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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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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We expected strong temperature influence so we tracked effects of thermal expansion on bearing effect2movement and cable tensions

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To get a fuller picture of the g pglobal response we installed a Total Positioning Systema Total Positioning System

Note: Top of Side Tower

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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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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%

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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

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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