given when the peak particle velocity in any direction exceeds the Response Values specified in Section 9.3.7. The warning emitted by the vibration-monitoring equipment shall be instantaneously transmitted to the responsible person designated by [ ] by means of warning lights, audible sounds and electronic transmission.
DATA REDUCTION, PROCESSING, PLOTTING AND REPORTING
A. Every 5 working days the Contractor shall submit to [ locations.
] a report documenting the results at each of the monitoring
C. All reports shall include the following: 1. Project identification 2. Location of the monitoring equipment, including GPS address. 3. Location of vibration sources (e.g. traffic, demolition equipment, etc.) 4. Summary tables indicating the date, time and magnitude and frequency of maximum single-component peak particle velocity measured during each one-hour interval of the monitoring period. The exact form of data presentation shall be agreed with [
] within 10 days of Appointment.
DAMAGE TO INSTRUMENTATION
A. The Contractor shall protect all instruments and appurtenant fixtures, leads, connections, and other components of vibration-monitoring systems from damage due to construction operations, weather, traffic, and vandalism. B. If an instrument is damaged or inoperative, the Contractor’s instrumentation personnel shall repair or replace the damaged or inoperative instrument within 72 hours at no additional cost to [ ]. The Contractor shall notify [ ]at least 24 hours prior to repairing or replacing a damaged or inoperative instrument. [ ] will be the sole judge of whether repair or replacement is required.
DISCLOSURE OF DATA
A. The Contractor shall not disclose any instrumentation data to third parties and shall not publish data without prior written consent of [ ]
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DATA INTERPRETATION AND IMPLEMENTING PLANS OF ACTION
A. The Contractor shall interpret the data collected, including making correlations between seismograph data and specific construction activities. The data shall be evaluated to determine whether the measured vibrations can be reasonably attributed to construction activities. B. The Response Values for vibration include a RED warning level – 10 mm/s (peak particle velocity in any direction) AMBER warning level – 7 mm/s (peak particle velocity in any direction) The actions associated with these Response Values are to be agreed with [ ] in writing. Plans for such actions are referred to herein as plans of action, and actual actions to be implemented are referred to herein as response actions. Response Values are subject to adjustment by [ ] as indicated by prevailing conditions or circumstances.
DISPOSITION OF INSTRUMENTS
A. The Contractor shall remove salvageable instruments only when directed by [ B. All salvaged instruments shall become the property of [
BASIS OF PAYMENT
A. The contract sum price paid for vibration monitoring shall include [
B. Any additional areas where vibration monitoring is required will be paid for as extra work as provided in the Standard Specifications.
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