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Structural Health Monitoring: The paradigm and the benefits
shown in some monitoring projectsProfessor James MW Brownjohn, Professor Aleksandar Pavic, Dr Paul Reynolds
Full Scale Dynamics Ltd (http://www.fullscaledynamics.com)
Dr Ki-Young KooVibration Engineering Section (http://vibration.shef.ac.uk)
Department of Civil and Structural Engineering, University of Sheffield
14th European Conference on Earthquake Engineering,Ohrid, Macedonia, August 2010
OUTLINE
Introduction to FSDLPurposes and components of SHM systemExamples:pTamar Bridge• combined static and dynamic monitoringRugeley Chimney• pure dynamic response monitoring Sheffield University Arts tower• further development of Rugeley with more comprehensive information• with Digitexx
Lessons/conclusions
What is FSDL?What is FSDL?
What is SHM?SHM systems for civil infrastructure have two broad purposes and
neither is about damage detection:For diagnosis, to:• Prove structural fitness for purpose
Ch k l t f t ti / t t l f• Check novel systems of construction/structural forms • Validate structural modifications & mitigation measures• Track structural loads/overloads/extreme responsesTrack structural loads/overloads/extreme responses• Evaluate ’servicability’ –e.g. user comfort/safety• Provide a feedback loop to design and loading codesFor prognosis, to:• Assess structural safety after trauma (e.g. earthquake/impact/bridge scour)• Track long term degradation to aid maintenance decisions• Detect ’damage’? –In rare cases outside lab and simulation: please tell me!• Provide warning of impending failure? (and then bury the incident)• Provide warning of impending failure? (and then bury the incident)
What are elements of a civil infrastructure SHM system? Automated continuous/long term monitoring Offline assessmentAutomated continuous/long term monitoring• Sensors (static/dynamic)• Local data storage
Offline assessment• (FE) modeling• (Dynamic) testing• Local data storage
• Local processing• Data transmission
• (Dynamic) testing• Model validation/updating→Physics –based/FE model• Data transmission
• System identification• Data reduction/mining
→Physics –based/FE model
Data reduction/mining• Performance/
load evaluationData-driven model
Using data-driven/physics-based model:• Anomaly detection• Reports/alerts/decisions
Example 1: Tamar Bridge: 335m span, opened 1961. Upgraded 2000, static monitoring system installed to
check performance due to upgradeg
SHM configuration with three sub-systems:1) 70 h F t * (2000 d d 2007)1) 70 ch Fugro system* (2000, upgraded 2007),2) 16 ch Sheffield dynamic system (2006), 3) Sheffield TPS system (2009)3) S e e d S sys e ( 009)Data fusion via MATLAB/MySQL database
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Temperature Temperature (Level Station) Humidity Accelerometer
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*Installed to evaluate effect of upgrade
2000 structural Lateral2000 structural upgrade i l d j
Lateral restraint←
involved major changes: Vertical
restraint
e g deck bearing system and longitudinal continuity rearranged
restraint→
e.g. deck bearing system and longitudinal continuity rearranged, ..
… additional stays (φ∼100mm) added,…cantilever road/walkways added
From initial vibration monitoring f d d k d f i
we found deck mode frequencies have obvious diurnal variations,
i t l t ithVS1 0.393Hz LS1 0.457Hz VA1 0.595Hz
appearing to correlate with temperature, but it’s not so simple
35
Deck vibration modes identified by full scale ambient vibration survey, April 2006
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0 200 400 600 800
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670 672 674 676 678 680
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days since 1 Jan 2007 days since 1 Jan 2007
We expected strong temperature
effect
We expected strong temperature influence so we tracked effects of thermal expansion on bearing effect2movement and cable tensions
effect
effect
cause?
Saltash tower expansion joint
(March 2007 data)expansion joint
To get a fuller picture of the g pglobal response we installed a Total Positioning Systema Total Positioning System
Note: Top of Side Tower
DeckTop of TowerWall of Tower
deflections too small for GPS
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Bridge exhibits a complex displacement patterng p p p
Static displacement based on measured TPS dataStatic displacement based on measured TPS data and updated FE model
However a main contributor to frequency variation i fi i l d i ffi (l )in first vertical mode is traffic (lorry) mass
Finite element model i l ti ithsimulations with
observed traffic loadsTraffic load estimates andTraffic load estimates and measured first mode frequency ranges for
l dseveral daysMode demonstrates a combined responsecombined response of traffic and thermal effects.effects.
Observations from Tamar Bridge study so far• Temperature is the dominant driver of static structural configuration.
M d l d t id tif th d t t h d
Observations from Tamar Bridge study so far
• Modal survey was used as a means to identify the modes to match mode shapes to varying frequencies
• Majority of dynamic loads derive from traffic (heavy vehicles)• Majority of dynamic loads derive from traffic (heavy vehicles).• Structural configuration variation leads to major effects on dynamic properties
making their use as sole measure for SHM a major challenge.g j g• Establishing a performance ‘baseline’ and developing ‘filters’ is critical but
complex aspect of SHM. We can now identify anomalies.• Instrumentation is far from simple, with heavy focus on static response• Even so this SHM configuration is a scaled down version of typical major long
b id j tspan bridge projects
Example 2: Rugeley chimneyR i f d t fl• Reinforced concrete flue gas chimney at Rugeley coal-fired power stationpower station
• Built in 1968• Built in 1968
• 183m high• 183m high
• Construction of new chimney in 2006 prompted study of ‘interference effects’ showing old chimney might be unsafe and should be monitoredeffects showing old chimney might be unsafe and should be monitored
• New chimney approx. 100m in SSW direction from old one!
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10 1212 - 1414 - 1616 - 18>=18
Wind Speed(m/sec)
S0 - 22 - 44 - 66 - 88 - 1010 - 12
What do in-line chimneys do?What do in line chimneys do?
FSDL vibration monitoring system installed:4 channel monitoring system online from March 20074-channel monitoring system online from March 2007-designed to give alarm for high response and provide response datap
High alert levelBiaxial QA750 accelerometers
P iPanasonic Toughbook+NI USB-6251 orNI USB 6251 orNI USB-9239
…followed quickly by installation of tuned mass damper (TMD b M ltit h) f M h 2007(TMD, by Multitech), from March 2007
Lower accelerometer box
Unique real time frequency and damping estimation sytem shows max performance of TMD around 4% (29 February 2008)max performance of TMD around 4% (29 February 2008)
Baseline Baseline damping 0.7%
This is odd: modal parameter variation over 28 monthsp0.35
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/Hz Zoom on frequency shift (March 2008):
330 340 350 360 370 380 390 400 410 420 4300.25
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330 340 350 360 370 380 390 400 410 420 430days since 31/3/2007
Why the change? We suspect ‘structural dust’ vertical sequence of masonry rings with small ‘dust’ filled expansion joint, suggests composite action of concentric cylinders
h i l d d li iwhen gap is closed due to liner expansion
What happened to the old chimney? Decommissioned…pp y
Result of SHM study on Rugeley Chimney
• This is a very rare example where vibration-based monitoring and frequency
Result of SHM study on Rugeley Chimney
This is a very rare example where vibration based monitoring and frequency changes have had direct value for structural assessment
• Safe-range performance of the chimney was proven at all times during co-existence
• TMD effectiveness was demonstrated in real time• Bizarre form ‘structural mechanism’ observed due to gross temperature
changes• Damping frequency and response level evaluation are critical for super tall• Damping, frequency and response level evaluation are critical for super-tall
buildings (e.g. Burj Kahlifa)• ‘Real-time’ evaluation is crucial for effective SHM; real time means results areReal time evaluation is crucial for effective SHM; real time means results are
needed soon enough to make informed decision on operation and intervention
O d i 1966Example 3 Sheffield University Arts Tower• Opened in 1966• 78m tall, 21 levels Li l d i• Lively dynamic response
• Nicknamed ‘Faulty Tower’• Grade II listed building• Grade II listed building• 2009: £15m upgrade started
I t t tiInstrumentation
• 2 Guralp CMG-5TD tri-axial accelerometers on the roof of th b ildithe building
• with time-stampingfrom GPS antenna
METAL WALKWAY
METAL WALKWAY
METAL PLATFORM
CRADLE
METAL WALKWAY
CRADLE TRACK CRADLE TRACK CRADLE TRACK
METAL PLATFORM METAL PLATFORM
METAL PLATFORM95E 96E
METAL PLATFORM
95Z 96Z
METAL WALKWAY METAL WALKWAY
CRADLE TRACK CRADLE TRACK CRADLE TRACK
Modal analysis10
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Modal analysis
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Modal parameters for first 6 modes10
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Modal parameters for first 6 modes
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 510
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Frequency (Hz)
Modal frequencies varied during de-construction (2009/2010)(2009/2010)…
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Oct Nov Dec Jan Feb Mar Apr0.5
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Time
…and with responseNatural
frequencies
…and with response
frequenciesvs RMS
(no effect of(no effect of temperature)
Damping tiratios
vs RMS
Mk 2 Arts Tower 16 ch Digitexx streaming
FSDL/Digitexx developments:g pPowerful on-line real time visualisation tool
There will be an on-line demonstrator launched very soon
FSDL/Digitexx developments:FSDL/Digitexx developments:Data integration and condensation
Lessons from Arts Tower
• Guralp seismometer-like acquisition system shows that using building internet
Lessons from Arts Tower
Guralp seismometer like acquisition system shows that using building internet backbone for monitoring is a sensible option
• Site conditions are a major problem for instrumentation: disruptions have been caused by pulled wires, moved sensors, threats of asbestos etc.
• Such systems depend heavily on intervention by client site engineer/staff: they have to really value your data!they have to really value your data!
• Major lesson: don’t expect an easy life in full scale instrumentation• The monitoring has shown clearly the amplitude dependence of modal• The monitoring has shown clearly the amplitude-dependence of modal
parameters• Combination of data from three systems will track the response during theCombination of data from three systems will track the response during the
complete upgrade period
Lessons & conclusions from our monitoring experience
• Dynamic performance alone is a very strong element of SHM
g p
Dynamic performance alone is a very strong element of SHM• It can tell you a lot about the structure, particularly the boundary conditions,
but don’t expect reliable information about ‘damage’ e.g. cracks• Normal environment and load variations can easily mask significant
anomalies: key challenge is (still) reliable filtering of normal effects• Static response monitoring helps to explain ‘why’, but is harder do to in
practice than sticking on accelerometers. • Complex instrumentation needs love and attention: hard to publish that!• Complex instrumentation needs love and attention: hard to publish that!• We’ve tried a whole lot of exotic sensors in challenging field applications.
Many of them aren’t effective or don’t survive, but we don’t give up easily.Many of them aren t effective or don t survive, but we don t give up easily.• Even after that big effort, still the biggest challenge is converting the data to
knowledge and presenting it for decision making.g p g g